Über die Autorin bzw. den Autor

Nikolaos Hadjiliadis is Professor in the Dept of Chemistry at the University of Ioannina, Greece. Einar Sletten is Professor in the Dept of Chemistry at the University of Bergen, Norway.

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The importance of metal-DNA interactions in living systems, and their potential applications especially in the treatment and diagnosis of diseases has stimulated considerable recent interest in this area. The most important application is the antitumour action of certain heavy metals, which work by binding to and distorting DNA, causing cell death. For example, Cisplatin (Cl2H6N2Pt ) is one of the most potent and widely used anticancer drugs in use today.

This book provides an overview of metal-DNA interactions, the mechanism of their interaction, metal-based drugs and metal ion toxicity. It is essential reading for academic researchers in bioinorganic chemistry, biochemistry, biology, biotechnology and medicine, and pharmaceutical industries involved in developing metal-based drugs.

Aus dem Klappentext

The importance of metal-DNA interactions in living systems, and their potential applications especially in the treatment and diagnosis of diseases has stimulated considerable recent interest in this area. The most important application is the antitumour action of certain heavy metals, which work by binding to and distorting DNA, causing cell death. For example, Cisplatin (Cl2H6N2Pt ) is one of the most potent and widely used anticancer drugs in use today.

This book provides an overview of metal-DNA interactions, the mechanism of their interaction, metal-based drugs and metal ion toxicity. It is essential reading for academic researchers in bioinorganic chemistry, biochemistry, biology, biotechnology and medicine, and pharmaceutical industries involved in developing metal-based drugs.

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Metal Complex - DNA Interactions

John Wiley & Sons

Chapter One

Einar Sletten and Nils ge Frystein

1.1 Introduction

The biological significance of the interaction between metal ions and nucleic acids has become a rather well-established fact. One may mention the observed necessity for the presence of metal ions in many natural processes where nucleic acids play the dominant role. The effect of platinum-based chemotherapeutic drugs probably originates from their attack on DNA. Another aspect of metals in biological systems is the increased flux of metals in the environment during the last decades. An assessment of the toxic effect of an unnatural metal ion concentration must include information on the processes in which the metal can participate. In a comparison of metal carcinogenicity in humans based on several experimental factors, Cr and Ni turned out to be the most potent carcinogens.

The nucleic acid monomers, guanine (G), adenine (A), thymine (T) and cytosine (C) have different metal ion affinities. The order of stability of 3d transition metal ion-nucleobase complexes are: G > A, C > T. At physiological pH the preferred binding sites on the nucleobases are: guanine N7, adenine N1 and/or N7, cytosine N3, thymine O4. For nucleotides the relationship between phosphate and base binding is dependent on the type of metal ion. Eichhorn and Shin studied the effect of various metal ions on the melting temperature of DNA (Figure 1.1). The authors suggest that magnesium ions increase [T.sub.m] by binding to phosphate and stabilizing the double helix, whereas copper ions decrease [T.sub.m] by binding to the bases and destabilizing the double helix. Based on the metal-induced variation in [T.sub.m] they suggested that the relative metal affinity to the phosphate backbone of DNA follows the order [Mg.sup.2+] > [Co.sup.2+] > [Ni.sup.2+] > [Mn.sup.2+] > [Zn.sup.2+] > [Cd.sup.2+] > [Cu.sup.2+].

This implies that the binding of an individual metal ion may involve phosphate and base on the same molecule or form a linkage between two different nucleotides. An example of the latter situation is the mononucleotide-metal ion binding pattern observed for the Cu-(GMP) complex, where [Cu.sup.2+] ions are bridging the GMP ligands through alternating N7-Cu-phosphate bonds (Figure 1.2).

When nucleobases are incorporated into a duplex DNA matrix, the affinities towards metal ions are modified. It has been shown that several divalent metal ions, like [Mn.sup.2+], [Cu.sup.2+] and [Pt.sup.2+] prefer GC-rich regions, while [Hg.sup.2+], for example prefer AT-rich regions. A more detailed picture indicates that metal binding to base residues is sequence-dependent, i.e. not all guanines in a particular sequence show identical affinity towards a specific type of metal ion. As a consequence, one may envisage designing metal complexes that can bind selectively to chosen sequences of DNA. Such complexes may be used as drugs that block specific gene expression associated with a certain disease.

