Solution speciation of bioactive Al(III) and VO(IV) complexes

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Polyhedron 19 (2000) 2389–2401

Review

Solution speciation of bioactive Al(III) and VO(IV) complexes

Tamas Kiss a,*, Tamas Jakusch a, Melinda Kilyen a, Erzsebet Kiss b, Andrea Lakatos c

a Department of Inorganic and Analytical Chemistry, Jozsef Attila Uni6ersity, PO Box 440, H-6701 Szeged, Hungaryb Department of Inorganic and Analytical Chemistry, Kossuth Lajos Uni6ersity, PO Box 21, H-4010 Debrecen, Hungary

c Biocoordination Chemistry Research Group, Jozsef Attila Uni6ersity, PO Box 440, H-6701 Szeged, Hungary

This paper is dedicated to the memory of Professor Kalman Burger.

Abstract

The speciation of the toxic Al(III) and the beneficial VO(IV) in various biofluids and tissues is discussed in order to describethe solution state of these metal ions in the organism. The importance of ternary complex formation with relevant biomoleculesis emphasized. The interactions of Al(III) and VO(IV) with oligopeptides are also dealt with. The importance of the biospeciationof Al(III) ion in its transport and involvement in neurological disorders, and of insulin mimetic VO(IV) complexes is discussed.© 2000 Elsevier Science B.V. All rights reserved.

Keywords: Biospeciation; Al(III) complexes; VO(IV) complexes; Insulin-mimetics

1. Introduction

The chemical speciation of an element, either essentialor toxic, may be of basic relevance as concerns itsbiology, as the chemical form in which an element entersthe body mostly determines its absorption and transportproperties, and hence in part its biological and physio-logical activities. It is also true that to some extentindependently of how this essential or toxic element wasabsorbed (or the form, e.g. a metal ion-containing drug,in which it was administered), it undergoes transforma-tion because of the possible ligand-exchange reactionswith the many potential metal ion-binding biomoleculesin the biofluids or tissues.

In biological fluids, a majority of the metal ions arebound to proteins. In fact, only a small proportion ofthem are bound to low-molecular-mass (l.m.m.) com-pounds, mainly amino acids, biophosphates, carboxy-lates or hydroxycarboxylates. Free metal aquaions existin biological fluids only in extremely low concentrations,and at these concentrations cannot play a significantrole in physiological processes. In contrast, metals

bound to l.m.m. compounds play major roles in manybiological processes, such as intestinal absorption, cellabsorption, transport and renal excretion. An under-standing of these processes requires an accurate knowl-edge of the proportions of the l.m.m. and protein-boundfractions of the metal ions and of the delicate equilibriawhich determine these proportions.

Specific experimental techniques such as gel filtration,ion chromatography, ultrafiltration, ultracentrifugation,atomic absorption spectrophotometry, etc., are avail-able to separate and determine l.m.m. and high molecu-lar mass (h.m.m.) bound metals. On the other hand,computer programs developed to deal with multiplechemical equilibria can be actively applied not only forspeciation calculations of laboratory solutions, but alsofor the simulation of naturally occurring mixtures ofmetal ions and ligands [1].

The present paper gives a brief literature survey andan insight into our recent research connected with somebiospeciation problems relating to the toxic aluminumand the beneficial vanadium. Several examples are pro-vided to emphasize the importance of a knowledge ofthe solution state of these metals, and it is demon-strated how this speciation information may be used fora better understanding of the effects of these metal ionsin biological systems.

* Corresponding author. Tel.: +36-62-54-4337; fax: +36-62-42-0505.

E-mail address: [email protected] (T. Kiss).

0277-5387/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S 0277 -5387 (00 )00537 -4

T. Kiss et al. / Polyhedron 19 (2000) 2389–24012390

2. Characterization of Al(III) binding to smallbiomolecules

Aluminium is a well-established neurotoxic element[2]. Scheme 1 gives the various routes through whichAl can enter the human organism from the environ-ment, the diet or medication. A total of 10–30 mg ofAl is estimated to be ingested each day, but only avery small proportion of this Al (5–10 mg) is ab-sorbed. In many countries, aluminum sulfate is usedas a coagulant in water treatment. Acid rain consider-ably increases the Al load from the environment. Alcompounds, e.g. alums, are used as additives in differ-ent food-producing processes, for instance as bakingpowder in some countries. Some plants, e.g. tea, canaccumulate an enormous amount of Al: the Al con-tent of old tea leaves can reach even 3%. A significantamount of Al(III) may be liberated from Al vesselsused in the kitchen when acidic meals are cooked inthem. Some antacids and buffered medicines also con-tain Al. Al can accumulate dangerously in childrenwith an impaired renal function who are treated withorally taken Al(III)-containing phosphate binders.Some forms of medical treatment, e.g. long-termhaemodialysis, can also elevate the Al level in humansif the water used has a high Al content. Under normalconditions, i.e. with a normal daily load and a healthyorganism, this amount is well tolerated (although thephysiological effects of long-term low aluminum load,e.g. from the drinking water are still not clear [3]), asa consequence of the efficient defence mechanism, in-volving the formation of poorly-soluble phosphatesand hydroxides in the gastro-intestinal tract, which areeasily excreted. However, an abnormally high Al loadand/or with an impaired renal function, an excess ofAl(III) can be absorbed and transported to various

target organs, where it may accumulate and exertharmful effects, e.g. osteomalacia in the bones, micro-cytic anemia in the red blood cells, or neurodegenera-tive diseases in the brain.

