Model of the most abundant DNA interstrand cross-link of Transplatin : X-ray structures of two modifications and H bonding behavior in the solid state and in solution of trans-[Pt(NH3)(2)(1-MeC-N3)(9-EtGH-N7)](ClO4)(2)center dot nH(2)O (1-MeC=1-methylcytosine; 9-EtGH=9-ethylguanine)