Despite its importance, little is known about the absolute performance and the mechanism for quercetin’s antioxidant activity in water solution. We have investigated this aspect by combining differential oxygen-uptake kinetic measurements and B3LYP/6311+g (d,p) calculations. At pH = 2.1 (30 °C), quercetin had modest activity ( k inh = 4.0 × 10 3 M −1 s −1 ), superimposable to catechol. On raising the pH to 7.4, reactivity was boosted 40-fold, trapping two peroxyl radicals in the chromen-4-one core and two in the catechol with k inh of 1.6 × 10 5 and 7.0 × 10 4 M −1 s −1 . Reaction occurs from the equilibrating mono-anions in positions 4′ and 7 and involves firstly the OH in position 3, having bond dissociation enthalpies of 75.0 and 78.7 kcal/mol, respectively, for the two anions. Reaction proceeds by a combination of proton-coupled electron-transfer mechanisms: electron–proton transfer (EPT) and sequential proton loss electron transfer (SPLET). Our results help rationalize quercetin’s reactivity with peroxyl radicals and its importance under biomimetic settings, to act as a nutritional antioxidant.

Microorganisms require protection against the potentially damaging effects of ultraviolet radiation exposure. Photoprotection is, in part, provided by mycosporine-like amino acids (MAAs). Previous reports have proposed that nonradiative decay mediates the impressive photoprotection abilities of MAAs. In this letter, we present the first ultrafast dynamics study of two MAAs, shinorine and porphyra-334. We demonstrate that, in aqueous solution, these MAAs relax along their S1 coordinates toward the S1/S0 conical intersection within a few hundred femtoseconds after photoexcitation and then traverse the conical intersection and vibrationally cool in approximately 1 ps through heat transfer to the solvent. This new insight allows a quintessential component of microbial life to be unraveled and informs the development of molecular photon-to-heat converters for a myriad of applications.

Compounds 10a–10e and 14a/b contain elements of the two established sunscreen molecules avobenzone and octocrylene. The compounds were prepared, characterised and were found to have broad-wavelength, UV-absorbing properties.

Sunscreens provide a frontline defense for our DNA against the damage caused by ultraviolet (UV) radiation. The active ingredients in topically applied sunscreens that provide this defense are UV filters, which preferentially absorb or reflect UV radiation before it penetrates the skin and interacts with photosensitive nucleic acids. However, there are concerns related to human and environmental toxicity of current UV filters, and consequently a shift toward nature‐inspired, particularly microbial, UV filters. In this paper, new physical insight is provided into the fundamental mechanisms of photoprotection in two synthetic analogs of mycosporine‐like amino acid‐type UV filters, demonstrating new methods of protection that are distinct from those of current commercial sunscreens, extending previous work in this area. Transient absorption measurements (both transient electronic absorption spectroscopy and transient vibrational absorption spectroscopy) are combined with steady‐state studies and high‐level computational results to aid our mapping of the experimentally derived lifetimes to real‐time photodynamic processes. The conclusions reached here pave the way toward developing new and more efficient biomimetic DNA photoprotectant materials.