The hydroxyl radical (OH*) is a highly reactive oxygen species that plays a salient role in aqueous solution. The influence of water molecules upon the mobility and reactivity of the OH* constitutes a crucial knowledge gap in our current understanding of many critical reactions that impact a broad range of scientific fields. Specifically, the relevant molecular mechanisms associated with OH* mobility and the possibility of diffusion in water via a H-transfer reaction remain open questions. Here we report insights into the local hydration and electronic structure of the OH* in aqueous solution from Car-Parrinello molecular dynamics and explore the mechanism of H-transfer between OH* and a water molecule. The relatively small free energy barrier observed (~4 kcal/mol) supports a conjecture that the H-transfer can be a very rapid process in water, in accord with very recent experimental results, and that this reaction can contribute significantly to OH* mobility in aqueous solution. Our findings reveal a novel H-transfer mechanism of hydrated OH*, resembling that of hydrated OH(-) and presenting hybrid characteristics of hydrogen-atom and electron-proton transfer processes, where local structural fluctuations play a pivotal role.

A detailed description of the local solvation structure and mobility of hydroxyl radicals (OH*) in aqueous solution near ambient conditions is provided by Car-Parrinello molecular dynamics simulations. Here, we demonstrate that for HCTH/120 and BLYP functionals, smaller systems (i.e., 31·H2O-OH*) are contaminated by system size effects, being biased for the presence of a three-electron two-centered hemibond structure between the oxygen atoms of a water molecule and the radical. Radial and spatial distribution functions of relatively large 63·H2O-OH* systems reveal the existence of a 4-fold coordinated "inactive" OH* structure with three H-bond donating neighbors and a strongly coordinated H-bond accepting neighbor. The local hydration structure around the radical exhibits more H-bond ordering than has been predicted by recent simulations employing classical force fields. Local structural fluctuations can end with spontaneous H-transfer reactions from the nearest H-bond donor water molecule, facilitated by the formation of an "active" OH* state, resembling the proton transfer mechanism of hydrated OH(-) (i.e., slight polarization of the (H3O2)* complex). A comparison of the free energy barriers for the H-transfer reaction obtained by both DFT functionals and for both system sizes is also provided, demonstrating that this can be a very rapid process in water.

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ABSTRACT:The interaction of two flavonoid species (resorcinolic and fluoroglucinolic) with the 20 essential amino acids was studied by the multiple minima hypersurface (MMH) procedures, through the AM1 and PM3 semiempirical methods. Remarkable thermodynamic data related to the properties of the molecular association of these compounds were obtained, which will be of great utility for future investigations concerning the interaction of flavonoids with proteins. These results are compared with experimental and classical force field results reported in the available literature, and new evidences and criteria are shown. The hydrophilic amino acids demonstrated high affinity in the interaction with flavonoid molecules; the complexes with lysine are especially extremely stable. An affinity order for the interaction of both flavonoid species with the essential amino acids is suggested. Our theoretical results are compared with experimental evidence on flavonoid interactions with proteins of biomedical interest.

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