Characterization of crystalline L-carnosine Zn(II) complex (Z-103), a novel anti-gastric ulcer agent: Tautomeric change of imidazole moiety upon complexation.

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Abstract
A novel crystalline Zn(II) complex of L-carnosine was prepared and evaluated for inhibitory activity against gastric ulceration in rats. The complex was found to be much more active than various other Zn(II) compounds. It was characterized by means of infrared (IR) spectroscopy, solid-state carbon-13 (13C) and nitrogen-15 (15N) 1H-cross-polarization (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The spectroscopic data indicated that L-carnosine coordinates to Zn(II) as a quadridentate ligand. A comparison of the 13C-NMR signals of the imidazole carbons of the complex with those of several histidine derivatives revealed that a tautomeric change of imidazole moiety had occurred upon complexation. The binding mode of the complex was considered to be analogous to that of the corresponding Cu(II) complex.