CHAPTER I

General Introduction

The area of inorganic chemistry, which most widely developed in the last few

decades is mainly due to coordination chemistry and applies very particularly to the

coordination compounds of transition metals. The chemistry of coordination compounds

has always been a challenge to the inorganic chemists as it has more branches now-a-

days.

Coordination compounds play a very significant role in our lives, the study of

them has contributed to the highest degree of understanding the chemical bond in

inorganic chemistry. As a whole classical coordination chemistry deals with the

formation of adducts by metal in their higher oxidation states bonded to inorganic or

organic ions or molecules. Interest in both basic and hi-tech research with these materials

continues at a rapid pace.

Metals play a vital role in an immense number of extensively differing biological

processes. Some of these processes are quite specific in their metal ion requirements, in

that only certain metal ions in specified oxidation states can accomplish the necessary

catalytic structural requirement. Metal ion dependent processes are found through out the

life science and vary tremendously in their function and complexity.

It is now appreciated that metal ions control a vast range of processes in biology.

Many new and exciting developments in the field of biochemistry create interest out of

inorganic chemists to court in the new area called “Bioinorganic Chemistry”.

1

One of the principal themes of bioinorganic chemistry is the synthesis of metal

complexes that have the ability to mimic the functional properties of natural

metalloproteins [1,2]. Proteins, some vitamins and enzymes contain metal ions in their

structure involving macromolecular ligands. The chemistry of metal complexes with

multidentate ligands having delocalized π-orbitals, such as Schiff bases or porphyrins has

recently gained more attention because of their use as models in biological systems.

Schiff bases

The condensation of primary amines with aldehydes and ketones give imines.

Imines that contain an aryl group bound to the nitrogen or to the carbon atom are called

Schiff bases, since their synthesis was first reported by Schiff [3].

Schiff bases are capable of forming coordinate bonds with many of metal ions

through both azomethine group and phenolic group or via its azomethine or phenolic

groups [4-19]. A large number of Schiff bases and their complexes are significant interest

and attention because of their biological activity including anti-tumor, antibacterial,

fungicidal and anti-carcinogenic properties [7-12] and catalytic activity [12-19].

Naphthylideneimine Schiff base complexes (Figure 1.1) possessing luminescence

property, catalyze oxidation of primary and secondary alcohols into their corresponding

carbonyl compounds in the presence of N-methylmorpholine-N-oxide (NMO) as the

source of oxygen have been reported recently [14]. The formation of high valent RuIV= O

species as a catalytic intermediate is proposed for the catalytic process.

2

Figure 1.1 Structure of the bidentate Schiff bases.

L-Amino acid Schiff bases with N,O donor system have been reported by Taqui

Khan et al [20]. and are used as catalyst of enantio selective epoxide of 1,2-di hydro-

naphthalene. Nitro substituted benzaldehyde Schiff bases were used in organic catalytic

reactions [21]. Schiff bases of N-methyl and N-acetyl isatin derivatives with different

arylamines have been prepared and screened for anti convulsant activities [22].

Antibacterial screening of monobasic bidentate Schiff base complexes

(Figure 1.2) with N,O donor have been reported [23].

C N

H R

OH

OCH3

(R = -CH3, -C5H4N, -C6H12)

Figure 1.2 Structure of monobasic bidentate Schiff base complexes with N,O donor.

3

Schiff bases of ethylenediamine/triethylenetetramine (salen) with benzaldehyde/

cinnamic aldehyde/salicylaldehyde as corrosion inhibitors of zinc in sulphuric acid have

been reported by Desai et al [24].

Recent report says, Schiff bases (Figure 1.3) are also employed as fluorescent

indicators by spectrofluorimetric monitoring of small changes of pH [25].

Figure 1.3 Structure of fluorescent Schiff bases.

Transition Metal complexes

The phenomenon of complex formation is really a very general one, but is

especially noted among the transition metal ions. For bonding, the metal must posses

vacant orbitals and these orbitals symmetrically must be correct, sterically available and

4

of reasonably low energy. Since transition metal ions generally meet these requirements

best, it is not surprising that they form complexes readily.

