Schiff Bases of Pyridoxal 5\u27-phosphate and 5\u27-deoxypyridoxal with Phenylglycine Derivatives and Their Metal Complexes

Potentiometric p[H] measurements are employed to evaluate protonation constants, metal complex stabilities, and Schiff base (SB) formation constants in solutions containing pyridoxal 5\u27-phosphate (PLP) 5\u27-deoxypyridoxal (DPL), phenylglycine and its derivatives, and divalent transition-metal ions at 25°C and at an ionic strength of 0.100M (KNO3). Protonation constants for phenylglycine (PG), (4-methoxyphenyl)glycine (MPG), and (4-sulfophenyl)glycine (SPG) indicate that these synthetic amino acids are more acidic than glycine and other natural amino acids. Stability constants are reported for 1:1 and 1:2 complexes of Mn2+, Co2+, Ni2+, Cu2+ and Zn2+ with PG, MPG, and SPG. Hydrolysis of these metal ions complicates the study of the nature of the metal complex formed at high p[H]. The vitamin B6 derivatives, pyridoxal 5\u27-phosphate and 5\u27-deoxypyridoxal, were found to form only 1:1 metal complexes. The equilibrium constants for the formation of Schiff bases by PLP and DPL with PG, MPG and SPG are reported. The monoprotonated form of the Schiff base was found to be the most stable species in each of the systems studied. The equilibrium constants are reported for protonated 1:1 Cu(II)-Schiff base complexes, CuSBHxn+ and for 1:1 and 2:1 Schiff base complexes of Mn(II), Co(II), Ni(II), and Zn(II). The formation of a number of hydroxometal Schiff base chelates in alkaline solutions is observed. The equilibrium constants determined are employed to calculate the distribution of complex species as a function of p[H] in solutions containing these ligands and the metal ions identified above