Coordination Modes of Histidine. 3. Stereochemistry of Copper(II) Complexes Related to Pyridoxal Catalysis

Article abstract of DOI:10.1021/ja00373a010

Copper(II) complexes of Shiff bases derived from pyridoxal, salicylaldehyde, or pyruvic acid and histidine, histidine methyl ester, and representative amino acids with nonpolar side chains have been prepared by metal ion template synthesis.The mode of coordination to copper(II) of the histidine residues in these complexes have been investigated by circular dichroism spectroscopy.The complexes derived from amino acids with nonpolar side chains provide appropriate references for the glycine-like coordination mode, while the derivatives of histidine methyl ester are appropriate references for the histamine-like mode.The histidine residues exhibit a striking tendency to bind copper(II) through chelate ring types complementary to those of the fused carbonyl residue.Thus, in the complexes derived from pyridoxal and salicylaldehyde the histidine residues bind glycine-like, whereas in those derived from pyruvic acid the histidine residues bind histamine-like.The conformation of the coordinated Shiff-base ligands has been deduced from the circular dichroism spectra of the complexes and discussed in relation to vitamin B₆ model reactions.The ESR spectra of these complexes were also investigated in different solvents to establish the donor sets and the ligand field simmetry in solution.The spectra show the pattern typical for tetragonal symmetry (g_** > g_*), and the magnetic parameters were used to compute the molecular orbital coefficients that describe the bonding character in the complexes.The electronic excitation energies required in the calculations were deduced from the circular dichroism spectra.