Abstract

Diabetes mellitus is a disease characterized by hyperglycemia, a result of

abnormally high concentrations of glucose in the blood. Type II diabetes, Non-Insulin

Dependent Diabetes, accounts for 90% of all diabetic cases and is generally associated

with obesity and physical inactivity. One of the major characteristics of type II diabetes is

the progression of insulin resistance, which ultimately requires diabetics to inject insulin

to control blood glucose levels. With the increasing waist-line of Americans, type II

diabetes is reaching epidemic proportions with estimated costs of nearly $132 billion a

year [1]. The current study uses molecular modeling software to analyze the structure and

associated energies of vanadium(IV) and vanadium(V) complexes. The molecular

modeling studies generate energies that can be correlated to stabilities of the vanadium

complexes in identifying target compounds to be synthesized. Once target compounds are

synthesized they can be characterized and studied for their potential use as orally active

insulin mimetic agents.

1. Introduction

Diabetes Mellitus (DM) is a disease characterized by hyperglycemia and affects

20 million people in the United States alone. Hyperglycemia occurs when there are

elevated levels of glucose in the blood stream. Diabetes is an extremely costly disease in

terms of human financial expenditures and is reaching epidemic proportions. Diabetes

develops when resistance to the glucose lowering actions of insulin and impaired insulin

secretion occurs [2]. There are two types of Diabetes Mellitus, Insulin-dependent

Diabetes Mellitus (IDDM) also known as Type I diabetes, and Noninsulin-dependent

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Diabetes Mellitus (NIDDM) or Type II diabetes. The onset of Type I diabetes usually

occurs at a young age, and for this reason, Type I diabetes is often also referred to as

Juvenile Diabetes. Type I diabetes develops as a result of an autoimmune response in

which the body loses immunological tolerance for its own beta cells and ultimately

destroys them. Beta cells are found in the Islets of Langherons in the pancreas, and

secrete insulin. In Type I diabetes, an insulin deficiency results in an increase in

gluconeogenesis. This, along with impaired glucose transport in the peripheral tissues,

leads to dramatic hyperglycemia [3].

Type II diabetes usually occurs later in life and accounts for ninety percent of

diabetic cases affecting more than 150 million people worldwide. Type II diabetes

adversely affects virtually every organ in the body, frequently affecting the eyes, nervous

system, kidneys, and cardiovascular system. Type II diabetes is generally associated with

obesity and physical inactivity. The metabolic and inflammatory stresses associated with

obesity disrupt the normal operation of the endoplasmic reticulum (ER). The ER

attempts to cope with this stress by activating a transcriptional regulator. If homeostasis

is not restored, an ER stress-sensor activates a serine kinase that opposes insulin action

[4].

Type II diabetes is characterized by the progression of insulin resistance. This

occurs when the pancreas cannot produce enough insulin or when the body’s tissues

become resistant to insulin. Insulin resistance leads to decreased glucose clearance from

the circulation and excessive glucose production despite the availability of insulin [3].

Both types of diabetes involve an insulin deficiency; therefore, it is important that there

are therapeutic agents available to treat patients suffering from this disease.

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Vanadium is a transition metal currently under investigation for its potential

glucose lowering effects and insulin mimetic action. First discovered in 1813 by Del Rio

[2], vanadium is thought to be essential for proper cell growth and development, and to

enhance the phosphoprotein formation, which is attributed to either the activation of

protein kinases or inhibition of protein phosphatases [5]. The insulin-like properties of

vanadium compounds have attracted a great deal of attention in recent years for several

reasons. One of the key reasons for this attention is due to the fact that vanadium

presents a promising complementary approach to the management of diabetes,

specifically Type II diabetes in which insulin resistance is present [6]. It is thought that

vanadium improves insulin sensitivity by mimicing and enhancing the metabolic effects

of insulin on peripheral tissues [3]. It may be possible that vanadium can effectively

preserve pancreatic beta cell function, and in turn slow the progression of insulin

resistance in Type II diabetic patients.

Three key properties critical to the mimetic properties of vanadium compounds

include: stability, lability, and redox chemistry. Vanadium can easily alter its oxidation

states and can exist in both anionic and cationic forms. In its anionic form, Vanadium

exists as metavanadate in the +5 oxidation state. In the cationic form, it exists as vanadyl

in the +4 oxidation state [7]. The insulin mimetic action of vanadium is proposed to occur

via its participation in physiological reactions. Vanadium is able to play a role in several

oxidation-reduction reactions within the body. One such reaction is the oxidation of

vanadyl (V+4) to vanadate (V+5), generating hydrogen peroxide as a by product.

