TERNARY METAL(II) COMPLEXES WITH TYROSINE-CONTAINING DIPEPTIDES - STRUCTURES OF COPPER(II) AND PALLADIUM(II) COMPLEXES INVOLVING L-TYROSYLGLYCINE AND STABILIZATION OF COPPER(II) COMPLEXES DUE TO INTRAMOLECULARAROMATIC RING STACKING

The structures and stabilities of metal(II) complexes of tyrosine- (ty r-) containing dipeptides (L), L-tyr-X [X = glycine (gly), L-/D-alanin e, -tyr, -tryptophan, and -phenylalanine] and diamines [DA = ethylened iamine (en), 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen)] ha ve been studied by crystallographic, spectroscopic, and potentiometric methods. The absorption spectra of the 1:1:1 Cu(DA)(L) systems exhibi ted a single d-d peak at 610-640 nm (pH 6-7) and at 620-640 nm (pH app roximately 9) with an additional peak at approximately 850 nm indicati ng the formation of a five-coordinate complex. The circular dichroism (CD) spectra showed magnitude anomaly resulting from conformational ch anges. The stability constants beta(pqrs) of the ternary complexes Cu( p)(DA)q(L)(r)H(s) have been determined by potentiometric titrations at 25-degrees-C and I = 0.1 M (KNO3). The complexes with DA = bpy or phe n are stabilized relative to Cu(en)(glycylglycine) by the stacking int eraction between the side-chain aromatic ring of L and DA. Two complex es with L = L-tyr-gly, [Pd(bpy)(L-tyr.gly)].3H2O (1) and [Cu(phen)(L-t yr-gly)].3H2O (2), were isolated as crystals, and the structures were determined by the X-ray diffraction method. Complex 1 crystallizes in the triclinic space group, P1, with one molecule in a unit cell of dim ensions a = 10.856(2), b = 8.114(1), c = 7.704(1) angstrom; alpha = 81 .58(1); beta = 112.89(1); and gamma = 117.48(1)-degrees. The Pd(II) io n is in a four-coordinate square-planar geometry with the two nitrogen s of bpy and two nitrogens of L-tyr.gly. The phenol ring Of L-tyr.gly is situated above the coordination plane and stacked with bpy with the average spacing of 3.28 angstrom. Complex 2 crystallizes in the ortho rhombic space group, P2(1)2(1)2(1), with four molecules in a unit cell of dimensions a = 10.765(2), b = 22.074(3), and c = 10.078(2) angstro m. The Cu(II) ion has a rive-coordinate square-pyramidal geometry; the two nitrogens and one oxygen Of L-tyr.gly and one of the two nitrogen s of phen occupy the equatorial positions in a slightly distorted squa re plane, and the other nitrogen of phen is coordinated at an axial po sition. Intramolecular aromatic ring stacking has been detected betwee n the phenol ring Of L-tyr.gly and the aromatic ring of phen perpendic ular to the Cu(II) coordination plane, the average spacing between the rings being 3.61 angstrom. The results confirm the stabilization of C u(DA)(L) (DA = bpy or phen) evaluated from log beta(pqrs) values and s uggest that the conformation of side chain aromatic rings and coordina tion structures can be regulated by intramolecular stacking.