Electrochemical study of the interaction of nicotinamide with tryptophan in the presence and absence of nickel(II)

The interaction of nicotinamide with tryptophan in the presence and absence of nickel(II) in 0.1 M ammonium tartrate (pH 8.94) has been examined by vo ltammetric techniques. Tryptophan gave a catalytic hydrogen peak at -1.12 V versus an Ag \ AgCl \ saturated KCl reference electrode while nicotinamide formed two peaks at -1.42 and -1.57 V, respectively. In the absence of nic kel(II), when the tryptophan concentration was increased, the peak of nicot inamide at -1.42 V increased gradually while its peak at -1.57 V decreased. It can be concluded that tryptophan plays an important role in the reducti on mechanism of nicotinamide at the mercury electrode surface. Voltammetric results show that the nickel(II)-tryptophan and nickel(II)-nicotinamide co mplexes on the mercury electrode surface reduce at -0.44 and -0.94 V, respe ctively. These complexes are reduced at a more positive potential than that of the Ni(II)tartrate (-1.18 V). From electronic spectral data of the comp lexes, their stoichiometries of 1:1 (metal/ligand) in aqueous medium are de termined. The interactions between tryptophan and nicotinamide were also st udied in the presence of nickel(II). Excess nickel(II) causes changes in th e voltammetric reduction peaks of nicotinamide and tryptophan and the forma tion of binary complexes. The nickel(II)-tryptophan (log beta = 2.2) comple x is more predominant than that of nickel(II)-nicotinamide (log beta = 2.2) complex. This probably results from the binding sites and the size of the catalyst. Further, when nicotinamide was added to a solution containing the nickel(If)-tryptophan complex, or tryptophan to the cell including the nic kel(II)-nicotinamide complex, the formation of a mixed-ligand complex was n ot seen. (C) 2001 Elsevier Science B.V. All rights reserved.