Al(III) is a typical hard metal ion, and the mostlikely binding sites of Al(H2O)3+ in biosystems aretherefore O donors, and especially negatively-chargedO donors. Carboxylate, phenolate, catecholate andphosphate are the strongest Al(III) binders.Biomolecules containing such functions may be in-volved in the uptake and transport processes, and alsoin the biological and physiological actions of Al(III)in living systems. These small biomolecules (given inthe frames in Scheme 1) are either of biological im-portance or can mimic larger biomolecules. For exam-ple, salicylate and catecholate derivatives can serve asgood model compounds to study the metal ion- bind-ing properties of the larger soil organic material hu-mic and fulvic acids. Small organic acids, such ascitric acid for example, are important small moleculeAl binders in biological fluids. Transferrin plays anessential role in the Al(III) transport in the plasma.2,3-Diphosphoglycerate is important in the bindingand accumulation of Al(III) in the red blood cells,resulting in a special kind of anemia. Other phos-phates, such as nucleosidephosphates, e.g. ATP, arewidely distributed in living systems, and a large vari-ety of the reactions which take place in living organ-isms involve ATP. Catecholamines occur in fairly highconcentration in the brain, where other potent Al(III)binders, e.g. transferrin or citrate, occur at relativelylow concentrations, so that catecholamines can be ofimportance in the binding of Al(III) in the brain.Phosphorylated proteins, which are rich in abnormalneurone degradation products occurring in variousneurological disorders, may play roles in the accumu-lation of Al(III).

In recent years, we have studied the interactions ofsuch small biomolecules with Al(III) and described thespeciation and solution structures of the complexesformed [4–18], in order to obtain the necessary stabil-ity constants and knowledge promoting characteriza-tion the solution state of Al(III) in biological fluidsand tissues. Because of the possibility of the ratherslow formation of complicated oligonuclear mixed hy-droxo complexes in many of these Al(III)–ligand sys-tems, pH-potentiometric solution speciationmeasurements were carried out at different metal ionconcentrations (usually at cAl=1–4 mmol dm−3) andat various metal ion to ligand ratios (usually at 1:1–1:10), and thermodynamic equilibrium was rigorouslychecked [14,17,18]. Those titration points, when 10min was not enough to reach thermodynamic equi-librium state were omitted from speciation calcula-tions.Scheme 1.

T. Kiss et al. / Polyhedron 19 (2000) 2389–2401 2391

Table 1Total concentrations of components in computer model a of serum

Concentration l−1Components

High molecular mass (h.m.m)Albumin 630 mmol

37.0 mmolTransferrin77.0 mmolHistidine

Lactate 1.51 mmol9.20 mmolOxalate

Low molecular mass (l.m.m.)Phosphate 1.10 mmol

99.0 mmolCitrateCarbonate 24.9 mmol

10.9 mmolCistine33.0 mmolCysteine2.30 mmolGlycine60.0 mmolGlutamate330 mmolSulfate39.8 nmolH+ (free)417 nmolOH− (free)

Al3+ 3.00 mmol1.37 mmolCa2+ (free)580 mmolMg2+ (free)

Zn2+ 10.0 mmol

a Data are taken from Ref. [19].

much weaker metal ion binder and is not assumed to beable to compete for a significant amount of Al(III) [25].Among the l.m.m. binders, citrate and phosphate arethe most important. Amino acids [17], lactic acid, oxalicacid [13] and inorganic anions are not competitivebinders in the presence of citrate and phosphate [13,24].The picture is much more controversial as concernsthese l.m.m. Al(III) binders. Dayde et al. [20], Jackson[21] and Harris [19] propose inorganic phosphate as themain Al(III) binder, while Clevette and Orvig [22],Duffield et al. [23] and O8 hman and Martin [24] suggestthat citrate (Cit) is the only important l.m.m. binder.Our earlier results [16] led us to conclude that both thephosphato and citrato complexes of Al(III) contributeto the l.m.m. fractions in the blood plasma, althoughabout 80% of the Al(III) binds to transferrin.

The reason of such contradictory results is the lack ofreliable speciation data [13,26]. In the Al(III)–phos-phate (A3−) system, under model conditions in themmol dm−3 concentration range precipitation occursat pH\3.5, and we need to extrapolate from thesedata to physiological conditions, which are entirelydifferent from the model conditions. We used LFERrelationships to obtain more reliable speciation data forthe Al(III)–phosphate system: log b ([AlAH]+)=17.6,log b (AlA)=13.5, log b ([AlAH–1]−)=7.2 at 25°Cand I=0.2 mol dm−3 (KCl) [14].