Hydrogen peroxide has been shown to mimic insulin action by playing a role in enhanced

glucose transport, and glucose oxidation. Vanadium also demonstrates several other

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physiological roles other than antidiabetic actions. Other physiological roles include:

stimulation of histamine release from mast cells, toxic and cytotoxic actions, and

stimulation of bone cell proliferation [8]. Vanadium interferes in various phosphate-

metabolizing reactions; some of the insulin-like effects have been related to inhibition or

stimulation of enzymes that participate in phosphate metabolism [23]. Vanadium is also

unique in that it forms low molecular weight compounds that are often analogous to

phosphate. Previous studies have shown that vanadium might mediate glucose lowering

effects by enhancing glucose uptake in skeletal muscle and adipose tissues. It is thought

that vanadium restores reduced levels of GLUT4 mRNA and enhances GLUT4

translocation in the muscles of diabetic animals. Vanadium has shown no effect on

GLUT4 expression or translocation in control animals, emphasizing that vanadium

restores diabetic-induced metabolic disorders while having no effect on normal

metabolism [6].

In order to develop an understanding as to how vanadium mimics insulin, it is

important to have an understanding as to how the insulin signaling cascade works in the

body. While it is not completely understood, it is believed that the insulin signaling

cascade begins with a series of signaling events emanating from the insulin responsive

glucose transporter GLUT4. In the absence of insulin, GLUT4 slowly recycles between

the plasma membrane and vesicular compartments within the cell. Insulin stimulates the

translocation of a pool of GLUT4 to the plasma membrane. An insulin receptor is also

involved in the signaling cascade; the insulin receptor is an activated tyrosine kinase.

The receptor is a heterotetrameric bifunctional complex consisting of two alpha subunits

and two beta subunits. The alpha subunits bind insulin while the beta subunits are

Passalacqua 4

involved with tyrosine kinase activity. Insulin binding to the alpha subunit induces the

transphosphorylation of one beta subunit on specific tyrosine residues in an activation

loop. The receptor also undergoes autophosphorylation resulting in the activation of

tyrosine kinase activity, ultimately leading to the phosphorylation of tyrosine [10].

There is a great interest in designing drugs that can be taken orally to decrease the

amount of diabetic patients that rely on insulin injections as a means of therapy.

Currently, there are therapeutic drugs available for oral administration to treat diabetic

patients. Some of these therapies work by increasing the sensitivity to the body’s own

insulin, while others work by causing the body to produce more insulin. There are no

current drug therapies available that mimic insulin. Drugs that mimic insulin action

would be extremely beneficial, especially for patients that are insulin resistant. Such

drugs could help maintain healthy levels of glucose in the body. There is a great interest

in designing drugs that can be taken orally to decrease the amount of diabetic patients

that rely on insulin injections as a means of therapy. Insulin injections are troublesome

and often can be painful especially for elderly patients. Drugs that can be taken orally

would alleviate the inconvenience that is associated with insulin injections. The design

of vanadium-containing compounds as insulin enhancing agents can take advantage of

vanadium’s many unique characteristics which render it ideal. In designing vanadium

complexes as insulin enhancing agents, it is important to improve the efficiency of the

drug by increasing gastrointestinal absorption as well as targeting insulin responsive

tissues, and minimizing toxicity [11].

For vanadium to be useful as an orally active insulin mimetic agent, it must be

able to cross biological membranes rather easily for the initial absorption process as well

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as for intracellular uptake. Most metal ions are assumed to cross cell membranes by

passive facilitated diffusion, which requires that the metallocomplex form low molecular

weight compounds, and have a fair degree of resistance to hydrolysis. Moreover, the

metal-ligand complex should possess adequate thermodynamic stability. Organic

ligands, complexed to vanadium in coordination compounds, present ways to fine-tune

the effects of vanadium. The choice of the coordinating ligands allow for any adverse

effects to be minimized without sacrificing important benefits [22].

One such compound Bis(maltolato)oxovanadium (BMOV), is a compound

currently under a great deal of investigation for its glucose lowering effects. BMOV is

considered to be the “benchmark” compound because of the success it has demonstrated

in lowering plasma glucose levels. BMOV is coordinated with two oxygen atoms from

each maltol unit giving it a coordination mode of VO4. The structure of BMOV is unique

in that it is coordinated with one neutral oxygen atoms as well as one negatively charged

oxygen atoms from each maltol unit.

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Figure 1: Structure of BMOV

Journal of Applied Physiology, 84; 569-575.

