Structural transitions of a GG-platinated DNA duplex induced by pH, temperature and box A of high-mobility-group protein 1

[H-1,N-15] and H-1 NMR, and CD spectroscopy are used to show that the duplex d(A-T-A-C-A-T-Pt(7)G-Pt(7)G-T-A-C-A-T-A) . d(T-A-T-G-T-A-C-C-A-T-G-T-A-T), where Pt(7)G is platinated guanine, containing the cis-[Pt(NH3)(2)](2+) adduct, undergoes reversible temperature-induced (T-0.5 310 K) and pH-induced (pk(a) approximate to 4.8) transitions between kinked-duplex and distorted forms, with the latter forms predominating at high temperature and low pH. A related pH-induced structural change was observed for the unplatinated duplex (pK(a) 4.69, Hill coefficient n = 1.4) but was less cooperative than for the platinated duplex (n = 2). The pH-induced transition is attributed to protonation of cytosine residues and has wider implications, since many reported NMR studies of DNA are carried out near pH 5 to minimize NH-exchange rates. The [Pt(en)](2+) (where en is 1,2-ethanediamine) GG chelate of the same duplex is shown to exist in kinked and distorted forms, and the [H-1,N-15]-NMR shifts for the kinked form are indicative of the presence of highly stereospecific interactions with the Pt-NH protons. On binding of the duplex platinated with [Pt(NH3)(2)](2+) to high-mobility-group protein 1 (HMG1) box A, similar changes in shifts of the Pt-NH3 resonances to those induced by raising the temperature or lowering the pH were observed. The specific changes in H-1-NMR chemical shifts of HMG1 box A are consistent with binding of the platinated duplex (intermediate exchange rate on the H-1-NMR time-scale) to the concave face of the protein via helices I and II and the intervening loop.