Further, there are rather slow oligomerization reac-tions in the Al(III)–citrate (A3−) system, which arestrongly concentration dependent. As first reported byO8 hman [27,28], at mmol dm−3 model concentrations atrinuclear species [Al3(CitH–1)3(OH)]4− predominatesin a wide pH range including the physiological pH,when the thermodynamic equilibrium state is moni-tored by potentiometric titration (equilibration maytake hours, especially in equimolar metal ion to ligandsolutions). This trinuclear species is formed, but in arather slow process, via the interactions of mononuclear1:1 and 1:2 complexes, which exist at the beginning ofcomplex formation and also predominates at high ex-cesses of ligand [29]. The slow formation of the trinu-clear complex through one or two intermediate speciesis nicely demonstrated by the changes in the 1H NMRspectra at pH �5 (see Fig. 1); after mixing, the signalsare poorly resolved, broad and merged, indicating dy-namic changes in the system. The complexation pro-cesses and/or ligand exchange between the initiallyformed mononuclear complexes and the intermediatespecies seem to be fairly fast. In the equilibrium statethe characteristic, but rather complicated pattern of thetrinuclear complex [Al3(AH–1)3(OH)]4− predominatesin the spectrum. It is a reasonable question whether thisapproach provides a realistic description of the Al(III)speciation under physiological conditions: when theAl(III) concentration really exceeds a few mmol dm−3,the ligand excess for potential binders is at least 1000–

Table 2Simulated species distribution of low molecular mass Al(III) in bloodserum a

Protein-bound Al(III) l.m.m. fraction b Refs.Al(III)

80% [Al(CitH–1)(OH)]2−: 94%; [23][Al(CitH–1)]−: 3%; Al(PO4): 1.5%;[Al(Oxalate)(OH)2]−: 1.4%

50% Al(PO4): 62%; [Al(CitH–1)]−: 23%; [21][Al(PO4)(CitH)]2−: 10%Al(OH)3: 51%; Al(PO4): 41.5%; [20]57%[Al2(PO4)(OH)2]+: 7.2%

83% Al-citrate: 98%; Al-hydroxide: 2% [22](phosphate not considered)

[19]81% [Al(PO4)(OH)]−: 80%; Al(PO4): 2%;[Al(CitH–1)]−: 10%; Al(OH)3: 4%;[Al(OH)4]−: 3%

[16]Al-citrate: 80%; Al-phosphate: 20%77%

a Based on models reported in Ref. [25].b Total is 100%.

3. Biospeciation of Al(III) in blood serum

Absorbed Al(III) is transported through the bloodstream, but its speciation in the blood serum is still notknown. The chemical composition of the serum given inTable 1 is taken from Ref. [19]. As listed in Table 2,different speciation models [16,19–24] have been re-ported in the literature.

As concerns the h.m.m. components, all models agreethat most of the Al(III) is bound to transferrin; albu-min, the other potential h.m.m. carrier protein, is a


Fig. 1. 1H NMR spectra of the Al(III)–citrate (A3−) system, 1:1, atpH 5.0 as a function of time: (a) 15 min, (b) 40 min, (c) 10 h and (d)30 h after mixing of the components, with cAl(III)=0.04 mol dm−3

(taken from Ref. [30]).

10 000-fold, and in consequence of the continuousmetabolism, the biological fluids are open systems,which never reach real thermodynamic equilibrium.The problem of time-dependent speciation is a realchallenge in Al(III) bioinorganic chemistry. Speciationmodel and stability constants for complexes formed inthe Al(III)–citrate system at 25°C and I=0.2 moldm−3 (KCl) are as follows: log b ([AlCitH]+)=10.15,log b (AlCit)=7.86, log b ([AlCitH–1]−)=4.15, log b

([AlCitH–2]2−)= –1.71, log b ([AlCit2]3−)=12.75,log b ([AlCit2H–1]4−)=7.94 and log b ([Al3(CitH–1)3-(OH)]4−)=16.35 [30,31].

With a single exception, the results listed in Table 2were obtained by ignoring the formation of ternarycomplexes between citrate and phosphate.

We recently have studied the Al(III)–citrate (A3−)–phosphate (B3−) ternary system and monitored thetime-dependent speciation by pH-potentiometry andmultinuclear 1H, 13C and 31P NMR spectroscopy. Adetailed description of the role of time in the speciationof this system is given elsewhere [32]. Here, we focusonly on the application of these results in order toobtain a reliable description of the speciation of Al(III)under serum conditions. The speciation curves obtainedby pH-metry at different times after mixing of thenecessary amounts of the stock solutions of Al(III) andcitric acid adjusted to the required pH are shown inFig. 2.

It can be seen that the ternary species [AlABH]2−,[AlAB]3− and [AlABH–1]4− predominate throughoutthe whole pH range from mixing until real thermody-namic equilibrium is reached. Time affects only theconcentration of the trinuclear species, which is formedin a rather slow process from the mononuclear species[AlAH–1]− and OH−:

3[AlAH–1]− +H2O= [Al3(AH–1)3(OH)]4− +H+

In the physiological pH range, Al(III) is bound in theternary species [AlAB]3− and [AlABH–1]4−, and thebinary species with phosphate, [AlBH–1]−, and citrate,[AlAH–2]2−. The results were rather surprising as themonodentate phosphate proved to be at least as effi-cient an Al(III) binder as the tridentate citrate. Multi-nuclear NMR measurements confirmed this result. Itcan be seen in Fig. 3, that when phosphate was addedin twofold excess to a solution of the trinuclear species,the phosphate slowly displaced citrate from theoligomeric complex and in a few days the characteristicquartet of citrate was observed in the 1H NMRspectrum.

