THE INVESTIGATION OF OXIDATIVE DECOMPOSITION OF PHENYL ALANINE BY TRANSITION METAL IONS

Abstract

The aim of this study is to comprehend the mechanism of the oxidative decomposition of phenylalanine (the simplest aromatic amino acid) by transition metals, in particular cobalt, using quantum mechanical tools. All the ground and transition state structures are optimized using the density functional method M06-2X. Additional calculations have been carried out using M06. Recently, a couple of works for profiling the transcriptomic data of the yeast Saccharomyces cerevisiae when exposed to transition metals, such as cobalt and nickel were done by Cakar and co-workers [1, 2]. According to their results, the cobalt-resistant mutants had a tendency to reduce the amount of aromatic amino acids, suggesting that Co3+, but not other metals, reacts specifically with aromatic amino acids, yielding harmful products. Although the in vitro decomposition of amino acids has been considered in numerous studies in the literature, the mechanism and intermediates formed during the reaction are not clear. The reactions use oxidizing agents such as diperiodatocuprate(III) in alkaline medium, or Mn3+ and its hydroxide form (Mn(OH)2+) in sulfuric acid medium [3, 4]. But the reaction of Co3+ with phenylalanine was not studied. In this study, it has been shown that cobalt (III) ions are powerful oxidizing agents against aromatic amino acids. Our study has revealed several pathways of the decomposition reaction of zwitterionic phenyl alanine at neutral pH and the effect of the Co3+ ion on the decomposition reaction. It was shown that the oxidation of phenylalanine by a Co3+ ion results in decarboxylation and deprotonation of phenylalanine. The presence of the aromatic ring facilitates the redox reaction. Then, oxidation by a second Co3+ in concert with the nucleophilic addition of an OH- ion leads to a pathway that yields phenyl acetaldehyde. A different pathway may proceed through oxidation by two more Co3+ ions and yield phenylacetamide. But this pathway has a higher activation free energy. Another possible decomposition pathway yielding phenylpyruvate also requires a considerably high reaction activation free energy. The reactions of Fe3+ and Ni3+ with phenylalanine have also been investigated. It has been found that these two transition metal ions are not suitable for the reactions mentioned above.