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 INTRAMOLECULAR AROMATIC RING STACKING

The structures and stabilities of metal(II) complexes of tyrosine- (tyr-) containing dipeptides (L), L-tyr-X [X = glycine (gly), L-/D-alanine, -tyr, -tryptophan, and -phenylalanine] and diamines [DA = ethylenediamine (en), 2,2'-bipyridine (bpy), and 1,10-phenanthroline (phen)] have been studied by crystallographic, spectroscopic, and potentiometric methods. The absorption spectra of the 1:1:1 Cu(DA)(L) systems exhibited a single d-d peak at 610-640 nm (pH 6-7) and at 620-640 nm (pH approximately 9) with an additional peak at approximately 850 nm indicating the formation of a five-coordinate complex. The circular dichroism (CD) spectra showed magnitude anomaly resulting from conformational changes. 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 phen are stabilized relative to Cu(en)(glycylglycine) by the stacking interaction between the side-chain aromatic ring of L and DA. Two complexes with L = L-tyr-gly, [Pd(bpy)(L-tyr.gly)].3H2O (1) and [Cu(phen)(L-tyr-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 dimensions 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) ion is in a four-coordinate square-planar geometry with the two nitrogens 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 orthorhombic 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) angstrom. 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 nitrogens of phen occupy the equatorial positions in a slightly distorted square plane, and the other nitrogen of phen is coordinated at an axial position. Intramolecular aromatic ring stacking has been detected between the phenol ring Of L-tyr.gly and the aromatic ring of phen perpendicular to the Cu(II) coordination plane, the average spacing between the rings being 3.61 angstrom. The results confirm the stabilization of Cu(DA)(L) (DA = bpy or phen) evaluated from log beta(pqrs) values and suggest that the conformation of side chain aromatic rings and coordination structures can be regulated by intramolecular stacking.