Metal-Modified Base Pairs Involving Different Donor Sites of Purine Nucleobases:  trans-[a2Pt(7,9-DimeG-N1)(9-EtGH-N7)]2+ and trans-[a2Pt(7,9-DimeG-N1)(9-EtG-N7)]+ (a = NH3 or CH3NH2; 9-EtGH = 9-Ethylguanine; 7,9-DimeG = 7,9-Dimethylguanine). Possible Relevance to Metalated DNA Triplex Structures

The X-ray crystal structures of four mixed-nucleobase complexes 1a, 1b, 2, and 4 of trans-a2PtII (a = NH3 or CH3NH2) with N1-bonded 7,9-dimethylguanine (7,9-DimeG) and N7-bonded 9-ethylguanine (9-EtGH) are reported. The compounds are discussed in terms of their possible model character for platinated nucleobase triplets of the pu × pu·pym (pu = purine, pym = pyrimidine) type and are compared with known guanine·guanine pairs. Structural differences resulting from a “metal modification” have been studied, as have been stacking and H bonding motifs. 1a and 1b are two modifications of the same compound, trans-[(NH3)2Pt(7,9-DimeG)(9-EtGH)](ClO4)2·H2O. X-ray data for 1a:  orthorhombic, Pca21, a = 13.833(3) Å, b = 14.357(3) Å, c = 13.293(3) Å, Z = 4, R = 0.027. 1b:  monoclinic, P21/n, a = 13.987(3) Å, b = 12.582(3) Å, c = 15.613(3) Å, β = 104.61(3)°, Z = 4, R = 0.034. trans-[(CH3NH2)2Pt(7,9-DimeG)(9-EtGH)](ClO4)2 (2):  orthorhombic, Pca21, a = 13.886(1) Å, b = 14.698(1) Å, c = 13.008(3) Å, Z = 4, R = 0.034. trans-[a2Pt(7,9-DimeG)(9-EtG)](ClO4nH2O (3, a = NH3, n = 2; 4, a = CH3NH2, n = 3). X-ray data for 4:  monoclinic, C2/c, a = 23.296(5) Å, b = 10.566(2) Å, c = 22.327(4) Å, β = 93.73(3)°, Z = 8, R = 0.047. The metalated base triplet 2·(1-MeC) (1-MeC = 1-methylcytosine) has been detected in DMSO-d6 solution through 1H NMR spectroscopy, and an ab initio optimized structure of an analogous platinated base triplet has been obtained.