Multiple metal binding to 6-oxopurine nucleobases as a source of deprotonation. The role of metal ions at N7 and N3

Simultaneous metal coordination to N7 (Pt(II)) and N3 (Pd(II)) of N9-blocked guanine leads to a 10(4) fold acidification of the guanine-N(1)H position and hence to a virtual complete deprotonation of the N(1)H position at neutral pH. The chelate-tethered nucleobase ethylenediamine-N9-ethylguanine was employed and relevant acid-base equilibria were studied by pD dependent 1H NMR spectroscopy. CH2 resonances of the tether were assigned on the basis of NOESY and COSY experiments. Our findings suggest a plausible method of formation of a previously reported trinuclear Pt(II) complex of 9-ethylguanine with metals coordinated to N1, N3 and N7. According to this, a sequence with the first metal binding to N7, the second one binding to N3, and only the third one binding to N1 with deprotonation of this site is proposed.