The transition elements play vital role in coordination complexes mainly because

of the following characteristics [26-29]:

variable oxidation state (electron transfer properties),

coordination geometries (octahedral, tetrahedral, square planar, pyramidal, etc.),

spectral and magnetic features, ligand field effects, unpaired d-electrons,

formation of chelated complexes,

most M2+ and higher oxidation states are borderline or hard acids and generally

prefer borderline or hard base such as O and N-donor groups; lower oxidation states

e.g. Cu(I) are softer acid will bind the soft bases such as O2, CO, N2, and S. and

formation of polynuclear metal species e.g. dimers, tetramers with bridging.

Metal coordination complexes have a wide diversity of technological and

industrial applications ranging from catalysis to anticancer drugs [30].

Ruthenium complexes

Ruthenium Schiff base complexes, particularly those containing oxygen and

nitrogen as donor atoms were found to be very efficient catalysts in the oxidation of

alcohols and alkenes. Electron transfer reactions are fundamental and play important role

in chemical and biological processes. As the coordination environment around the central

metal ion directs properties of the complexes, complexation of ruthenium by ligands of

different types has been of significant importance.

5

Ruthenium(II) complexes

Ruthenium(II) compounds display long luminescence life time and are extremely

photo stable [31,32].

There is a continuing interest in the development of probes for nucleic acid

structure determination. In recent years, a number of metal chelates have been used as

DNA structural probes and as chemotherapeutic agents. In particular, ruthenium(II)

complexes of the type [Ru(LL)3]n+, where LL is a bidentate ligands of various nature,

have been extensively studied as probes for the determination of nucleic acid

structure.[33-38] The application of these complexes as DNA structural probes is due to

their water solubility, coordinatively saturated nature and substitution inertness.

Catlytic activity and antibacterial screening of Ru(II) Schiff base complexes of

the type [Ru(CO)(EPh3)(B)(L)] (E = P or As; B= PPh3, AsPh3, py or pip; L = Schiff

bases have been reported by Balasubramanian et al [39].

DNA binding and antibacterial screening of dehydroacetic acid complexes

(Figure 1.4 A&B) of Ru(II) and Ru(III) containing PPh3/AsPh3 have been recently

reported by Chitrapriya et al [40].

Figure 1.4(A) Structure of Ru(III) dehydroacetic acid complexes.

6

Figure 1.4(B) Structure of Ru(II) dehydroacetic acid complexes.

Recent studies indicate that ruthenium complexes (Figure 1.5) are promising

candidates for NLO materials because of their rich photochemical properties and varied

coordination form [41-46].

7

Figure 1.5 Structures of the ruthenium(II) complexes possess NLO property.

Ruthenium(II) complexes containing triphenylphosphine/triphenylarsine and tetra

dentate Schiff bases are found to be effective catalysts in the oxidation of primary and

secondary alcohols using N-methylmorpholine-N-oxide as oxidant [47]. The catalytic

activity of these triphenylarsine complexes have been compared with that of

triphenylphosphine complexes and with similar ruthenium(III) complexes.

Ruthenium(III) complexes

Prabhakaran et al [11] synthesized ruthenium(III) complexes (Figure 1.6)

containing tetradentate Schiff base to test its antibacterial activity.

Figure 1.6 Structure of tetradentate Schiff base ligand.

8

Ramesh et al [18] reported catalytic oxidation of primary alcohols by some of the

ruthenium(III) Schiff base complexes (Figure 1.7) was carried out in CH2Cl2 in the

presence of N-methylmorpholine-N-oxide.

Figure 1.7 Ruthenium(III) Schiff base complexes.

Kannan et al [48] synthesized a series of new ruthenium(III) Schiff base

complexes (Figure 1.8) incorporating triphenylphosphine/triphenylarsine and chloride/

bromide ligands, and have been reported as efficient catalyst for the oxidation of both

primary and secondary alcohols to the corresponding carbonyl compounds with excellent

yields in the presence of NMO. Further, the possible explanations for the mode of action

of these complexes against two different microbes S. aureus and E. coli are described.

Figure 1.8 Structure of Ru(III) Schiff base complexes.

