Oxidation of l -leucine amino acid initiated by hydroxyl radical: are transition metal ions an enhancement factor?

Abstract

Hydroxyl radical (HO·) formation initiated by the Fenton-type reactions of Fe and Cu complexes of l -leucine (Leu) amino acid as well as its oxidation reaction by HO· was computationally investigated by using the density functional theory method at the M05-2X/6-311++G(3df,2pd)//M05-2X/6-311++G(d,p) level of theory in the aqueous phase. The results showed that dipole-salt is the main form of Leu in the physiological condition. Leu exhibits high chelating potential towards both Fe(III)/Fe(II) and Cu(II)/Cu(I) ions with the most favourable coordinating positions at two oxygen atoms of the –COO functional group. Furthermore, the Leu-ions complexes show a high risk of HO· formation via Fenton-like reactions, especially when ascorbate anion exists in the environment as a reducing agent. Finally, the oxidation reaction of l -leucine by HO· demonstrated a relatively high overall apparent reaction rate, k overall , being 1.18 × 10 9 M −1 s −1 , in which formal hydrogen transfer reactions of the dipole-salt form occur as the primary mechanism. Consequently, the Leu oxidation by HO· radical can be promoted by the Fenton reaction enhancement of its transition metal complexes.

Revisiting the HO ● -initiated oxidation of L-proline amino acid in the aqueous phase: influence of transition metal ions

“…The structures of [Fe(H 2 O) 6 ] 3+ , [Fe(H 2 O) 6 ] 2+ , [Cu(H 2 O) 4 ] 2+ and [Cu(H 2 O) 4 ] + were employed as a recommendation by previous works [40,41,45–47]. The high-spin states of hydrated Fe(II) and Fe(III) ions in this study were equal to 5 and 6, respectively [23,41,46,48–51].…”

“…The influence of the aqueous media was mimicked using the solvation model based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent (SMD) [42][43][44], which has also been widely used in the literature [39,41] [40,41,[45][46][47]. The high-spin states of hydrated Fe(II) and Fe(III) ions in this study were equal to 5 and 6, respectively [23,41,46,[48][49][50][51].…”

Influence of transition metal ions on the oxidation of L-proline amino acid initiated by HO● radical in the aqueous phase

“…[38][39][40][41][42] The high-spin states of hydrated Fe(II) and Fe(III) ions in this study were equal to 5 and 6, respectively. 23,38,41,[43][44][45][46] The kinetics of the HO-initiated oxidation of Pro, which occurs as FHT reactions in water, were studied using the pre-reactive complexes scheme proposed by Singleton and Cvetanovic 47 . Briefly, a two-step mechanism was considered:…”

“…Indeed, several previous studies show that some organic molecules exhibit the reactive HO  radical via the Fenton-like reactions of their transition metal ion complexes. 23,[38][39][40][41][42] The formed HO  radicals have a high potential to return oxidizing the Pro amino acid. The whole phenomenon is called the prooxidant risk of an organic molecule.…”

“…Furthermore, Cys also plays as a pro-oxidant in the Fe(II)/EDTA system. Truong et al 23 theoretically investigated the antioxidant activity of L-leucine (Leu) based on its chelating ability toward Fe and Cu ions. Leu had highly trapping potential to the iron and copper ions via its -COO functional group of dipole-salt form.…”