DOI: 10.5061/dryad.k6djh9wbb
title: Revisiting the HO●-initiated oxidation of L-proline amino acid in the aqueous phase – Influence of transition metal ions

description: The oxidation of L-proline (Pro) by HO● radical in water and the influence
 of transition metal ions on this process has been revisited by using the
 density functional theory (DFT) method at the
 M05-2X/6-311++G(3df,3pd)//M05-2X/6-311++G(d,p) level of theory at the
 temperature of 298.15 K. HO●-initiated oxidation of Pro via hydrogen atom
 transfer (HAT) reactions is more favourable at the b- and g-carbon than at
 the a- and d-carbon. The total branching ratios are 44.6% and 39.5% for
 the β- and γ-carbon and 11.5% and 4.2% for the a- and d-carbon,
 respectively. The overall rate constant at 298.15 K is 6.04 × 108 M-1 s-1.
 In addition, Pro tends to form the stable mono- and bi-dentate complexes
 with both Fe and Cu ions via the –COO functional group of dipole-salt
 form. The most stable Cu(II)-Pro complexes have high potential oxidant
 risks by enhancing the HO● radical formation in the presence of reducing
 agents such as superoxide anion or ascorbate anion. Besides, the complexes
 of the metal at a high oxidation state, i.e., Fe(III)-Pro and Cu(II)-Pro,
 show their ability to be oxidized by HO● radical via HAT reactions but
 with a lower rate constant than that of free Pro. Unsurprisingly, the
 oxidation enhancement is insignificant for SET reactions with almost
 negligible rate constants. In contrast, the rate constants of SET
 reactions of low oxidation state complexes (i.e., Fe(II)-Pro and
 Cu(I)-Pro) are faster than the oxidation rate of free ligands, and thus,
 these complexes promote the oxidation of Pro.
description: All geometry optimizations and vibrational frequency calculations
 for reactants, transition states, pre-reactive complexes, post-reactive
 complexes and products were performed by Gaussian 16 Rev. A.03 package in
 aqueous phase using M05-2X functional and 6-311++G(d,p) basis set. The
 accuracy of the energy values was then improved by single point
 calculations at M05-2X/6-311++G(3df,2pd) level of theory.
 The standard enthalpies
 (Δ<sub>r</sub>H<sup>0</sup>) and Gibbs free
 energies (Δ<sub>r</sub>G<sup>0</sup>) of the
 reactions forming complexes were calculated as the differences between
 total reactant and total product molar enthalpies and Gibbs free energies,
 respectively. The kinetics calculations of the redox reactions as well as
 the oxidation reaction of Pro in free and complexation form by
 HO<sup>●</sup> radical were determined using the pre-reactive
 complexes scheme proposed by Singleton and Cvetanovic for FHT reactions
 and the conventional transition state theory for SET ones.
description: - The ESI file (Proline-ESI.docx) contains the Cartesian
 coordinates, thermochemistry properties and the figures of two forms of
 L-proline (Pro); the 1:1 metal-to-ligand stoichiometric ratio mono-dentate
 and bi-dentate Pro complexes with iron and copper ions; and the transition
 states of FHT reaction of Pro in free form as well as complexation ones
 towards HO<sup>●</sup> radical. All structures are optimized
 and calculated for thermochemical parameter values in water at the
 M05-2X/6-311++G(3df,3dp)//M05-2X/6-311++G(d,p) level of theory.
 - the data file (thermo-kinetics-Proline.xlsx) contains the
 thermodynamic and kinetic data.