A model compound of the second most abundant DNA adduct of the antitumor agent cisplatin has been synthesized and structurally and spectroscopically characterized and its conformational behavior examined : cis-[(NH3)(2)Pt- (9-MeA-N7)(9-EtGH-N7)](NO3)(2) . 2H(2)O (9-MeA = 9-methyladenine; 9-EtGH = 9-ethylguanine) crystallizes in the monoclinic system, space group P2(1)/n (No. 14) with a = 7.931(2), b = 11.035(3), c = 26.757(6) Angstrom, beta = 94.94(2)degrees, and Z = 4. The two purine bases adopt a head-to-head orientation, with NH2 of 9-MeA and CO of 9-EtGH being at the same side of the Pt coordination plane. A theoretical conformational analysis of the complex cis-[(NH3)(2)Pt(Ade)(Gua)](2+) (Ade = adenine; Gua = guanine) based on molecular mechanics calculations of the nonbonded energy has revealed four minimum-energy zones similar to those derived previously for cis-[(NH3)(2)-Pt(Gua)(2)](2+) (Kozelka; et al. Ear. J. Biochem. 1992, 205, 895). This conformational analysis has allowed, together with the calculation of chemical shifts due to ring effects, the attribution of the two conformers observed for cis-[(NH3)(2)Pt{d(ApG)}](+) by Dijt et al. (fur. J. Biochem. 1989, 179, 344) to the two head-to-head conformational zones. The orientation of the two nucleobases in the crystal structure of cis-[(NH3)(2)Pt(9-MeA)(9-EtGH)](2+) corresponds, according to our analysis, roughly to that preferentially assumed by the minor rotamer of cis-[(NH3)(2)-Pt{d(ApG)}](+).