BMOV exhibits several potentially useful properties including: significant water

solubility, neutral charge, and lipophilicity and does not produce any overt toxic side

effects [28]. It is thought that BMOV enhances gastrointestinal absorption through a

passive diffusion process [12]. BMOV has proven to be effective in lowering plasma

glucose levels in Streptozotocin induced diabetic rats [19]. Current research in this field

includes the investigation as to whether inorganic or organic bound vanadium compounds

are more efficient in stimulating glucose uptake and lowering glucose levels as well as

exploring the many different binding motifs of organically bound vanadium complexes

[13]. Some of the binding motifs that are under investigation include: N2O2, O4, N2S2, N4,

O2S2, and S4.

2. Materials and Methods

2.1 Molecular Modeling

Spartan molecular modeling software was used to analyze the structure and

associated energies of vanadium(IV) and vanadium(V) complexes. The molecular

modeling studies generate energies relative to the stabilities of the vanadium complexes

in identifying target compounds to be synthesized. Molecular modeling allows for the

analysis of the stability of vanadium complexes in relation to their calculated energies as

well as their coordination modes. The energies were calculated using the Molecular

Mechanics Force Field equation.

anglesbond

A

anglestorsion

A

bondednon

A

atoms

B

bondednonAB

torsionA

bendA

bonds

A

StretchA

strain EEEEE_ _

Equation 1: Equation used to obtain MMFF calculations

Passalacqua 7

The wave equation is used to find the allowed energy levels of quantum mechanical

systems (such as atoms, or transistors). The associated wavefunction gives the probability

of finding the particle at a certain position. The solution to this equation is a wave that

describes the quantum aspects of a system [14]. The importance of the calculations

performed is to minimize strain energy between all bonds involved in each of the

complexes. The calculations are based on six tests of convergence.

Once the molecular modeling has been completed, any correlations that may exist

between the complexes and their effectiveness in lowering plasma glucose levels can be

distinguished. The current study investigates eleven different complexes which are

shown in Table 1:

Bis(N’N’dimethylbiguanidato) oxovanadium [VO(metf)2]Vanadyl bis(cysteinate methyl ester) [VCME]Oxobis(pyrrolidine N-carbodithioato) [VP]Bis(3-ethyl-2,4-pentanedionato) oxovanadium [VO(etacac)2]Bis(kojato)oxovanadium [VO(KA)2]Bis(1-oxido-2-pyridinethiolato)oxovanadium [VO(OPT)2][N’,N’-Bis(salicylidene)ethane-1,2 diaminato] oxovanadium [VOSALEN]Oxobis(picolinato)vanadium [VOPA]Bis(methylpicolinato)oxovanadium [VOMPA]Bis(maltol)oxovanadium [BMOV]Imidazole(oxo)bis(peroxo)vanadate [Imidazole]

Table 1: Vanadium Complexes included in molecular modeling study.

Passalacqua 8

The energy values of these eleven complexes were determined using the Spartan program

and then evaluated based on their glucose lowering potential as determined in previous

studies.

2.2 Infrared Spectroscopy

Infrared spectroscopy is one of the most common spectroscopic techniques used

in both organic and inorganic chemistry. An infrared or diffuse reflection spectrum is

obtained by passing infrared radiation through a sample. The main goal of infrared

analysis is to determine the chemical functional groups in a sample. Different functional

groups absorb characteristic frequencies of infrared radiation relative to the frequency of

the bending, stretching mode. Absorption of infrared is restricted to compounds with

small energy differences in the possible vibrational and rotational states. For a molecule

to absorb infrared radiation the vibrations and rotations within a molecule must cause a

net change in the dipole moment of the molecule. Infrared absorption is usually

represented as either wavenumbers or wavelengths. Wavenumber defines the number of

waves per unit length and is directly proportional to the frequency as well as the energy

of the infrared absorption. Wavelengths are inversely proportional to frequencies and

associated energy. Infrared spectra are obtained by detecting changes in transmittance

intensity as a function of frequency.

Infrared spectroscopy allows for the characterization of complexes as well as any

ligands that may be attached to the complex. Using infrared spectroscopy, the V=O

double bond stretching frequencies of synthesized vanadium complexes will be examined

and compared to spectrums of known vanadium complexes. The stretching frequencies

Passalacqua 9

of known vanadium complexes have been determined in prior studies. Stretching

frequencies will appear in different areas of the spectrum depending on the type of bond

vanadium is involved in. Vanadium-oxygen single bonds appear in the range of 520-

210cm-1, while vanadium oxygen double bonds are found in the range of 970-955cm-1

[16]. Finally, bands located in the range of 455-443cm-1 are a result of a vanadium-

nitrogen bond [17]. These bands can sometimes be shifted up field or down field

depending on the ligands attached. Different ligands can affect the absorption of the

infrared radiation and therefore cause the bands to be shifted up field or down field [15].

