Molecular complexes of l-phenylalanine with substituted 1,4-benzoquinones in aqueous medium: Spectral and theoretical investigations

Abstract

Various spectral techniques such as UV–Vis, FT-IR, and fluorescence have been employed to investigate the charge transfer interaction of l-phenylalanine (LPA) with substituted 1,4-benzoquinones (MQ1–4). Kinetic and thermodynamic properties of the complexes were determined in aqueous medium at physiological condition (pH=7). The interaction of MQ1–4 with l-phenylalanine (LPA) was found to proceed through the formation of donor–acceptor complex, yielding a radical anion. The stoichiometry of the complexes was determined by Jobs continuous variation method and was found to be 1:1 in all the cases. Fluorescence quenching studies showed that the interaction between the donor and the acceptors is spontaneous. The results indicated that the progressive replacement of chlorine atom (−I effect) by methoxy group (+M effect) in the quinone decreased the electron acceptor property of the quinone. The order of the experimentally measured association constant of these complexes was well supported by DFT/B3LYP calculations.