9

Copper(II)/Cobalt(II)/Nickel(II) complexes

The transition metals especially first row transition metal ions are well known for

their ability to form wide range of coordination complexes in which octahedral,

tetrahedral, and square planar geometries predominate. Copper(II) is a typical transition

metal ion to form complexes, but less typical in its reluctance to take up a regular

octahedral (or) tetrahedral geometry. The magnitude of the splitting of the electronic

energy levels in copper(II) complexes tend to be larger than other first row transition

metals due to the presence of large Jahn-Teller distortion.

Copper is one of the essential trace elements present in living organisms. A

number of important redox enzymes like hemocyanins, superoxide dismutase, blue

copper proteins, etc., contains copper atoms bound to protein molecules.

Copper(II) complexes with amino acids are cited as having potent anti-

inflammatory and anti- ulcer activity [49]. Copper ions are found to present in the active

sties of large number of metalloproteins, which involved in important biological electron

transfer reactions as well as in the molecular oxygen redox reactions [50,51].

There has been a substantial interest in the rational design of novel transition metal

complexes, which bind and cleave duplex DNA with high sequence or structure

selectivity. The characterization of DNA recognition by small transition-metal complexes

has been substantially aided by the DNA cleavage chemistry that is associated with

redox-active or photoactivated metal complexes.

10

Non–covalent interactions between positively charged metallointercalators and

the base–pair stack of DNA has been an area of interest for some time. It is clear that a

number of factors affect both the sequence selectivity of intercalators binding to DNA

and the resulting twist angle, and therefore it is necessary to understand the structural

features of intercalators that control the specificity of their binding. This, inturn should

lead to the design of more specifically targeted intercalators.

The synthesis and investigation of synthetic reversible dioxygen carriers have

attracted substantial interest in recent years and their physico-chemical properties are

sufficiently favorable for application in industries and medicine [52,53]. Synthetic

oxygen carriers have been studied extensively over several decades for two main reasons

viz (i) to understand the mechanism of oxygen binding proteins and (ii) to design

complexes suitable for practical applications. Co(II) complexes of porphyrins, Schiff

bases and tetraaza systems (Figure 1.9(A-C) are usually studied as models for oxygen

carriers. Schiff bases used for the studies of oxygen carrying properties are generally

tetradentate, of which at least two of the ligating atoms should be nitrogen, with the

others being nitrogen, oxygen, sulphur (or) combination of the three.

Tian et al [54] reported the nuclease activity (DNA cleavage) of Cobalt(II)

complexes with pBR 322 DNA in the absence of any external agents.

11

(A)

(B)

(C)

Figure 1.9(A-C) Co(II) complexes of porphyrins, Schiff bases and tetraaza systems.

12

The intercalative DNA-Binding studies of a Nickel(II) coordination compound

Ni(bpy)2dppz2+ have been reported [55]. The calculated intrinsic binding constant for the

same is 1.5 × 104 M−1. These features are equivalent to those observed with

Ru(bpy)2dppz2+ and suggest that nickel complex binds by intercalation in a manner that

parallels Ru(bpy)2dppz2+ [56].

Square planar nickel(II) complexes were studied owing to their known catalytic

activity towards olefin epoxidation.

Biological studies

Nucleic acids viz. ribonucleic acid (RNA) and deoxyribonucleic acid (DNA),

contain three types of basic structural nucleotide units: (a) pentose sugar,

(b) nitrogenous base (pyrimidins/purines) and (c) phosphate residue. They are long,

thread-like polymers, consisting of a linear array of monomers called nucleotides.

Nucleotides are the phosphate ester of nucleosides, which are the basic components of

DNA. The stacks of DNA contain A, G, C, T while RNA contains A, G, C, U. The types

of pentose also distinguish the nucleic acids 2-Deoxyribose is found in DNA while it is

ribose in RNA.

DNA usually consists of two complementary polymeric chains twisted about each

other in the form of a right-handed helix, making a complete turn every 34 0A (3.4 nm),

with a diameter of 20 0A (2 nm). Since the distance between adjacent nucleotides is

3.4 0A, there must be 10 nucleotides per turn. The constant diameter of the helix can be

explained if the bases in each chain face inward and are restricted so that a purine is

13

always opposite to a pyrimidine avoiding organization of purine-purine (too thick) or

pyrimidine-pyrimidine (too thin). Irrespective of the actual amounts of each base, the

proportion of G is always the same as the proportion of C in DNA, and the proportion of

A is always the same as that of T. The composition of any DNA can be described by the

proportion of its bases ic., G+C, which ranges from 26% to 74% for different species.

