VISIBLE CIRCULAR DICHROISM OF PLANAR NICKEL ION COMPLEXES OF PEPTIDES AND CYSTEINE AND DERIVATIVES

Planar, diamagnetic nickel ion complexes of peptides composed of L-amino acid residues typically exhibit negative circular dichroism extrema from 450 to 490 mμ. These minima appear about 50 mμ to longer wavelengths than the absorption maxima at 410-450 mμ due to d-d transitions on the nickel ion. Positive CD extrema also appear near the absorption maxima if histidyl residues are involved in the chelate rings or when bulky amino acid side chains are present in the amino terminal residue. The identity of CD sign for adjacent positions about a chelate ring observed in nickel ion complexes of Gly-Gly-L-Ala and Gly-L-Ala-Gly cannot be accounted for by any octant rule. A hexadecant rule accommodates the results for planar complexes by dividing the coordination plane perpendicularly into eight sectors centering on the metal ion and assigning opposite signs to adjacent sectors. Small side chains for L-amino acid residues then fall into sectors of the same sign regardless of position in a peptide. Four d-d transitions are identified in the circular dichroism for planar complexes of nickel ion and cysteine and derivatives.