9-Ethylguanine complexes

The crystal structure and photophysics of arhenium(I) bipyridine 9-ethylguanine complex [Re(bpy)(CO)3(EtG)]+ (2) have been reported by Thorp and co-workers [19]. In CH2C12 solution, the complex emits at ca. 600 nm with a lifetime of 0.17 ps and an emission quantum yield of 0.054. Electrochemical measurements reveal that EtG is similar electronically to electron-rich pyridines, such as py l-NMe, and py-4-NH2, and is a much better n-donor ligand than pyridine, trimethylphosphine, and acetonitrile. 

In the case of complexes such as (21) and (23) which have an extended planar ligand, a significantly higher proportion of interstrand cross-links in DNA is formed in comparison to either cis- or trans-platin.172 The steric effects of these planar ligands result in the formation of structurally unique 1,2-interstrand cross-links like those formed by cisplatin, a unique example of how steric effects may alter a nonactive lesion into an active one (Figure 13).173,174 Model studies predicted this outcome by preparation of the monofunctional models trans-[PtCl(9-ethylguanine) (NH3)(quinoline)] and comparison of substitution rates of the Pt—Cl bond by G or C mononucleotides.175 176 Interestingly, the iminoether compound (25) appears to form predominantly monofunctional adducts with DNA.177... 

Fig. 14. Dynamic chiral recognition of 9-ethylguanine by chiral ruthenium-arene complex 9.

The complex [Ru(tpy)Cl3] binds to DNA, and is active as a cytostatic in L1210 leukemia cells. Its activity lies between those of cisplatin and carboplatin. Model complexes [Ru(tpy)L(H20)] + in which L = 9-ethylguanine or 9-methylhypoxanthine, have been prepared and characterized using NMR spectroscopy. " Proton NMR spectroscopy has been applied to a study of hydrogen bonding between 2, 3 -isopropylidine adenosine and [Ru(tpy)2] derivatives bearing a thymine group. 

To rule out specific recognition by the ethylguanine ligand as the mechanism of thymidine specificity, the complex Ru(bpy)2(dmap)OH22+ was prepared (dmap = 4-dimethylaminopyridine). The spectral and... 

EGMCYT10. 9-Ethylguanine-l-methylcyt0sine complex (C7H9NsO, C5H7N30). O Brien EJ (1967) Acta Crystallogr 23 92... 

It has been demonstrated that the tran5 -[(NH3)2PtCl] adduct of 1-methylcytosine, bound via N-3, can form crosslinks with either 9-ethylguanine or 9-methyladenine as a result of Cl substitution by purine N-7 atoms. Additional H-bonds between exocyclic groups of cross-linked nucleic acid bases provide additional stabihzation of the individual complexes (Figure 13d). Binding of the related complex [(NH3)2Pt(N-3-cytosine)Cl]+ with d(GG) leads to the formation of a major product in which the platinum is bound only to N-7 of the 5 -nucleotide. ... 

The effect of Pt coordination on the acid-base properties of the purine ligands / -ethylguanine, N -methylhypoxanthine, inosine, Af -methyladenine, and trimethyladenine has been quantitatively evaluated The acidity constants of the complexes were calculated by curve-fitting procedures, using the pH (pD) dependence of the H NMR shift data. It was found that Pt coordinated to the N -site acidifies the N-7 unit to the same extent as N -coordinated Pt acidifies the N-1 site. 

Mixed Purine/Pyrimidine Complexes. Unfortunately, only very limited structural information is available (15, 16) for mixed purine/pyrimidine complexes of Pt(II). In fact, so far only complexes containing N(7)-bound 9-ethylguanine (or its N(l)-depro-tonated monoanion) and N(3)-bonded 1-methyIcytosine have been structurally characterized. Conformational drawings for these complexes are presented in Figure 5. 

There is evidence for interligand interactions, without inner-sphere metal-nucleic acid binding, in three different types of complexes. In the first, the potential metal-binding sites are blocked by a proton, for example, in saltlike structures of the complexes of protonated bases 9-ethylguanine,... 

Ag complexes with all purine and pyrimidine bases are known. Ag forms a binary 2 1 complex with 9-ethylguanine through N7 coordination. A distorted trigonal planar Ag is reported for a complex in which Ag is coordinated to both the N7 atom of 9-methyladenine and the N3 atom of... 

The crystal structure of [Re bipy(CO)3(9-ethylguanine)] coordinated to a guanine base has been reported, with a 2.220 A distance between Re and the guanine N7. The electronic properties of the nucleobase strongly affect the luminescent properties of the Re complex. 

Table 2.2 Comparison of arene hinge-bending and orientation in [r -arene)Ru(en)X complexes for arene = dihydro- or tetrahydro-anthracene and X = Cl, H2O or NZ-bound 9-ethylguanine [86,106. ...

Table 4 Extent of the (N1)H acidification by (N7)-coordinated Cd " at inosine (hio) and 2 -deoxyguanosine (dGuo) as defined by equation (12). The corresponding data of the Ni complexes including those for 9-ethylguanine (9EtG) are given for comparison (aqueous solution 25°C 7 = 0.1 M, NaNOj)".

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