The infrared spectrums were obtained using a Shimadzu FTIR-8300 infrared

spectrometer. All chemicals were purchased from Sigma Aldrich and used without

purification. A background spectrum was obtained using Potassium Bromide [KBr]. A

background must initially be run to get rid of any outside interference such as particles in

the air. Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4] were used to

obtain infrared spectrums that can then be compared to spectrums from vanadium

complexes in previous studies. The infrared spectra from complexes synthesized in

previous studies will allow for the determination of the type of vanadium-oxygen bond

that exists in Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4].

3. Results

3.1 Molecular Modeling

Molecular modeling studies have been useful in determining the stabilities of the

vanadium complexes in question. The associated energy of each complex is directly

Passalacqua 10

related to the stability of the complex. Stable complexes will have lower energies, while

less stable complexes will have higher energies. Analysis of the energy of each complex

studied allows for the determination of the most stable as well as the least stable complex.

Each complex can then be examined based on the coordination mode to determine if

there is any correlation between stability and coordination mode. Each complex has been

included in a previous study performed by Thompson et al (2000); in determination of

the glucose lowering effects of each complex. The results of this study have been used to

compare the glucose lowering effects of each complex with the stability as well as the

coordination mode. Graph 1 depicts the MMFF calculated energy of the complexes

studied.

Graph 1: MMFF Calculated Energy of Known Vanadium Diabetic Complexes

Passalacqua 11

MMFF Calculated Energy of Known Vanadium Diabetic Complexes

-1000

-800

-600

-400

-200

0

200

400

VO

(met

f)2

VC

ME

VP

VO

(eta

cac)

2

VO

(KA

)2

V(O

PT

)2

VO

SA

LEN

VO

PA

VO

MP

A

BM

OV

Imid

azol

e

Complex

En

erg

y (k

J/m

ol )

The coordination mode can also be considered when analyzing the structure and energy

of the complexes. Coordination modes may play a role in the stability as well as the

effectiveness of the complex. The most stable complex in the set of complexes is

bis(N’,N’-dimethylbiguanidato)oxovanadium(IV) [VO(metf)2], with a calculated energy

value of -843.034 kcal/mol. This complex exhibits the coordination mode N4 and

demonstrates a square pyramidal geometry [18]. Figure 3 shows the structure of

VO(metf)2 as produced by Spartan.

Figure 3: Structure of VO(metf)2 as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

While VO(metf)2 is extremely stable, it has not shown promising results in effectively

lowering plasma glucose levels. In previous studies, VO(metf)2 has shown to decrease

plasma glucose levels to a point of euglycemia. Euglycemia is defined as having normal

glucose concentrations in the blood. While VO(metf)2 produced a state of euglycemia in

test animals, plasma glucose levels increased significantly upon termination of treatment

Passalacqua 12

[11]. Vanadyl bis(cysteinate methyl ester) [VCME] is another vanadium complex that is

relatively stable with a calculated energy of -97.649 kcal/mol. VCME has a coordination

mode of N2S2 and demonstrates a square pyramidal geometry [11].

Figure 4: Structure of VCME as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

Similar to VO(metf)2, VCME as shown in Figure 4, has nitrogen atoms bound to the

vanadium which may suggest that the nitrogen may play a role in the stability of these

compounds but does not contribute to its ability to effectively lower plasma glucose

levels. Like VO(metf)2, VCME has been shown to have little effect in lowering plasma

glucose levels. Results from previous studies have concluded that VCME is not very

effective because it is water insoluble and demonstrates high lipophilicity. This is

problematic in that the body can not easily metabolize drugs that are water insoluble and

are too highly lipophillic [30].

The current study has also rendered some fairly unstable complexes as well. The

compound BMOV, as previously mentioned is a relatively unstable complex with an

Passalacqua 13

energy of +123.227 kcal/mol. While this complex is relatively unstable compared to

other vanadium complexes modeled in this study, it has been proven to be the most

effective complex in terms of lowering plasma glucose levels in rats, and for this reason it

is the “benchmark” compound [19].

Figure 5: Structure of BMOV as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

The redox chemistry of BMOV demonstrates notable lability in oxidation and reduction.