The two chains of DNA (Figure 1.10) are complementary to each other.

Figure 1.10 Structure of right handed double helical DNA (B-Type).

14

Since DNA is the basis for the storage, transmission and expression of genetic

information, any reaction or damage caused to it will have important consequences.

There has been substantial interest in exploring the factors that determine kinship and

selectivity in binding of small molecules with DNA [57-59]. A quantitative

understanding of such factors that determine recognition of DNA sites would be valuable

in designing small molecules that binds to specific sites in DNA and finds application in

chemotherapy. A number of metal chelates mostly polypyridyls have been used as

probes of DNA structure in solution [60], as agents for mediation of strand scission of

duplex DNA [61] and as chemotherapeutic agents [62].

The interaction of small molecules like metal complexes with DNA has been an

active area of research at the interface of chemistry and biology [63-68]. These small

molecules are stabilized in binding to DNA through a series of weak interactions, such as

the p-stacking interactions associated with intercalation of a planar aromatic group

between the base pairs, hydrogen-bonding and van der Waals interactions of

functionalities bound along the groove of the DNA helix [69], and the electrostatic

interaction of the cation with phosphate group of DNA [70]. Studies directed toward the

design of site- and conformation- specific reagents provide rationales for new drug design

as well as a means to develop sensitive chemical probes of nucleic acid structure.

Small molecules (metal complexes) bind to DNA double helix by three

distinguished binding modes Figure 1.11. They are,

A. Electrostatic binding / External binding

15

B. Groove binding

C. Intercalative binding / Intercalation

A. External binding / Electrostatic binding

Complexes are positively charged and the DNA phosphate sugar backbone is

negatively charged and their interaction is known as electrostatic. This association mode

was proposed for [Ru(bpy)3]2+, due to the luminescence enhancement of this complex

upon binding to DNA. Cations such as Mg2+ usually interact in this way [71].

B. Groove binding

The molecules approach within van der Waals contact and reside in the DNA

groove. Hydrophobic and/or hydrogen-bonding are usually important components of this

binding process, and provide stabilization. The antibiotic netropsin is a model groove-

binder where the methyl groups prevent intercalation [72].

16

C. Intercalation

This association involves the insertion of a planar fused aromatic ring system

between the DNA base pairs, leading to significant �-electron overlap. Stacking

interactions stabilizes this mode of binding and is thus less sensitive to ionic strength

relative to the two other binding modes. This mode of binding is usually favored by the

presence of an extended fused aromatic ligand like DPPZ [73]. Indeed with less extended

aromatic systems, the intercalation is usually prevented through clashing of the ancillary

ligands with the phosphodiester backbone, so that only partial intercalation can occur as

in the case for [Ru(phen)3]2+ [74].

17

Figure 1.11 Binding modes of small molecules with DNA.

Many metal complexes can bind to DNA in noncovalent modes such as

electrostatic, intercalative and groove binding [75,76]. Varying the substitutive group or

substituent position in the intercalative ligand can create some interesting differences in

the space configuration and the electron density distribution of transition metal

complexes, which will result in some differences in spectral properties and the DNA-

binding behaviors of the complexes and will be helpful to more clearly understand the

binding mechanism of transition metal complexes to DNA [77,78].

18

Scope of the present work

Schiff bases, a class of chelators, considered as privileged ligands, attractive due

to their stability, the ease by which modified variations can be obtained, a diverse range

of applications and are flexible both in terms of size and charge.

Schiff base complexes of transition metals having O and N donor atoms

containing phosphine/arsine especially ruthenium complexes, find application in classical

catalytic processes such as hydrogenation, isomerisation, decarbonylation, reductive

elimination, oxidative addition and in making C–C bonds. Transition metal carbonyl

complexes are reactive species in homogeneous catalytic reactions such as

hydrogenation, hydroformylation and carbonylation.

