Keto–enol tautomerism in micro-hydrated acetylacetone: an atoms-in-molecules study

Theoretical Study of Radical Inactivation, LOX Inhibition, and Iron Chelation: The Role of Ferulic Acid in Skin Protection against UVA Induced Oxidative Stress

Ferulic acid (FA) is used in skin formulations for protection against the damaging actions of the reactive oxygen species (ROS) produced by UVA radiation. Possible underlying protective mechanisms are not fully elucidated. By considering the kinetics of proton-coupled electron transfer (PCET) and radical-radical coupling (RRC) mechanisms, it appears that direct scavenging could be operative, providing that a high local concentration of FA is present at the place of ^•OH generation. The resulting FA phenoxyl radical, after the scavenging of a second ^•OH and keto-enol tautomerization of the intermediate, produces 5-hydroxyferulic acid (5OHFA). Inhibition of the lipoxygenase (LOX) enzyme, one of the enzymes that catalyse free radical production, by FA and 5OHFA were analysed. Results of molecular docking calculations indicate favourable binding interactions of FA and 5OHFA with the LOX active site. The exergonicity of chelation reactions of the catalytic Fe²⁺ ion with FA and 5OHFA indicate the potency of these chelators to prevent the formation of ^•OH radicals via Fenton-like reactions. The inhibition of the prooxidant LOX enzyme could be more relevant mechanism of skin protection against UVA induced oxidative stress than iron chelation and assumed direct scavenging of ROS.

Theoretical insight into the on-water catalytic effect in the biogenesis of triterpene dimers: from one-step to two-step hetero Diels–Alder reactions

An alternative pathway to the hetero Diels–Alder reaction for the biogenic origin of triterpene dimers is presented here. In this new pathway, the explicit water molecules take a fundamental role.

The effect of deuteration on the keto-enol equilibrium and photostability of the sunscreen agent avobenzone

The remarkable properties of deuterium have led to many exciting and favourable results in enhancing material properties, for applications in the physical, medical, and biological sciences. Deuterated isotopologues of avobenzone, a sunscreen active ingredient, were synthesised to examine for any changes to the equilibrium between the diketone and enol isomers, as well as their UV photostability and photoprotective properties. Prior to UV irradiation, deuteration of the diketone methylene/enol moiety (i.e. avobenzone-d2) led to an increase in the % diketone compared to non-deuterated, determined by 1H NMR experiments in CDCl3 and C6D12. This can be rationalised from two angles; mechanistically by a deuterium kinetic isotope effect for the CH vs. CD abstraction step during tautomerisation from the diketone to the enol, and a weaker chelating hydrogen bond for the enol when deuterated allowing increased equilibration to the diketone. Avobenzone-d2 was further examined by solid state 13C NMR. The higher % diketone for avobenzone-d2 was postulated to favour increased photodegradation by a non-reversible pathway. This was investigated by UV irradiation of the avobenzone isotopologues in C6D12, both in real time in situ within the NMR by fibre optic cable as well as ex situ using sunlight. An increase in the relative amount of photoproducts for avobenzone-d2 compared to non-deuterated was observed by 1H NMR upon UV irradiation ex situ. Overall, the study demonstrates that deuteration can be applied to alter complex equilibria, and has potential to be manifested as changes to the properties and behaviour of materials.