Metal ions can interact with nucleic acids in two distinct modes of binding: diffuse binding and site binding, both of which are important for the structure and function of nucleic acids. In the diffuse binding mode the metal and the nucleic acid retain their hydration layer and the interaction is through water molecules. This is a long-range Coulombic interaction, in which positive metal ions accumulate around the nucleic acid in a delocalized manner; for example, the counterion atmosphere that all nucleic acids possess is made up of diffusely bound positive ions. In the site-binding mode the metal is coordinated to specific ligands on the nucleic acid; the coordination can either be direct (termed inner-sphere) or through a water molecule (termed outer-sphere). In the outer-sphere binding mode only the innermost hydration layer of the metal is kept intact, and the metal and the nucleic acid ligand(s) to which the metal is coordinated share solvation shells. In inner-shell binding there is direct contact between the metal and the nucleic acid. Dehydration of the metal ion and the nucleic acid binding site therefore has to occur before an inner-shell bond is formed.

The mechanism of inner-sphere binding is likely to be initiated by a diffuse binding mode, in which the metal and the nucleic acid are separated by no more than two layers of solvent molecules. This step is diffusion controlled. The next step is that the metal ion and the nucleic acid form an outer-sphere complex, separated only by one layer of solvent molecules. This step primarily depends on electrostatic attractions and hydrogen bonding between the metal and the nucleic acid. In the final step the metal and the nucleic acid come into direct contact (inner-sphere binding). Here the nucleophilicity of the coordination site plays a crucial role. In the last two steps steric effects are also important. Several attempts have been made to quantify the importance of accessibility and molecular electrostatic potential (MEP) at the site where the inner-sphere complex/covalent bond is formed. In these studies a reasonable correlation between these two important factors exists, and has been used to predict which DNA site is the most reactive to metalation or methylation.

In this chapter we present data on sequence-selective interactions between metal complexes and nucleic acids. In the outline we will distinguish between (i) site-selective inner-sphere metal coordination of nucleobases, and (ii) the selectivity of fully hydrated species located in the minor or major groove through hydrogen bonding and electrostatic interaction. In the former case a further distinction will be made between labile and nonlabile metals.

1.2 Ab initio Calculations and Photo-Cleavage Studies

The highest occupied molecular orbital (HOMO) of DNA nucleobases plays a crucial role in metal coordination by interacting with the lowest unoccupied molecular orbital (LUMO) of metal ions. The calculations of HOMOs of macromolecules such as duplex DNA are extremely difficult. Consequently, there has been little focus on the role of the HOMO/LUMO in studies of DNA-metal ion interactions. Theoretical calculations of DNA bases have mostly focused on ionization potentials (IP) of monomeric nucleobases and the stability of the nucleobase pair in neutral and radical cation states. About ten years ago the Saito group published the first extensive studies on the variation of nucleobase IPs and localization of HOMOs as a function of base stacking, using high-level ab initio calculations. The IPs of four base monomers and 16 sets of nearest-neighbour stacked nucleobases in the B form were calculated. It was found that the GG/CC system has the lowest IP among ten possible stacked nucleobase pairs and that approximately 70% of the HOMO is localized on the 5'-G of 5'-GG-3'. These calculations indicate that the 5'-G of 5'-GG-3' is the most electron-donating site in B-DNA. The origin of IP lowering as a result of base stacking was further investigated by calculations of HOMO energy distribution as a function of the twist angle of a GG dimer. Within a normal range of twist angles for B-DNA (-25 to -45), IP values of GG are between 7.2 and 7.4eV and the HOMO is predominantly localized on 5-G. This implies that in B-form DNA the 5'-side G of the 5'-GG-3' sequence is the most strongly interacting site with electrophiles. This...