Similarly, the 31P NMR spectrum (Fig. 4) of theAl(III)–citrate–phosphate system at pH �7 exhibitsthree signals, a sharp resonance at �2 ppm, ascribedto free phosphate and two broad resonances at −3.3and −13 ppm, ascribed to Al(III)-bound phosphates[14]. A comparison of the NMR spectrum with thespeciation curves calculated for the experimental condi-

Fig. 2. Time-dependent speciation curves of Al(III)–citrate (A3−)–phosphate (B3−) system at a 1:1:4 metal ion to ligand ratio: (a) 5min, (b) 40 min and (c) 30 h after mixing of the components, withcAl(III)=0.04 mol dm−3.


Fig. 3. 1H NMR spectra at pH 7.4 of: (a) [Al3(CitH–1)3(OH)]4−, 10mmol dm−3; (b) 10 h after the addition of phosphate to the samplein 20 mmol dm−3 concentration; and (c) 5 days after the addition ofthe phosphate to the solution (equilibrium state) (taken from Ref.[32]).

tions of the NMR measurements indicates practicallycomplete agreement between the speciation results ob-tained by the two independent methods. Fig. 5 depictsthe species distribution of Al(III) under plasma condi-tions. At physiological pH, phosphate seems to be themore efficient Al(III) binder, although citrate-boundAl(III) also occurs in significant concentration, in ac-cordance with the results of Sadler et al. [33], whodetected Al(III)-bound citrate in native serum by meansof 1H NMR.

4. Interactions of Al(III) with oligopeptides

Another Al(III) speciation problem relates to theinteractions of Al(III) with proteins or peptides andtheir phosphorylated derivatives. This is connected withthe involvement of Al(III) in Alzheimer’s disease (AD),which is again a rather controversial issue [34]. Thereare probably a majority who deny any connectionbetween Al(III) and AD. The crucial question iswhether Al(III) is present at elevated concentrations inneurofibrillary (NF) tangles and/or senile plaques,which are common intracellular and extracellular mark-ers of AD. According to our rather modest view,Al(III) is certainly not a causative factor in AD but itcan be a risk factor as it can enhance plaque and tangleformation [35,36]. It has been clearly demonstrated byCD and FT-IR methods that Al(III) can interact withO-donors, such as Asp–COO−, Glu–COO− or Ser–OH, Thr–OH or Tyr–O− rich fragments of b-amyloidsor NF-proteins either via intramolecular complexation,causing conformational changes, or via intermolecularcomplex formation, linking peptide chains togetherthrough the carboxylates or phosphorylated Ser or Tyrsites. And this leads to the aggregation of neuronaldegradation products.

The above mentioned results led us to attempt toobtain quantitative equilibrium-speciation and solutionstructural information concerning these interactions.Accordingly, we first studied the interactions of Al(III)with the building blocks of these compounds, i.e. aminoacids, such as Asp, Glu, Ser, etc. [17], and also withphosphorylated Ser and Tyr [18]. Then we continuedwith small peptide model systems, first the Al(III)–Asp-Asp (H3A) system. As demonstrated in Fig. 6, in theweakly acidic pH range the formation of mono com-plexes in different protonation states, [AlAH2]2+,[AlAH]+, AlA and [AlAH–1]− was detected, but, de-pending on the metal to ligand ratio, precipitationoccurred at pH �5–6. This indicates that the terminalCOO− groups of Asp (or Glu) are not sufficient in sucha small peptide to keep Al(III) in solution and toprevent precipitation of Al(OH)3. To achieve this, amore specific arrangement of suitable side-chain donorsis necessary.

Fig. 4. 31P NMR spectrum of the Al(III)–citrate (A3−)–phosphate(B3−) system, 1:1:2, at pH 7.4 in the thermodynamic equilibriumstate, with cAl(III)=0.04 mol dm−3.

Fig. 5. Speciation curves for the complexes formed in the Al(III)–ci-trate (A3−)–phosphate (B3−) system mimicking serum conditions:cAl(III)=0.6 mmol dm−3 (20% of the total cAl(III) in the serum, whichis bound to l.m.m. ligands), ccitrate=99.0 mmol dm−3, andcphosphate=1.1 mmol dm−3.


We have studied several penta- and heptapeptides.Only preliminary data are available on these systems atpresent and a full speciation and structural descriptionof the Al(III) complexes formed with oligopeptides willbe reported later. In the first group, pentapeptides werestudied, which were rich in Glu and Ser, and had aprotected C-terminus and a free or protected N-termi-nus, e.g. Ac–ProGluValSerGly–NH2 (HA) and its ana-logue HN–ProGluValSerGly–NH2 (HA)(see Scheme2). We found that the Al(III)-binding capabilities of thepeptides were stronger when they had a free Pro–NHfunction at the N-terminus. This is rather surprising asAl(III), is known [17,37] to have a low affinity foramines, and the N-terminal Pro–NH group is thereforenot expected to be a strong binding site. As the C-ter-minal COO− was blocked in both peptides, we wouldexpect coordination via the side-chain Glu–carboxylateand Ser–alcoholate groups. Instead, coordinationseems to be much more likely through the N-terminaldonor groups. In a second group the peptides had afree C-terminus, but a blocked N-terminus. With theseheptapeptides, 1:1 complexes could again be detectedby pH-metry; for one of these peptides, Ac–LysSer-ProValValGluGly (H2A, the formula is given in Fig. 8),the stability constants obtained were as follows:log b ([AlAH2]3+)=17.11, log b ([AlAH]2+)=13.30,log b ([AlA]+)=9.36 log b (AlAH–1)=4.72,log b([AlAH–2]−)= −0.42. 1H NMR indicated a weakinteraction of the peptide with Al(III) in the slightlyacidic pH range. Fig. 7 depicts the 1H NMR spectra ofthe peptide at pH �4 in the absence and the presenceof equimolar Al(III).

