Binding modes of metal complexes

Figure 7 Binding modes of metal complexes with DNA (a) groove binding, (b) intercalation, and (c) insertion. (Reproduced from Ref. 119. Royal Society of Chemistry, 2007.)...

The radical anions of PQ and PTQ also form complexes with metal ions. The ESR spectra provide valuable information on the binding modes of metal ions. Figure 28 shows ESR spectra of PQ and metal ion complexes, which were formed by photoinduced ET from (BNA)2 (29) to PQ or ET from decamethylferrocene to... 

Section III.B illustrated the complex and unusual structures that can be generated from the different binding modes of metal-alkyne constructs. Clearly, metal-alkyne Jt-complex chemistry is rich and in some ways has a distinct... 

D. Z. M. Coggan, 1. S. Haworth, P. J. Bates, A. Robinson, and A. Rodger, DNA binding of ruthenium tris[l,10-phenanthroline] Evidence for the dependence of binding mode on metal complex concentration, Inorg. Chem. 38[20], 4486-4497 [1999]. 

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

NMR spectroscopy. NMR is a powerful tool for the analysis of the structure and dynamics of drug-nucleic add complexes [59], and has been widely used for characterising the binding modes of organic molecules with oligonucleotides. It has been less applied so far for metal complexes [60-63]. 

For the purposes of this chapter, which focuses on comparisons of isocyanide binding in transition metal complexes and isocyanide adsorption on metal surfaces, we first summarize known modes of isocyanide binding to one, two and three metals in their complexes. In such complexes, detailed structural features of isocyanide attachment to the metals have been established by single-crystal X-ray diffraction studies. On the other hand, modes of isocyanide attachment to metal atoms on metal surfaces are proposed on the basis of comparisons of spectroscopic data for adsorbed isocyanides with comparable data for isocyanides in metal complexes with known modes of isocyanide attachment. 

In the following sections we will review the copper complexes of the various classes of aminoglycosides described earlier. Discussion will focus on complexes that are formed at physiological pH (7.4) but will take into account the differing modes of coordination at other pH values, as well as accounting for the distinct binding modes of other transitions metal ions. 

Fig. 1. The common binding modes of dioxygen as a ligand in mononuclear and dinuclear metal complexes.

Possible consequences of the irregular mode of metal binding for the reactivity of the complex have been fully discussed by Vallee and Williams (17). It is perhaps worth emphasizing, however, that the cooperation with other reactive groups not directly associated with the metal ion seems to be of decisive importance for the catalytic action of the metalloenzymes discussed here. 

The binding mode of uracils and thymines in neutral and deprotonated forms has been reviewed up to 1987 [13]. They coordinate hard, and relatively few soft metal ions, through 0(4) (preferentially) and 0(2). Uracil (thymine) behaves as a weak dibasic acid in alkaline media with the more basic site N(3) at pKa 9.69 (10.16), as compared to N(l) at pKa 14.2. At high pH the monoanions of uracil and thymine bind the metal ions preferentially via N(l). However, the N(3) linkage isomer of the Ptn complex has also been obtained [24]. The relatively few examples of complexes with soft metal ions, containing monodentate uracilate anions, are due to the high tendency of the ligand to bind additional metal ions to form polynuclear species [13]. 

The ability of metal complexes to unwind DNA has been put forth as an important criterion for proving an intercalative binding mode and has been observed with other complexes of phen, dppz, and phi (21, 28,30,33). The enzyme topoisomerase can be used to determine if small molecules unwind DNA, according to published procedures (28). We find that by using this assay, Ru(tpy)(dppz)OH22+ unwinds DNA by 17°, which is consistent with intercalative binding. 

Table I shows examples of the steady-state and time-resolved emission characteristics of [Ru(phen)2(dppz)]2+ upon binding to various DNAs. The time-resolved luminescence of DNA-bound Ru(II) is characterized by a biexponential decay, consistent with the presence of at least two binding modes for the complex (47, 48). Previous photophysical studies conducted with tris(phenanthroline)ruthenium(II) also showed biexponential decays in emission and led to the proposal of two non-covalent binding modes for the complex (i) a surface-bound mode in which the ancillary ligands of the metal complex rest against the minor groove of DNA and (ii) an intercalative stacking mode in which one of the ligands inserts partially between adjacent base pairs in the double helix (36, 37). In contrast, quenching studies using both cationic quenchers such as [Ru(NH3)6]3+ and anionic quenchers such as [Fe(CN)6]4 have indicated that for the dppz complex both binding modes...

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