Stereoselective binding of copper(II), zinc(II), cobalt(II), and ni(II) to the optically active amino acids beta-(2-pyridyl)-alpha-alanine and beta-(6-methyl-2-pyridyl)-alpha-alanine, analogs of histidine and phenylalanine.
In order to study the metal ion binding of optically active beta-(2-pyridyl)-alpha-alanine, NH2CH(CH2C5H4N)CO2H, Pyala, and beta-(6-methyl-2-pyridyl)-alpha-alanine, NH2CH(CH2C5H3NCH3)CO2H, Mepyala, these pyridine analogs of histidine were synthesized and resolved; absolute configurations were determined for the isolated enatiomers. Protonation constants and formation constants for the binding of L-Pyala, D,L-Pyala, D-Mepyala, and D,L-Mepyala with Cu(II), Ni(II), Co(II), and Zn(II) were determined by potentiometrictitration. They show that the formation constant (Kx) for the reaction, M(L-Pyala) + D-Pyala in equilibrium M(L-Pyala)(D-pyala), is larger (up to 8.7 times larger) than that (K2) for the coordination of the same enantiomer, M(L-Pyala) + L-Pyala in equilibrium M(L-Pyala)2. Although the difference between Kx and K2 can be explained on a statistical basis for Cu(II), the larger differences for Ni(II), Co(II), and Zn(II) demonstrate that their M(L-Pyala)complexes bind D-Pyala more favorably than they do L-Pyala. A similar trend was found for the optical isomers of Mepyala except that the stereoselectivity is less than in the Pyala system. This presumably results from the reduced coordinating ability of the 6-methylpyridyl group as compared to the less crowded pyridyl donor. Using ideas previously applied to the analogous histidine system, the observed stereoselectivities may be explained in terms of the structures of the complexes.
D001665:Binding Sites; D003035:Cobalt; D003300:Copper; D006639:Histidine / Q000031:analogs & derivatives*; D008968:Molecular Conformation; D009532:Nickel; D010649:Phenylalanine / Q000031:analogs & derivatives*; D011199:Potentiometry; D013237:Stereoisomerism; D015032:Zinc
D003035:Cobalt; D010649:Phenylalanine; D006639:Histidine; D003300:Copper; D009532:Nickel; D015032:Zinc