The significant selective line broadening indicatesinteractions between the peptide and the metal ion. Therate of the ligand-exchange reaction is roughly com-parable with the NMR time scale; hence, there is noshift in the position of the resonances, and there is noincrease in the number of signals. The preliminaryassignments of the signals indicate mostly that thesignals of the protons near the C-terminus undergobroadening mostly, which suggests that coordinationstarts at the terminal-COO−, (complex [AlAH2]3+) andAl(III) then chelates through the terminal-COO− of thepeptide and the side chain-COO− of Glu with partici-pation of the central peptide-carbonyl, through theformation of a joint chelate system (complex [AlAH]2+),and then parallel overlapping deprotonation of thenon-coordinating protonated amino donor and a coor-dinated water molecule subsequently takes place([AlA]+, AlAH–1 and [AlAH–2]−) (see Fig. 8). Two-di-mensional carbon–proton correlation NMR measure-ments are in progress in order to achieve a completeassignment of the protons. Molecular dynamic calcula-tions indicate favored arrangements of the terminalcarboxylate and the side-chain-COO− of Glu for thechelation of Al(III). It is interesting, however, that at

Fig. 6. Speciation curves for the complexes formed in the Al(III)–As-pAsp (A2−) system at a 1:4 metal ion to ligand ratio, with cAl(III)=1.0 mmol dm−3.

Scheme 2.

Fig. 7. 1H NMR spectra of Ac–LysSerProValValGluGly (H2A) at4.0 mmol dm−3 at pH 4.0 (a) in the absence and (b) in the presenceof 4.0 mmol dm−3 Al(III).

Fig. 8. Suggested binding modes in the Al(III) complexes of Ac–LysSerProValValGluGly (H2A).


Scheme 3.

been obtained for VO(IV)-promoted amide deprotona-tion and coordination, although this was strongly sug-gested in some glycine/alanine dipeptide complexes byCD and EPR measurements [46]. Among various bind-ing isomers, a species with the usual (NH2, N−, COO−)coordination involving deprotonated amide-N− wasalso assumed at pH �8. Interestingly, no amide depro-tonation was observed with the tripeptide GlyGlyGly[47]. It has been found for many metal ions that thepresence of a suitable anchoring donor which can bindmetal ions strongly enough and promote deprotonationof the amide plays a crucial role in amide deprotona-tion [48]. In aqueous solution, the terminal-NH2 is nota particularly good anchoring donor for the hardVO(IV) ion.

In order to make the arrangement of the donorgroups more favorable for VO(IV) binding, we studiedhow the presence of an extra negatively charged O-donor side-chain COO− group at the C- or N-terminusof simple dipeptides affects the VO(IV) binding ofGlyAsp and AspGly (H2A, Scheme 3) [49]. The resultswere very similar to those for simple dipeptides: noamide deprotonation could be detected with GlyAsp,but was suggested with AspGly at pH �8 after disso-lution of the precipitate VO(OH)2. The side-chain O-donors near the terminal -NH2 or COO− groups didnot favor amide deprotonation and coordination, asnew competitive binding sites in chelating positionswere developed in the molecules, and these bound themetal ions.

In the next step, the terminal –NH2 group wasreplaced by a hard phenolate as anchoring donorgroup: the dipeptide analogue salicylglycine (SalGly)(HOC6H4C(O)NHCH2COOH, 2-hydroxyhippuric acid,H2A) proved to be an efficient VO(IV) binder: noprecipitate occurred in the pH range 1–12, and EPRunambiguously indicated the presence of a N donoraround the metal ion [50]. As may be seen in Table 3,the fairly high g values and low A values stronglysuggest the presence of a coordinated amide-N− in theequatorial plane of the species [VOAH–1]− and[VOAH–2]2− (Scheme 4).

We recently started investigations of the tripeptideanalogue SalGly–L–Ala (H2A) too, where the terminalamino acid is an L enantiomer, and thus we can alsouse CD to obtain information regarding the bindingmodes of the ligand. Our preliminary results indicatethat both CD and EPR suggest coordination of thepeptide-amides in the pH range 4–9. For example, afurther decrease in the coupling constants A and anincrease in the g factor of the complex [VOAH–2]2−

(go=1.983, Ao=85, g��=1.963, A��=157 and gÞ=1.985, AÞ=51) is a fairly unambiguous indications ofthe presence of N donors in the equatorial coordinationsphere of VO(IV) (Scheme 5). Interestingly, with Sal-GlyAla the formation of various dimeric species pre-

pH �7 the two spectra (in the presence and absence ofAl(III)) are barely different and direct coordination ofAl(III) is therefore not confirmed. However, since noprecipitation was observed in the solution, it is verylikely that Al(III)-hydroxo species in some metastablestate (which occurs fairly commonly in aqueous Al(III)solutions), form outer-sphere complexes through hydro-gen-bonding with the hydrated oligopeptides. We arenow going further with similar oligopeptides, and alsowith their phosphorylated derivatives, as interchainlinks involving Al(III) between oligopeptides can playan even more important role in the aggregation ofneuropeptides, than intramolecular Al(III) binding.