The complexes ability to take part in a number of redox reactions may be a key factor in

its ability to effectively lower plasma glucose levels [11]. BMOV as shown in Figure 5

has a coordination mode of O4. There are several other vanadium complexes that exhibit

this same coordination mode. It is important to analyze these other complexes in terms of

their stability, as well as their effectiveness, to determine if there is a correlation between

the O4 coordination mode and glucose lowering effects. Imidazole, show in Figure 6 is

another fairly unstable compound that also has a coordination mode of O4. Imidazole has

Passalacqua 14

also proven to be effective in lowering glucose levels in animals. The calculated energy

for this compound was found to be +163.367 kcal/mol.

Figure 6: Structure of Imidazole as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

The imidazole complex coordinates to peroxovanadate has been shown to enhance

insulin receptor autophosphorylation in human liver cell culture, as well as increase

glucose transport in rat adipocytes. Peroxovanadate’s are not considered to be

hydrolytically stable and are subject to redox processes ultimately resulting in radical

formation, the formation of radicals raises the potential for increased intracellular

oxidative stress [11].

Bis(kojato)oxovanadium(IV) [VO(ka)2] shown in Figure 7 is a close analog to

BMOV that is synthesized using kojic acid. According to the calculated energy of

VO(ka)2, it is observed that the compound is somewhat more stable than BMOV. The

calculated energy of this complex was determined to be -38.296 kcal/mol. Like BMOV,

VO(ka)2 has the O4 coordination mode. This may be of importance in its effectiveness in

Passalacqua 15

lowering glucose levels however; it is also thought that the ligand involved may

contribute to its effectiveness [20].

Figure 7: Structure of VO(ka)2 as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

The ligand used in BMOV is maltol while the ligand used for VO(ka)2 is kojic acid.

Kojic acid is a close analog to maltol which may be the reason for the similarities

between the two complexes and their effectiveness. In previous studies, VO(ka)2 has

shown to reduce elevated plasma glucose levels to a state of euglycemia in forty-six

percent of test animals. These results were not sustained; twenty-four hours after

administration animals that had been treated had reverted to hyperglycemia [20].

The complex known as bis(3-ethyl-2,4-pentanedionato)oxovanadium

[VO(Etacac)2] is another complex with the O4 coordination mode as shown in Figure 8.

This complex is somewhat stable with a calculated energy of -44.678 kcal/mol. In

previous studies, VO(Etacac)2 lowered plasma glucose in streptozotocin induced diabetic

rats and closely paralleled results seen with BMOV [11].

Passalacqua 16

Figure 8: Structure of VO(Etacac)2 as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

The compound known as bis(1-oxido-2-pyridinethiolato)oxovanadium(IV)

[VO(OPT)2] is a complex with a coordination mode of O2S2 as shown in Figure 9.

VO(OPT)2 has a calculated energy of +24.334 KJ/mol. This complex has been

previously studied in vitro only, by free fatty acid release from adipocytes. VO(OPT)2

was 4.7 times as effective as vanadyl sulfate [VOSO4] [11].

Figure 9: Structure of VO(OPT)2 as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

Passalacqua 17

The complex VO(OPT)2 is compared to another sulfur containing complex known as

Oxobis(pyrrolidine-N-carbodithioato)vanadium(IV) [VP].

Figure 10: Structure of VP as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

While these two complexes do not exhibit the same coordination mode they do both

contain sulfur and may demonstrate some similarities. The complex VP as shown in

Figure 10, is insoluble in a variety or solvents [11]. The calculated energy for VP was

determined to be -51.382 Kcal/mol. Based on the calculated energy VP is a somewhat

stable complex. Comparisons between VO(OPT)2 and VP can be made based on their

energy values and the fact that they each contain sulfur. Upon evaluation of their energy

values, VP is the more stable of the two complexes. The complex VP has previously been

proven to be more effective than VO(OPT)2 when tested for insulin mimetic activity by

inhibition of free fatty acid release [11].

Passalacqua 18

The final coordination mode studied is the O2N2 coordination mode. There are

three complexes that fall into this coordination mode and can be compared based on their

calculated energy values as well as their glucose lowering effects seen in previous

studies. The first complex examined is [N,N’-bis(salicylidene)ethane-1,2-

dianinato]oxovanadium(IV) [VOSALEN] shown in Figure 11 [11]. The energy for this

complex was determined to be +34.067 kcal/mol.

Figure 11: Structure of VOSALEN as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

This compound has previously shown to be orally effective in lowering glucose levels in

diabetic induced rats; however, rats tended to become hypoglycemic. Previous studies

have also determined that upon withdrawal of VOSALEN a hyperglycemic state was

rendered [11]. Oxobis(picolinato)vanadium(IV) [VOPA] shown in Figure 12 is another

complex with a coordination mode of O2N2 examined in this study. The energy for VOPA

was found to be +45.503 kcal/mol.

