UNUSUAL HYDROGEN-BONDING PATTERNS OF N-7 METALATED, N-1 DEPROTONATED GUANINE NUCLEOBASES - ACIDITY CONSTANTS OF CIS-[PT(NH3)(2)(HEGUA)(2)](2-STRUCTURES OF CIS-[PT(NH3)(2)(EGUA)(2)]CENTER-DOT-4H(2)O AND CIS-[PT(NH3)(2)(EGUA)(2)]CENTER-DOT HEGUA-CENTER-DOT-7H(2)O (HEGUA=9-ETHYLGUANINE)() AND CRYSTAL)

Three closely related complexes of cis-Pt-II(NH3)(2) with neutral 9-et hylguanine (Hegua) and anionic (N-1 deprotonated) 9-ethylguanine (egua ), respectively, have been prepared and studied: cis-[Pt(NH3)(2)(Hegua )(egua)] ClO4 . 2H(2)O 1, cis-[Pt(NH3)(2)(egua)(2)]. 4H(2)O 2 and cis- [Pt(NH3)(2)(egua)(2)]. Hegua . 7H(2)O 3. The crystal structures of com plexes 2 and 3 have been determined. Platinum coordination in all case s is through N-7 Of neutral (1) or anionic (1-3) guanine. The Pt-II el ectrophile coordinated at N-7 acidifies the N-1 proton of neutral 9-et hylguanine (pK(a) = 9.57 +/- 0.04), giving pK(a1) = 8.02 +/- 0.01 and pK(a2) = 8.67 +/- 0.01 for the two Hegua ligands in cis-[Pt(NH3)(2)(He gua)(2)](2+). All three compounds display interesting hydrogen-bonding patterns either in the solid state and/or in solution. According to H -1 NMR interaction shifts of guanine NH and NH2 resonances of complex 1 in Me,SO solution. this compound forms three intermolecular hydrogen bonds between neutral and deprotonated guanine ligands. Far complex 2 , hydrogen bonding between two deprotonated guanines in the solid stat e is via N-1 and (NH2)-H-2. In complex 3. the neutral 9-ethylguanine i s hydrogen bonded to one of the two anionic guanine ligands through th ree hydrogen bonds, involving N-1, (NH2)-H-2 and O-6. This pattern rep resents a mismatch between two guanines, brought about by initial coor dination of a Pt-II electrophile to N-7 and subsequent deprotonation o f the N-1 position of the metallated nucleobase. The model character o f these compounds with regard to metal-induced mutagenicity briefly di scussed.