Second-sphere 'communication' between two cis-bound guanine nucleotides. Factors influencing conformations of dynamic adducts of cis-type platinum anticancer drugs with guanine nucleotides as deduced by circular dichroism spectroscopy

Adducts of Pt anticancer drugs bound to N7 of guanine are fluxional, regard less of whether the guanine is part of a larger species like DNA or of a sm all analog like 3'- or 5'-guanosine monophosphate (3'- or 5'-GMP). This dyn amic motion problem makes it impossible for NMR methods to reveal the solut ion conformation of the simple adducts studied here, cis-PtA(2)G(2) (G = gu anine derivative, A = NH3, and A(2) = trimethylenediamine (tn), or ethylene diamine (en)). Each adduct has three possible atropisomers (Delta and Lambd a HT (head-to-tail) and HH (head-to-head) forms). Recently, we demonstrated using NMR methods that the dynamic motion problem could be minimized by th e bulky CCC ligands of (CCC)PtG(2) complexes (CCC = chirality-controlling c helate ligand). We identified the atropisomers and determined their conform ation. We showed that the chirality of the major HT form correlated with th e sign of the CD peaks of the mixture. In the present study, CD spectroscop y was used to evaluate the conformation of the dominant HT atropisomer of d ynamic cis-PtA(2)G(2) adducts. All simple cis-PtA(2)G(2) adducts examined h ad the Delta type CD signal for G = 3'-GMP and the Lambda type CD signal fo r 5'-GMP. The CD signal type did not change when the phosphate group was pr otonated (pH 3), but the intensity decreased. However. the sign changed and the intensity decreased when the guanine ring NH (N1H) was deprotonated (p H similar to 10). Interestingly, the 1-methylguanosine 3'-monophosphate (1- Me-3'-GMP) and 1-methylguanosine 5'-monophosphate (1-Me-5'-GMP) adducts had a Lambda and Delta CD signal, respectively. 1-Me-3'-GMP was synthesized by phosphorylation of 1-methylguanosine (1-MeGuo) using sodium cyclo-triphosp hate hexahydrate. For cis-PtA(2)G(2) adducts of G's lacking a 3' or 5'-phos phate group (e.g. guanosine, guanosine 2'-monophosphate, and 9-ethylguanine ), the CD signals were always weak. The inosine 3'-monophosphate adduct, ci s-Pt(NH3)(2)(3'-IMP)(2) has a very strong CD similar to that of the 3'-GMP adduct. These results indicate that the stability of the dominant atropisom er of cis-PtA(2)G(2) complexes depends on the presence and the position of the phosphate group and on the G NIH groups, but not the G amino group. We conclude that intramolecular phosphate-N1H hydrogen bonds formed between tw o cis G nucleotides favor the Delta HT and Lambda HT conformers for the 3'- GMP and 5'-GMP adducts, respectively. The dominance of one chiral form acco unts for the characteristic enhanced CD signal. (C) 2000 Elsevier Science S .A. All rights reserved.