Complexes formed between zinc(II) and L-histidine: a carbon-13 nuclear magnetic resonance study

Abstract

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_–1^2–, His^–, HHis, H₂His⁺, H₃His²⁺, [Zn(His)]⁺, [Zn(His)₂], [Zn(His)(HisH_–1)]^– and [Zn(HisH_–1)₂]^2–. The ¹³C 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.