cis-[Pt(NH3)(2)(9-MeA-N7)(9-EtGH-N7)](PF6)(2).1.5H(2)O (9-MeA=9-methyladenine; 9-EtGH=9-ethylguanine): a right-handed helicoidal model compound for the intrastrand A,G cross-link in duplex DNA