Literature says, though there is a considerable growth of Schiff base complexes

of first row transition metals, the chemistry of ruthenium complexes is less well

developed. But, there has been considerable interest in ruthenium complexes now-a-days

because of their redox stability, excited state life time, excited state reactivities their

ability to act as probes in investigating the structure of DNA and its antimicrobial

activity. The interaction of ruthenium complexes with DNA has received a great deal of

attention during the past decade. Also metal complexes of ruthenium containing nitrogen

and oxygen donor ligands are found to be very effective catalysts for oxidation,

reduction, hydrolysis and other organic and inorganic transformations especially

regioselective oxidation. The redox property of the central ruthenium can be tuned by

changing the substituents in the ligand. The oxidation states of ruthenium complexes can

vary from –II to +VIII. The chemotherapeutic Schiff bases are now attracting the

19

attention of inorganic chemists. Reports show that some drugs show increased activity

when it is administered as metal complexes rather than as organic compounds. Yet the

mechanism of antitumour activity of ruthenium compounds is not fully understood, it is

believed that, similar to platinium drugs, the chloride complexes can hydrolyze in vivo,

allowing the Ru to bind to the nucleobases of the DNA. However a deep survey of

literature on Schiff base transition metal complexes of N and O donors reveal that

antimicrobial and DNA binding studies have been largely ignored.

In the present project, which has originated from our interest in the chemistry of

transition metals especially ruthenium in different coordination environments. We have

chosen different types of phenolic ligands to synthesis, characterize and to study their

antibacterial activity and DNA binding properties.

20

References

[1] Z. Lu, L. Yang, J. Inorg. Biochem. 95 (2003) 31.

[2] J.Z. Wu, H.Li, J.G. Zhang, H. Xu Ju, Inorg. Chem. Commun. 5 (2002) 71.

[3] H. Schiff, Ann. Chim.(Paris) 131 (1864) 118.

[4] S.A. Sadeek, M.S. Refat, J. Korean Chem. Soc. 50 (2) (2006) 107.

[5] A. Bigotto, V. Galasso, G. Dealti, Spectrochim. Acta 28A (1972) 1581.

[6] S. Ren, R. Wang, K. Komatsu, P. Bonaz-Krause, Y. Zyrianov, C.E. Mckenna,

C. Csipke, Z.A. Tokes, E.J. Lien, J. Med. Chem. 45 (2002) 410.

[7] N. Chitrapriya, V. Mahalingam, L.C. Channels, M. Zeller, F.R. Fronczek,

K. Natarajan, Inorg. Chim. Acta 361 (2008) 2841.

[7] M.S. Refat, S.A. El-Korashy, D.N. Kumar, A.S. Ahmed, Spectrochim. Acta

Part A 70 (2007) 898.

[9] R. Prabhakaran, R. Huang, K. Natarajan, Inorga. Chim. Acta 359 (2006) 3359.

[10] K.P. Balasubramanian, K. Parameswari, V. Chinnusamy, R. Prabhakaran,

K. Natarajan, Spectrochim. Acta Part A 65 (2006) 678.

[11] R. Prabhakaran, A. Geetha, M. Thilagavathi, R. Karvembu, V. Krishnan,

H. Bertagnolli, K. Natarajan, J. Inorg. Biochem. 98 (2004) 2131.

21

[12] S. Kannan, R. Ramesh, Polyhedron 25 (2006) 3095.

[13] S. Kannan, K. Naresh Kumar, R. Ramesh, Polyhedron 27 (2008) 701.

[14] M. Sivagamasundari, R. Ramesh, Spectrochim. Acta Part A 67 (2007) 256.

[15] S. Kannan, R. Ramesh, Y. Liu, J. Organomet. Chem. 692 (2007) 3380.

[16] V. Mahalingam, R. Karvembu, V. Chinnusamy, K. Natarajan, Spectrochim. Acta

Part A 64 (2006) 886.

[17] G. Venkatachalam, R. Ramesh, Inorg. Chem. Commun. 9 (2006) 703.

[18] R. Ramesh, Inorg. Chem. Commun. 7 (2004) 274.

[19] R. Karvembu, S. Hemalatha, R. Prabhakaran, K. Natarajan, Inorga. Chem.

Commun. 6 (2003) 486.

[20] M.M. Taqui Khan, R.I. Kureshy, N.H. Khan, Tetrahedron: Asymmetry, 2 (1991)

101.

[21] W.T. Gao, Z. Zhing, Molecules 8 (2003) 788.

