Guanidine Is a Zn2+-Binding Ligand at Neutral pH in Aqueous Solution
journal contributionposted on 20.04.2002, 00:00 by Shin Aoki, Kenta Iwaida, Noriko Hanamoto, Motoo Shiro, Eiichi Kimura
We have found the first well-characterized coordination of guanidine with Zn2+ in a 1:1 complex (ZnL) with cyclen (= 1,4,7,10-tetraazacyclododecane) functionalized with guanidinylethyl group (L1 = (2-guanidinyl)ethyl-cyclen). The X-ray structure analysis of the 1:1 complex crystallized at pH 7.5 revealed an apical coordination of the pendant guanidinyl group to Zn2+ ion in ZnL1. By potentiometrtic pH titration, initial formation of a 1:1 Zn(L1·H+) complex was indicated, where only the cyclen N's bind to Zn2+ with the complexation constant, log Ks (Ks = [Zn(L1·H+)]/[Zn2+][L1·H+] (M-1)), being 12.4 ± 0.1. Facile deprotonation of the guanidinium pendant in the Zn(L1·H+) occurred with a pKa value of 5.9 ± 0.1 at 25 °C with I = 0.1 (NaNO3) to yield the guanidine-coordinating complex ZnL1. 4-Nitrophenyl phosphate dianion (NPP-) interacted with ZnL1 through a new Zn2+-phosphate coordination, as indicated by 31P NMR titration and potentiometric pH titration. An apparent complexation constant for this new species, log Kapp(Zn(L1·H+)−NPP), was 4.0 ± 0.1, which is larger than the log Kapp(ZnL2−NPP) value of 3.1 for the 1:1 complex of Zn2+-cyclen (ZnL2) with NPP at the common pH 5.6. The interaction of ZnL1 with a phosphate dianion was proven by the X-ray crystal structure analysis of the 1:1 ZnL1−PP2- complex (PP2- is a dianion of phenyl phosphate) obtained from an aqueous solution at pH 6.5. At higher pH, the pendant guanidinium cation is deprotonated to displace the phosphate to yield the Zn2+−guanidine bond.