To summarize, although the results are not alwaysstraightforward, these studies confirm that neurofila-ment non-phosphorylated peptides can bind Al(III),and this can trigger conformational changes resulting inaggregation processes of b-amyloids and NF tangles.

5. Characterization of VO(IV)–peptide interactions

Vanadium is an important trace metal with a greatvariety of physiological effects. It can act, for instance,as an enzyme regulator in various phosphate-metaboliz-ing reactions, suggesting binding between vanadiumand the protein side-chains [38]. In oxidation states III,IV and V, vanadium binds to transferrin, the essentialmetal ion transport protein in the serum [39]. Its inter-actions with oligopeptides in the physiological pHrange, via coordination of the deprotonated peptide-N− group, have been proved to involve vanadium inoxidation state V [40–42].

Vanadium in its high oxidation states +4 and +5 isa rather hard metal ion, like Al(III), and forms com-plexes of fairly high stability with ligands containingO-donor atoms, but it binds more weakly to N- orS-donor containing biogenic ligands [43] in solution. Itwas seen above that Al(III) is able to bind to peptidesat either the N-terminal or the C-terminal end, involv-ing mostly O-donor groups, but is not able to inducedeprotonation and subsequent coordination of the pep-tide amide. And what about VO(IV)? The amide coor-dination of a few synthetic ligands to VO(IV) has beenobserved in the solid state [44]. Tasiopoulos et al. [45]recently isolated the first VO(IV) complexes of dipep-tides containing a deprotonated peptide-N− group. Inaqueous solution, however, no unambiguous proof has


Table 3Speciation and EPR data on VO(IV)–SalGly system a

go Ao (10−4 cm−1)Species g��log b A�� (10−4 cm−1) gÞ AÞ (10−4 cm−1)

[VOAH]+ 10.24(3)1.964 101VOA 1.9387.05(2) 1751.971 92 1.9492.29(2) 165[VOAH–1]− 1.980 571.971 89 1.951 163[VOAH–2]2− 1.980−5.28(3) 55

5.55(6)[VOA2H–1]−

a Data are taken from Ref. [50].

dominates in the pH range 4–10 and through stepwisedeprotonation of the species [(VO)2A2]2− to[(VO)2A2H–4]6− the coordination mode changes from2× (COO−,�O,�O,O−) to 2× (COO−,N−,N−,O−).The driving force of dimer formation is presumably thestacking interaction between the aromatic rings. TheCD results suggest that the stepwise deprotonation andcoordination of the amide start at the C-terminal end ofthe ligand. Accordingly, the terminal COO− and notthe phenolate seems to be the primary anchor. Furtherstudies with various peptides, pseudopeptides andderivatives in a search for efficient anchors which canpromote amide deprotonation are currently in progressin our laboratories. For example, in connection withthe phenolate and carboxylate anchors, we have beenstudying the VO(IV) complexation with N,N %-bis(5-sulphosalicyl)ethylenediamine and N,N %-bis(malonyl)-ethylenediamine, when phenolate or carboxylate an-chors, respectively, are available at both ends of themolecules. With various N-D-gluconylamino acids,EPR and CD measurements indicated the formation ofspecies containing a deprotonated peptide-N− donor.Alcoholic-OH groups of the sugar moiety in appropri-ate steric arrangements proved to be efficient anchors[51]. Similarly, thiolate-S− in mercaptopropyonyl-glycine [52,53] and gluthathione [52,54,55] seem to besuitable anchoring donors towards VO(IV) to induceamide deprotonation not only in the solid state [52], butalso in solution [53–55]. Phosphonic (–PO3

2−) groupsof diphosphono-dipeptides has been shown to act asefficient anchoring donors towards Cu(II) [56], and weare now studying their interactions with VO(IV).

6. Vanadium complexes as insulin mimetics

Vanadium is a nutritional element present in mam-malian tissues. It is available in anionic and cationicforms, the most common ones under physiological con-ditions being vanadate(V) (H2VO4

−) and oxovanadium-(IV) (VO2+). Many studies have been performed on thebiological significance of vanadium compounds duringthe past two decades. The insulin-like effect of vana-dium is one of the most noteworthy findings and therelationship between vanadium and diabetes mellitus

has been extensively studied [57–60]. Since the firstreport of the insulin-mimetic activities of vanadiumcompounds in vitro in 1979 [57], and in vivo in 1985[60], great efforts have been made to prepare vanadium-(IV) and vanadium(V) complexes of high biologicalactivity and low toxicity which are readily absorbed.Among others Orvig et al. and Sakurai et al. preparedmany oxovanadium(IV) complexes with various coordi-nation modes, such as VO(O4) [61–63], VO(S2N2) [64],VO(S4) [65,66], VO(N3O) [67], and VO(N2O2) [68–70],and examined the relationship between their structuresand insulin-mimetic activities by evaluating both the invitro and the in vivo experimental results.