[22] M. Verma, S.N Pandeya, K. Singh, T.P. Stables, Acta Pharma. 54 (2004) 49.

[23] K. Naresh Kumar, R. Ramesh, Spcetrochim. Acta Part A 60 (2004) 2913.

[24] M.N. Desai, J.D. Talati, N.K. Shah, Anti-Corrosion Methods and Materials 55

(2008) 27.

22

[25] M.N. Ibrahim, S.E. A. Sharif, E-Journal of Chemistry 4 (4) (2007) 531.

[26] J.C. Wu, N. Tang, W.S. Liu, M.Y. Tanchan, A.S. Chin, Chem. Lett. 12 (2001)

757.

[27] L. He, S.H.Q.F. Gou, J. Chem. Crystallogr. 29 (1999) 207.

[28] P. Bhattacharya, J. Parr, A.J. Ross, J. Chem. Soc. Dalton (1998) 3149.

[29] C.M. Liu, R.G. Xiong, X.Z. You, Y.J. Liu, K.K Chiung, Polyhedron 15 (1996)

4564.

[30] A.D. Burrows, Science Progress 85 (3) (2002) 199.

[31] F.N. Castellano, J.D. Dattelbaum, J.R. Lakowicz, Anal. Biochem. 255 (1998) 165.

[32] H.Wolpher, O. Johansson, M. Abrahamson, M. Kritikos, L. Sun, B. Akermark,

Inorg. Chem. Commun. 7 (2004) 337.

[33] T. Urathamkul, J.L. Beck, M.M. Sheil, J.R. Aldrich-Wright, S.F. Ralph, Dalton

Trans (2004) 2683.

[34] H. Chao, J.-G. Liu, L.-N. Ji, X.-Y. Li, J. Biol. Inorg. Chem. 6 (2001) 143.

[35] J.-G. Liu, B.-H. Ye, Q.-L. Zhang, X.-H. Zou, Q.-X. Zhen, X. Tian, L.-N. Ji,

J. Biol. Inorg. Chem. 5 (2000) 119.

[36] A. Anagnostopoulou, E. Moldrheim, N. Katsaros, E. Sletten, J. Biol. Inorg. Chem.

4 (1999) 199.

23

[37] R. Hartmann, A. Robert, V. Duarte, B.K. Keppler, B. Meunier, J. Biol. Inorg.

Chem. 2 (1997) 143.

[38] D.B. Hall, R.E. Holmin, J.K. Barton, Nature 382 (1996) 73.

[39] K.P. Balasubramanian, R. Karvembu, R. Prabhakaran, V. Chinnusamy,

K. Natarajan, Spectrochim. Acta Part A 68 (2007) 50.

[40] N. Chitrapriya, V. Mahalingam, M. Zeller, R. Jayabalan, K. Swaminathan,

K. Natarajan, Polyhedron 27 (2008) 939.

[41] C. Dhenaut, I. Ledoux, I.D.W. Samuel, J. Zyss, M. Bourgault, H. Le Bozec,

Nature (London) 374 (1995) 339.

[42] B.J. Coe, G. Chadwick, S. Houbrechts, A. Persoons, J. Chem. Soc., Dalton Trans.

(1997) 1705.

[43] B.J. Coe, S. Houbrechts, I. Asselberghs, A. Persoons, Angew. Chem., Int. Ed.

Engl. 38 (1999) 366.

[44] I.R. Whittall, M.P. Cifuentes, M.G. Mark, B. Luther-Davies, M. Samoc,

S. Houbrechts, A. Persoons, G.A. Heath, D.C.R. Hockless, J. Organomet. Chem.

549 (1997) 127.

[45] A.M. McDonagh, M.G. Humphrey, M. Samoc, B. Luther- Davies, S. Houbrechts,

T. Wadw, H. Sasabe, A. Persoons, J. Am. Chem. Soc. 121 (1999) 1405.

24

[46] H. Chao, R.H. Li, B.H. Ye, H. Li, X.L. Feng, J.W. Cai, J.Y. Zhou, L.N. Ji,

J. Chem. Soc., Dalton Trans. (1999) 3711.

[47] R. Karvembu, S. Hemalatha, R. Prabhakaran, K. Natarajan, Inorg. Chem.

Commun. 6 (2003) 486.