Passalacqua 19

Figure 12: Structure of VOPA as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

Previous studies have determined VOPA to be an inhibitor of free fatty acid release [11].

Bis(methylpicolinato)oxovanadium(IV) [VOMPA] has the same backbone as VOPA,

they differ only in a substituent. VOMPA also has the coordination mode of N2O2 and

has an energy value of +46.526 kcal/mol. VOMPA has also previously been studied for

its insulin mimetic effects and like VOPA has also been proven to inhibit free fatty acid

release. VOMPA has actually been proven to be more efficient than VOPA.

Passalacqua 20

Figure 13: Structure of VOMPA as produced by the minimization of the energies using MMFF calculations in Spartan ‘04

The difference in energy as well as the effectiveness may possibly be due to the

difference in the substituent. The substituent in VOPA is a CH3 group while the

substituent in VOMPA is a C2H5 group [11].

The energy values of all three complexes with the coordination mode N2O2

shown in Table 2 are very similar. In previous studies, these three complexes have

produced similar results in lowering glucose levels. This strongly suggests that there is a

correlation between coordination mode and effectiveness. Based on the results obtained

from Spartan ’04, the energy values suggest that there is a correlation between energy

and coordination mode.

VOSALEN VN2O2 34.06715453 kcal/mol

VOPA VN2O2 45.50345086 kcal/mol

VOMPA VN2O2 46.52685028 kcal/mol

Table 2: Comparison of complexes with coordination mode N2O2 and their respective energy values

Passalacqua 21

3.2 Infrared Spectroscopy

Infrared Spectroscopy was used to analyze the V=O stretching frequency of two

samples including: Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4].

These spectra were then used to compare synthesized complexes. Sodium Metavanadate

[Na3VO4] and Vanadyl Sulfate [VOSO4] can be used as standard spectra because the type

of bond exhibited by vanadium is known and can be used to determine the type of bond

exhibited by the synthesized complexes. Both, Sodium Metavanadate [Na3VO4] and

Vanadyl Sulfate [VOSO4] exhibit V=O bonds; the spectrums obtained from these

compounds can be used to determine if the synthesized compounds exhibit a V=O bond.

The infrared spectrum of Sodium Metavanadate [Na3VO4] as shown in Figure 14

indicates a medium intensity peak at just about 1000 cm-1, indicating a V=O bond. This

peak is slightly up field from where V=O bonds are expected to be seen. The peak is

slightly up field due to the fact that Sodium Metavanadate [Na3SO4] exhibits a very

strong V=O double bond. The stronger bond is a result of increased energy as well as

increased wave number. The spectrum also contains a broad beak around 3300 cm-1; this

peak represents an OH functional group. The presence of this peak indicates that the

compound is not completely dry and that water is present.

Passalacqua 22

50075010001250150017502000225025002750300032503500375040001/cm

30

40

50

60

70

80

90

100

%T

sodium metavanadate

Figure 14: Infrared spectrum of Sodium Metavanadate [NaSO4]

The infrared spectrum of Vanadyl Sulfate [VOSO4] as shown in Figure 15 was

also used as a standard to compare the spectrums of synthesized complexes. The

spectrum depicts a sharp peak just to the left of 1000 cm-1. This peak is representative of

the V=O bond that exists in this compound. The peak is represented on the spectrum

where it is expected to be seen for this type of bond. A previous study performed by

Sakurai et al determined the V=O bond for vanadyl sulfate [VOSO4] to show a peak at

980cm-1 [22]. The spectrum also shows that there is an OH functional group present. The

OH functional group is represented by the broad peak found between 3000-3250 cm-1.

The presence of this peak means that water is also present in this compound.

Passalacqua 23

Figure 15: Infrared spectrum of Vanadyl Sulfate [VOSO4]

[(VO)L2(bipy)] is complex synthesized and studied for its infrared characteristics

in a previous study performed by P. Sreeja. The complex was synthesized using an aroyl

hydrazone and a bidentate 2,2’-bipyridine as an auxiliary ligand. The spectra of Sodium

Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4] were compared to the spectrum of

[(VO)L2(bipy)] to determine if the peaks indicating V=O bonds were in agreement. The

spectrum for [(VO)L2(bipy)] as shown in Figure 16 indicates a strong peak at 963 cm-1

which is representative of the V=O bond that is present [17]. This strongly supports that

V=O bonds are represented by peaks between 970-955 cm-1, it also strongly supports that

Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4] both demonstrate V=O

bonds.

