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

Abstract

The tris(nucleobase) complex [Pt(NH₃)(Hmgua-N⁷)₃]²⁺1 (Hmgua = 9-methylguanine) has been synthesised and crystallised as the nitrate (1a) and a mixed perchlorate, chloride salt (1b). In the cation of 1a the three bases are oriented head–tail–head, as in one of the two crystallographically different cations of 1b. The second cation of 1b displays a head–head–tail arrangement 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 guanine bases associate to a novel guanine quartet structure. If 1b is co-crystallised with an excess of 1-methylcytosine (mcyt) from weakly acidic aqueous solution, a supramolecular ensemble 2 of ten nucleobases is obtained which consists of four Watson–Crick base pairs arranged as two base quartets as well as a Hmgua/mgua self-pair.