STRUCTURES AND STABILITIES OF TERNARY COPPER(II) COMPLEXES CONTAINING AN ACIDIC AND A BASIC-AMINO-ACID - EVIDENCE FOR ARGININE SIDE-CHAIN INVOLVEMENT IN INTERMOLECULAR INTERACTIONS AND ITS BIOLOGICAL IMPLICATION

Ternary copper(II) complexes containing an acidic amino acid (A) and a basic amino acid (B), Cu(A)(B), where A refers to ethylenediamine-N-monoacetic acid (EDMA) and DL-2,3-diaminopropionic acid (DAP) and B to L-arginine (L-Arg) and L-lysine (L-Lys), have been investigated by absorption and circular dichroism (CD) spectral, solution equilibrium, and X-ray diffraction methods with emphasis on ligand-ligand interactions. Deviations of the CD spectral magnitudes from additivity were observed for the systems Cu(A)(B) where ligand-ligand hydrogen bonds or electrostatic interactions exist between the oppositely charged side chains. Evaluation of the stability enhancement by such intramolecular interactions has been made by considering the following hypothetical equilibrium (charges are omitted): Cu(A)(B') + Cu(A')(B) half arrow right over half arrow left Cu(A)(B) + Cu(A')(B') (K), where A' and B' are ligands without an interacting side chain group such as valine (Val) and ethylenediamine, respectively, and ligand-ligand interactions are possible only in Cu(A)(B). The log K value which is defined to be equal to zero in the absence of interactions was calculated for each Cu(A)(B) from the overall stability constants of the relevant ternary complexes determined at 25-degrees-C and I = 0.1 M (KNO3) to be in the order Cu(DAP)(L-Arg) (0.63) > Cu(EDMA)(L-Arg) (0.15) approximately Cu(EDMA)(L-Lys) (0.14) approximately Cu(DAP)(L-Lys) (0.16) > Cu(EDMA)(L-Val) approximately Cu(DAP)(L-Val) approximately 0. The stability difference between Cu(EDMA)(L-Arg) and Cu(DAP)(L-Arg) was inferred to be due to steric requirements for intramolecular ligand-ligand interactions. X-ray crystal structure analysis has been performed on [Cu(EDMA)(L-Arg)ClO4].1/2C2H5OH(1)and[CU(L-Arg)2](NO3)2.3H2O(2). Complex 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1) with four molecules in a unit cell of dimensions a = 7.824(3) angstrom, b = 24.979(3) angstrom, and c = 10.617(4) angstrom. The Cu(II) ion is in a slightly distorted square-pyramidal geometry with the two nitrogen atoms of EDMA and the nitrogen and oxygen atoms of L-Arg coordinated at the equatorial positions and the carboxylate oxygen atom of EDMA coordinated at an axial position. A perchlorate oxygen atom weakly coordinates at the other axial site. The coordinated carboxylate group of EDMA is hydrogen-bonded to the positively charged guanidinium group of L-Arg of a neighboring complex molecule with the N...O distances of 2.89 and 2.85 A. Complex 2 crystallizes in the monoclinic space group C2 with four molecules in a unit cell of dimensions a = 26.680(3) angstrom, b = 7.320(1) angstrom, c = 12.772(1) angstrom, and beta = 92.08(1)-degrees. The Cu(II) ion has a square-planar geometry with the two nitrogen and two oxygen atoms of two coordinated L-Arg molecules in a cis configuration with respect to the amino groups, which are hydrogen-bonded to an uncoordinated nitrate ion. The side chain guanidinium group is also hydrogen-bonded to two uncoordinated nitrate ions with the N...O distances of 2.84-2.97 angstrom. On the basis of the log K values and the crystal structures, intermolecular interactions involving the arginine guanidinium group were inferred to be more specific than those involving the lysine ammonium group, and their biological implication was discussed.