Hydrogen bonding patterns of N(7) platinated guanine: Watson-Crick and different self-pairing motifs in a tris(9-methylguanine) complex of Pt-II

The tris(nucleobase) complex [Pt(NH3)(Hmgua-N-7)(3)](2+) 1 (Hmgua = 9-methy lguanine) has been synthesised and crystallised as the nitrate (1a) and a m ixed perchlorate, chloride salt (1b). In the cation of 1a the three bases a re oriented head-tail-head, as in one of the two crystallographically diffe rent cations of 1b. The second cation of 1b displays a head-head-tail arran gement of the three bases. Of special interest is the H bonding pattern of the head-tail-head rotamer of 1b in that pairs of the two trans-oriented gu anine bases associate to a novel guanine quartet structure. If 1b is co-cry stallised with an excess of 1-methylcytosine (mcyt) from weakly acidic aque ous solution, a supramolecular ensemble 2 of ten nucleobases is obtained wh ich consists of four Watson-Crick base pairs arranged as two base quartets as well as a Hmgua/mgua self-pair.