Passalacqua 24

Figure 15: Infrared Spectrum of [(VO)L2(bipy)] Spectrochimica Acta, 61; 331-336.

4. Discussion

4.1 Molecular Modeling

Molecular modeling studies have helped to gain a better understanding of the

stability of several vanadium complexes by calculating the MMFF energy associated with

each complex. The stability of the complex along with its effectiveness in lowering

glucose levels as determined in previous studies can be correlated to find an ideal

complex that may be used as orally active insulin mimetics. The ligands attached to the

complex also play a role in the stability as well as the effectiveness. For a ligand and

vanadium to act synergistically, particularly in vivo, certain requirements must be

fulfilled. It is important that the appropriate kinetics of complexation and

Passalacqua 25

decomplexation, which ensures delivery of the metalo-oxide ion to key sites of glucose

metabolism [21].

The most stable complexes in the series of complexes studies are those in which

contain either nitrogen, sulfur, or nitrogen and sulfur together. These complexes include

VO(metf)2, VP, and VCME and all had energies that were relatively low making them

relatively stable complexes. While the vanadium complexes do not share the same

coordination modes they do have similarities. VO(metf)2 is the most stable complex and

has a coordination mode of N4. VMCE is the next stable complex and has a coordination

mode of N2S2. Finally while VP is not unstable, it is the least stable of these three

complexes and has a coordination mode of S4. In terms of stability, vanadium complexes

that are coordinated with nitrogen, sulfur, or both seem to render stable complexes.

Although these complexes are stable they are not the most effective in lowering glucose

levels based on studies that have been previously performed.

The next coordination mode investigated is the O4 coordination mode. The results

obtained with complexes in this coordination mode are not in agreement. There are four

complexes studies that demonstrate the O4 coordination mode. These complexes include:

VO(Etacac)2, VO(ka)2, BMOV, and imidazole. While these four compounds are not

extremely stable they have previously shown to be rather effective in lowering plasma

glucose levels. All complexes with this coordination mode have rendered desired

glucose lowering effects however; BMOV and imidazole have been more effective than

VO(Etacac)2 and VO(ka)2. Although BMOV and imidazole have been more effective,

they are less stable than VO(Etacac)2 and VO(ka)2. The results make it difficult to make

any direct correlations between stability and effectiveness in complexes with the O4

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coordination mode. Further studies would need to be performed to determine how the

ligands attached play a role in effectiveness as well as stability.

The final coordination mode studied is that of O2N2. These complexes are

somewhat stable and include: VOSALEN, VOPA, and VOMPA. These complexes have

similar energy values in addition to being coordinated with the same atoms. These

complexes have also demonstrated similar glucose lowering effects in previous studies.

While these complexes have shown to have some effect in lowering glucose levels these

effects were not very significant and were not sustained upon termination of treatment.

The fact that these complexes have the similar energy values and have produced similar

results in lowering glucose levels suggests that there is a possible correlation between

stability and coordination mode.

Complexes in each coordination mode seem to be at least somewhat effective in

lowering glucose levels and therefore suggest that there may be a possible correlation

between coordination mode and effectiveness. However, there is no correlation between

stability and effectiveness. Complexes that were shown to be stable were not always

very effective while complexes that are less stable have shown to be effective. The

results suggest that there is no trend in the stability of a complex and its ability to lower

glucose levels.

4.2 Infrared Spectroscopy

Infrared spectroscopy is a technique used to study different functional groups of a

compound. This study uses infrared spectroscopy to determine what functional groups

are present in vanadium compounds that have been synthesized. The spectrums obtained

are compared to the spectrums of known vanadium containing compounds. The spectra

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of Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4] are analyzed and then

compared to the spectrum of a previously synthesized complex to determine where V=O

bonds will appear on a spectrum. The V=O bonds appear on the spectrums anywhere

from 970-955 cm-1 and may be up or down field from this range depending on any

interference that may be present or due to the ligands coordinated to the vanadium [26].

The spectra for Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4]

both revealed a V=O bond within the expected range. These spectra are in agreement

with spectrums from a previous study that has also examined the V=O stretching

frequency for vanadium containing compounds. The infrared spectrum of [(VO)L2(bipy)]

is a synthesized complex from a previous study that examines the V=O bond using

infrared spectroscopy. The spectrum is in agreement with the spectrums of the studied

complexes, Sodium Metavanadate [Na3VO4] and Vanadyl Sulfate [VOSO4]. This

strongly supports that the V=O bond is found between 970-955cm-1. The infrared

spectrum of [(VO)L2(bipy)] also represents a V-O single bond with a peak at 510 cm-1.

