Protolytic properties and complexation of DL-alpha-alanine and DL-alpha-valine and their dipeptides in aqueous and micellar solutions of surfactants

In this work we investigated the effect of the micellar media of anionic (sodium dodecylsulfate, SDS), cationic (cetylpiridinium chloride, CPC) and non-ionic (Brij-35) surfactants on the protolytic properties of amino acids DL-alpha-alanine, DL-alpha-valine and dipeptides L-alpha-alanyl-L-alpha-alanine, DL-alpha-alanyl-DL-alpha-valine and complex formation of copper(II) with L-alpha-alanyl-L-alpha-alanine by using potentiometric, spectrophotometric and mass-spectrometric methods, as well as by applying semi-empirical quantum chemical calculations to study the structure of copper(II) complexes with dipeptide. It was observed that the effect of the surfactant micelles on pK(a1) is decreasing in the following order: SDS > Brij 35 > CPC, and on pK(a2) the order is reversed: CPC similar to Brij 35 > SDS. In aqueous solutions and SDS micellar solutions the formation of two complexes: CuA(+) and CuH(-1)A was observed. However, in CPC and Brij 35 solutions the presence of the hydroxyl complex CuH(-1)OHA(-) was also detected. The stability of the complexes depends on the charge of the micelle surface and binding of the particular species by the micellar pseudophase. (C) 2013 Elsevier B.V. All rights reserved.