COMPLEXES FORMED BETWEEN ZINC(II) AND L-HISTIDINE - A C-13 NUCLEAR-MAGNETIC-RESONANCE STUDY

Carbon-13 NMR spectral data are reported as a function of pH for mixtures of L-histidine and zinc nitrate, and of L-histidine + zinc + 1,2-diaminoethane (en), 1,3-diaminopropane (pn), or glycine (gly), and also for L-histidine alone. The pH profiles have been analysed by computer and the chemical shifts of the carbon atoms determined for the following species; HisH-t2-, His-, HHis, H2His+, H3His2+, [Zn(His)]+, [Zn(His)2], [Zn(His)(HisH-1)]- and [Zn(HisH-1)2]2-. The C-13 chemical shifts have also been determined for en, pn and gly (methylene only) in the complexes [Zn(His)2X] (X = en, pn or gly), [Zn(His)2(HX)]+ (X = en or pn), [Zn(His)2(glyO)]-, [Zn(His)(HisH-1)X\ (X - en or pn) and [Zn(His)(HisH-1)(glyO)]2. Stability constants have been determined for the complexes. Potentiometric titrations with zinc and histidine solutions provide support for the interpretation of the chemical shift data. It is argued that [Zn(His)2] contains distorted octahedrally co-ordinated zinc, and that deprotonation occurs at the pyrrole-ring nitrogen rather than zinc-bound water.