Speciation Studies of Some Metal Complexes with L-Aspartyl-L-phenylalanine Methyl Ester by Potentiometry

The stoichiometry and stability constants of the complexes formed between nickel(II), cobalt(II), copper(II) and manganese(II) with L-aspartyl-L-phenylalanine methyl ester (aspartame, LH) have been determined potentiometrically at 25° C and m=0.1. Nickel(II) and cobalt(II) react with aspartame to form ML, ML2 and ML3 type complexes. Copper(II) forms ML and ML2 type complexes. On the other hand, manganese(II) forms ML type complexes. The Irving-Williams’ order, Co(II)<Ni(II)<Cu(II), was found to be satisfied for ML+ complexes. Complex distribution depending on pH and metal/ aspartame ratio is given for these systems.