With regard to the coordination chemistry ofVO(IV), fairly stable complexes can be expected withO-donor ligands, but not with N,S donors. These com-plexes will presumably decompose and liberate most ofthe carrier ligands when they dissolve after oral admin-istration. Even if they form strong complexes at neutralpH, at the pH �2 of the gastric juice in the stomachthey will almost completely dissociate, being labile com-plexes. Accordingly, other biomolecules that are poten-tial metal ion binders present in biological fluids maypartially or completely displace the original VO(IV)-carrier molecules from the coordination sphere of themetal. Ternary complex formation should therefore betaken into account in a speciation description of thesecomplexes in biological fluids. Such ternary complexes

Scheme 4.

Scheme 5.


Fig. 9. Speciation curves for complexes formed in the systems: (a)Al(III)–maltolate (A−), (b) VO(IV)–picolinate (A−) and (c)VO(IV)–6-Me-picolinate (A−) systems at a 1:4 metal ion to ligandratio, with cVO(IV)=0.002 mol dm−3. Calculated with stability con-stants reported in Refs. [71,72].

forms the most stable complexes with the VO(IV) ionvia the formation of five-membered chelates throughthe oxo and alcoholate groups. The neutral bis com-plex, which may have good membrane transport prop-erties, exists in the pH range 5–7.5. Picolinic acid and6-methylpicolinic acid also form mono and bis com-plexes via the pyridine N and carboxylate groups. How-ever, the methyl group in 6-methylpicolinic acid has avery marked effect on the VO(IV)-binding capabilitiesof the ligands: presumably because of the steric effect ofthe methyl group, the stabilities of these complexes aresignificantly lower. This is more pronounced for the 1:2complexes. In the VO(IV)–picolinic acid system, the biscomplex is predominant in the pH range 2–7, while for6-methylpicolinic acid VOA2 exists in the pH range4–6. The VO(IV)-binding abilities of the ligands stud-ied exhibit the sequence maltol\picolinic acid\6-methylpicolinic acid. The EPR parameters of the biscomplexes listed in Fig. 10 also support this sequence.The value of the hyperfine constant, A�� increases, whilethe g�� value decreases in this direction, which revealsthe formation of bonds with different strengths in thesesystems. The EPR results show that the bis complexes

Fig. 10. Geometric isomers and EPR parameters of VO(maltolate)2,VO(picolinate)2 and VO(6-Me-picolinate)2.

might be of great importance in the absorption andtransport processes of the various potential insulin-mimetic drugs, such as VO(maltolate)2, VO(picolinate)2

and VO(6-Me-picolinate)2 and even in their physiologi-cal activity. In order to describe the speciation of theseinsulin mimetics in various biological systems, we arecurrently studying their interactions with l.m.m.biomolecules in the organism, such as phosphate (gas-tro-intestinal tract), adenosinenucleotides (intracellularfluids), hydroxycarboxylic acids, lactic acid and citricacid (blood serum) and catecholamines, e.g. dopamine(brain). Here, mostly results on the ternary complexformation of VO(maltolate)2 are presented [71].

7. Biospeciation of insulin-mimetic VO(IV) complexesin biofluids

We shall consider first the solution speciation of theinsulin-mimetic compounds: VO(maltolate)2, VO(picoli-nate)2 and VO(6-Me-picolinate)2 [71–73]. As may beseen in Fig. 9, mono and bis complexes are formed inthese systems, but there are great differences betweenthe stability constants of the complexes formed. Maltol


Fig. 11. Predominance diagram of the systems (a) VO(IV)–maltolate (A−)–phosphate (B3−), (b) VO(IV)–picolinate (A−)–phosphate (B3−), and(c) VO(IV)–6-Me-picolinate (A−)–phosphate (B3−) systems at a 1:5:5 metal ion to ligand ratio, with cVO(IV)=0.001 mol dm−3.

form two geometric isomers, depending on the positionof the solvent molecule in the coordination sphere ofthe VO(IV): the water molecule can be either cis ortrans to the oxo group of the vanadium ion (see Fig.10). In aqueous solution at room temperature and atneutral pH, the cis form of the bis complexes has beenfound to predominate with these three ligands. 6-Methylpicolinic acid proved to be the weakest VO(IV)binder, but it has the most hydrophobic character andhence its complex may provide a more efficient absorp-tion via passive transport across the hydrophobic mem-branes [69].

Phosphate, as the most important l.m.m. componentin the gastro-intestinal tract, may play a role in theabsorption of these insulin-mimetic compounds. Pre-dominance curves (the sum of the VO(IV) fractionsbound in the binary complexes with ligand A or B andthose bound in the ternary complexes with ligand Aand B) for the systems VO(IV)–insulin mimetic (A−)–phosphate (B3−) are depicted in Fig. 11. It is seen thatthe parent complexes [VOA]+ and [VOA2]° of maltol

are the predominant species at pH \4, a range includ-ing the physiological pH, and the presence of phos-phate affects the speciation of the VO(IV)–maltolsystem only slightly. Ternary complexes occur only inthe pH range 2–4, but they never become predominantspecies. They are favored, however, with picolinic acidand 6-methylpicolinic acid. In the weakly acidic pHrange and at neutral pH, mixed ligand complexes arethe predominant species [72]. Accordingly, phosphatewill certainly not affect the absorption of VO(malto-late)2, but might have some (though not necessarilybeneficial) influence on the absorption of the two picol-inates. The ternary complexes are certainly chargedspecies (the coordinating hydrogenphosphate HPO4

2−

a charge 2–, while the picolinate displaced by thephosphate molecule is mononegatively charged), in con-trast with the neutral binary complex, and this mayhinder at least the passive transport of VO(IV) throughthe membrane.