This peak is found where it is expected to be for a V-O single bond [17].

Previous studies have also examined compounds such as: VO(acac),

VO(Cl)DMF, and VO(acac)2 all of which demonstrate a V=O bond. These complexes all

revealed spectrums in which the V=O peak was between 980-955 cm-1 [24]. The ligands

that are coordinated to the vanadium also play a role in the spectrum of the complex [25].

The infrared spectrum of [(VO)L2(bipy)] represents a synthesized complex from a

previous study that examines the V=O bond using infrared spectroscopy. The spectrum

is in agreement with the spectrums of the studied complexes, Sodium Metavanadate

[Na3VO4] and Vanadyl Sulfate [VOSO4]. This strongly supports that the V=O bond is

Passalacqua 28

found between 970-955cm-1. Infrared spectroscopy has proved useful in determining

where particular bonds are expected to be seen on an infrared spectrum.

5. Conclusion

Molecular Modeling software allows for the determination of the stability of

vanadium complexes in question as orally active insulin mimetics. The stability of a

complex is determined by minimizing the total energy of the complex. Stable complexes

have low energies, while unstable complexes have high energies. Modeling studies have

allowed for a better understanding of the structure of the complexes in question. Having

an understanding of the structure of a complex will allow further research to be done that

can lead to the determination of whether or not a complex will deem useful. In this case,

molecular modeling has allowed for the structure and the energy of the complexes being

studied to be determined. This information can help to lead to the determination of

whether or not these vanadium containing complexes will be successful in lowering

glucose levels in animals and potentially humans.

The results of this study along with results from previous studies suggest that

there is no clear correlation between the stability of a complex and its glucose lowering

effects in animals. The results show that the most stable complex VO(metf)2 is not very

effective in lowering glucose levels in rat adipocytes. Complexes that rendered stable

energies have shown to be fairly effective in lowering glucose levels. Knowing that there

is no correlation between the stability and effectiveness of a complex turns our attention

to the coordination mode of a complex. Analysis of the coordination mode of the studied

complexes will show whether a correlation exists between coordination mode and energy.

Passalacqua 29

Analysis of the coordination mode of the complexes shows that in some cases

there is a correlation between the coordination mode of a complex and the energy. For

example, complexes with the O2N2 coordination mode render fairly stable complexes.

The energy values for complexes with this coordination mode are fairly similar. These

complexes also have shown to have similar glucose lowering effects in previous studies

[11]. There are no correlations that can be made for the coordination mode O4, some

complexes with this coordination mode are fairly unstable while some are fairly stable.

However, these complexes have all proven to be fairly successful in lowering glucose

levels. The varying stability of these complexes could be due to the different ligands that

are coordinated to the vanadium. More research would need to be performed to

determine the effect of the different ligands on the energy of the complex. Further

research could lead to more correlations and the possibility of finding a suitable orally

active insulin mimetic.

Infrared spectroscopy is a technique that allows for the analysis of different

functional groups that may be present in a complex. Infrared spectroscopy studies have

allowed for the determination of the presence of V=O bonds in vanadium containing

compounds The complexes studied include: Sodium Metavanadate [Na3VO4] and

Vanadyl Sulfate [VOSO4]. These complexes were both studied for their spectral

characteristics and were both determined to contain a V=O bond. This was determined

by the presence of a peak located within the range that a metal oxide would be expected

to be found, this peak is expected to be found between 970-955 cm-1 [27]. The presence

of the V=O bond was confirmed with a spectrum of [(VO)L2(bipy)] which is known to

exhibit a V=O bond. These results strongly suggest that a V=O bond is without a doubt

Passalacqua 30

represented by a peak between 970-955 cm-1. Other synthesized complexes also support

these findings and have demonstrated similar peaks.

Infrared spectroscopy can be further used to determine other functional groups

that may be found in vanadium containing compounds. Infrared analysis of synthesized

complexes can be performed to determine other functional groups present and how they

may or may not contribute to the effectiveness of a complex. Knowing which functional

groups are present in a complex gives a lot of insight to the structure of a complex. The

functional groups of a complex can be further analyzed to determine their role and

whether or not they contribute to the effectiveness of a complex. Further studies also

include, the manipulation ligands coordinated to the vanadium in order to determine what

effects certain ligands have on a complex’s glucose lowering effects. Manipulation of

ligands can help to determine which if any ligands are ideal in rendering a useful drug

[29].

Passalacqua 31

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