“Button-on-a-String” Mechanism in Water, the Ultrafast UV-to-Heat Conversion by Mycosporine-like Amino Acid Porphyra-334 of Natural Sunscreen Compound

Mycosporine-like amino acids (MAAs) are a family of hydrophilic sunscreen compounds synthesized by aquatic organisms, such as algae and cyanobacteria. In this study, we demonstrate that porphyra-334, which is a common MAA, decays nonradiatively within several hundred femtoseconds after ultraviolet (UV) light absorption in water by rotating the intramolecular cyclohexenimine ring. The ring rotation resulted from the UV excitation of the intramolecular π-conjugation, and the ring rotation proceeded while preserving the hydrogen bonds with the surrounding water molecules. The hydrogen bonds were preserved due to the structural flexibility of the ring-attached amino acids of porphyra-334. The amino acids maintained their center of mass positions during the ring-rotating nonradiative decay. The amino acids and cyclohexenimine ring are analogous to the string and button of a button-on-a-string spinner, otherwise known as button whirligigs. Thus, we refer to the ring-rotating nonradiative decay of porphyra-334 as a button-on-a-string mechanism. We also show that this mechanism results from porphyra-334 itself rather than from the surrounding water molecules. The present results indicate that the molecular "spinner" exists in aquatic organisms and protects them from UV with the aid of water.