Adenosinenucleotides occurs in mmol dm−3 concen-tration in cellular fluids, and their interactions with


‘exogenous’ compounds in the organism are very likely.The predominance diagram for the VO(IV)–maltol(A−)–ATP (B3−) system (see Fig. 12) clearly indicatesthat maltol forms ternary complexes in the weaklyacidic pH range (4.5–5.5), as detected by both pH-me-try and EPR spectroscopy. The EPR spectra depictedin Fig. 13 show that a new species is present in practi-cally the same pH range, and is presumably the mixed-ligand complex [VOAB]2−. However, at physiological

Fig. 15. Predominance diagram of the VO(IV)–maltolate (A−)–cit-rate (B3−) system at a 1:2:2 metal ion to ligand ratio, with cVO(IV)=0.002 mol dm−3.

Fig. 12. Predominance diagram of the VO(IV)–maltolate (A−)–ATP(B3−) system at a 1:2:2 metal ion to ligand ratio, with cVO(IV)=0.002mol dm−3.

pH, maltol seems to be more competitive and VO(IV) isbound mostly to maltol. At pH \10, maltol is dis-placed and the ribose part of the nucleotides binds themetal [71].

The concentrations of catecholamines in the brainare fairly high. Catechol derivatives are extremelystrong VO(IV) binders; they can even displace the oxogroup of VO(IV) and form non-oxo 3× (O−,O−)-coor-dinated tris complexes [74–76]. The high affinity ofcatecholates for VO(IV) is hindered in the acidic pHrange because of the high proton competition on thephenolate functions. At the same time, maltol, which isalso a strong VO(IV) binder, has a high affinity forVO(IV) in the acidic pH range, owing to the weakerproton competition on the maltolate chelating function.Accordingly, in the VO(IV)–maltol (A−)–dopamine(B2−) system, maltol is the predominant metal ionbinder at pH B4.5; the ternary complexes of VOABHand [VOAB]− then become predominant up to pH �6,while at around the physiological pH range the cate-chols are more efficient VO(IV) binders and displacemaltol completely (see Fig. 14).

As discussed above, citrate is an important con-stituent of the blood serum: its concentration is �0.1mmol dm−3 (see Table 1). It generally behaves as atridentate ligand in its complexes and is regarded as thestrongest l.m.m. hard metal ion binder in the serum. Asshown in Fig. 15, pH-metric speciation measurementsand spectral studies unambiguously suggest enhancedternary complex formation in the pH range 4–8. Aconsiderable intensity decrease in the EPR spectrum ofVO(IV)–citrate (B3−) in neutral solution, due to theformation of oxo-bridged dinuclear species [77], is notobserved, but the spectrum indicate the formation ofnew mononuclear complexes, presumably the ternary[VOABH]−, [VOAB]2− and [VOABH–1]3−, in a widepH range. These data strongly suggest that, if theymeet, citrate will certainly react with VO(maltolate)2,and this potential drug may be transported in the bloodserum, presumably in the form of its ternary specieswith citrate. A more detailed study of the biospeciationof various insulin-mimetics in the serum is at present inprogress in our laboratory.

Fig. 13. High field parallel region of frozen solution EPR spectra (Xband) recorded at 140 K: (a) VO(IV)–maltolate, 1:2, pH 5.5; (b)VO(IV)–maltol–ATP, 1:1:1, pH 4.5; (c) VO(IV)–maltol–ATP, 1:1:1,pH 5.5; (d) VO(IV)–ATP, 1:2, pH 5.5; cVO(IV)=0.004 mol dm−3

(taken from Ref. [71]).

Fig. 14. Predominance diagram of the VO(IV)–maltolate (A−)–do-pamine (B2−) system at a 1:2:2 metal ion to ligand ratio, withcVO(IV)=0.002 mol dm−3.


Acknowledgements

This research was carried out in the frame of a COSTD8 project. The authors wish to express their thanks toProfessors G. Micera (University of Sassari, Italy), J.Costa Pessoa (University of Lisbon, Portugal), R.Bertani (C.N.R., Padova, Italy), M. Hollosi (EotvosUniversity, Budapest, Hungary), Zs. Majer (EotvosUniversity, Budapest, Hungary), Gy. Dombi (MedicalUniversity Szeged, Hungary), G. Toth (Medical Uni-versity Szeged, Hungary) and P. Buglyo (Kossuth Uni-versity, Debrecen, Hungary) for their valuablecontributions to this work. Financial support was pro-vided by the Hungarian Research Fund (OTKAT23776/97) and the Hungarian Ministry of Education(FKFP 0013/97).

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Documents

Solution speciation of bioactive Al(III) and VO(IV) complexes