COMPLEXATION OF HISTIDINE AND ALANYL-HISTIDINE BY VANADATE IN AQUEOUS-MEDIUM

At physiological pH and ionic strength, histidine (H-His-OH) and vanad ate form at least two weak complexes with the following characteristic s: delta(V-51) (ppm) (K (M-1)): 1 -546 (0.7), 2 -561 (1.6). K is the f ormation constant, and a 1:1 stoichiometry with the ligand coordinatin g through the amino group is assumed. Type 1 complexes are also formed with H-His-OR (R=Me, Bz), but not with BOC-His-OH. An additional, ver y weak complex 3 (delta=-591 ppm at pH 7.2, -573 ppm at pH 8.8) is pre sent at c(V) > 5 mM. Alpha-alanyl-histidine gives rise to a relatively strong, possibly dinuclear monoligate complex 4 (delta(V-51)=-519 ppm , K=5 x 10(5) M-2 at pH 7.2). Crucial for the formation of 4 are the d eprotonated amide of the peptide linkage, the NH2 group (as part of a chelate-5 ring; beta-alanyl-histidine does not provide a type 4 comple x), and the carboxylate unit. The complex stoichiometries and modes of coordination have been evaluated on the basis of C-13, N-14 and quant itative V-51 NMR spectroscopy.