[48] S. Kannan, R. Ramesh, Polyhedron 25 (2006) 3095.

[49] J.R.J. Sorenson, Progress in Medicinal Chemistry, G.P. Ellis, and G.B. West,

Ed., Elsevier, New York, Vol. 26 (1989) 437 and references cited therein.

[50] R.H. Holm, P. Kennepohl, Solomon, Chem. Rev. 96 (1996) 2239.

[51] K.D. Karlin, A.D. Zuber buhler, Bioinorganic Catalysis, 2nd Ed., J. Reediijk,

E. Bouwman, Ed., MarcelDekker, NewYork (1999) 469.

[52] S. Srinivasan, J. Annaraj, PR. Athappan, J. Inorg. Biochem. 99 (2005) 876.

[53] D.H. Busch, N.W. Alcock, Chem. Rev. 94 (1994) 585.

[54] J.-L. Tian, L. Feng, W. Gu, G.-J. Xu, S.-P. Yan, D.-Z. Liao, Z.-H. Jiang,

P. Cheng, J. Inorg. Biochem. 101 (2007) 196.

[55] L. Jin, P. Yang, Microchemical Journal 58 (1998) 144.

[56] A.E. Friedman, J.C. Chambron, J.P. Sauvage, J. Am. Chem. Soc. 112 (1990)

4960.

[57] W.S. Wade, P.B. Dervan, J. Am. Chem. Soc. 109 (1987) 1574.

25

[58] J.K. Barton, Science 233 (1986) 727.

[59] K. Kissinger, J.b. Dabrowiak, J.W. Lown, Biochemisty 26 (1987) 5590.

[60] J.K. Barton, Inorg. Chem. 7 (1985) 321.

[61] D.S. Sigman, Acc. Chem. Res. 19 (1986) 180.

[62] S.J. Lippand, Acc. Chem. Res. 11 (1978) 211.

[63] P. Nordell, P. Lincoln, J. Am. Chem. Soc. 127 (2005) 9670.

[64] C.C. Cheng, W.C.H. Fu, K.C. Hung, P.J. Chen, W.J. Wang, Y.T. Chen, Nucl.

Acid Res. 31 (2003) 2227.

[65] A.A. Mokhir, R. Kraemer, Bioconjugate Chem. 14 (2003) 877.

[66] W.C. Tse, D.L. Boger, Acc. Chem. Res. 37 (2004) 61.

[67] P. Yang, R. Ren, M.L. Guo, A.X. Song, X.L. Meng, C.X. Yuan, Q.H. Zhou, H.L.

Chen, Z.H. Xiong, X.L. Gao, J. Biol. Inorg. Chem. 9 (2004) 495.

[68] A.Th. Chaviara, P.J. Cox, K.H. Repana, A.A. Pantazaki, K.T. Papazisis,

A.H. Kortsaris, D.A. Kyriakidis, G.S. Nikolov, C.A. Bolos, J. Inorg. Biochem. 99

(2005) 467.

[69] A.M. Pyle, J.P. Rehmann, R. Meshoyrer, C.V. Kumar, N.J. Turro, J.K. Barton,

J. Am. Chem. Soc. 111 (1989) 3051.

26

[70] S. Satyanarayana, J.C. Dabrowiak, J.B. Chaires, Biochemistry 31 (1992) 9319.

[71] J. Kelly, A.Tossi, D. McComell, Nucl. Acids. Res. 13 (1985) 6017.

[72] H. Mei, J. Barton, J. Am. Chem. Soc. 108 (1986) 7414.

[73] I. Haq, P. Lincoln, D. Suh, B. Norden, B. Chowdhry, J. Chaires, J. Am. Chem.

Soc. 117 (1995) 4788.

[74] P. Lincoln, B. Norden, J. Phys. Chem. B. 102 (1998) 9583.

[75] J.A. Cowan, Curr. Opin. Chem. Biol. 5 (2001) 634.

[76] L.-N. Ji, X.-H. Zou, J.-G. Liu, Coord. Chem. Rev. 216 (2001) 513.

[77] C. Metcalfe, J.A. Thomas, Chem. Soc. Rev. 32 (2003) 215.

[78] H. Chao, L.N. Ji, Bioinorg. Chem. Appl. 3 (2005) 15 and references cited therein.

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