The deactivation pathways of the excited-states of the mycosporine-like amino acids shinorine and porphyra-334 in aqueous solution

In vitro studies on the structurally related mycosporine-like amino acids (MAAs) porphyra-334 and shinorine in aqueous solutions were carried out aiming at their full photochemical and photophysical characterization and expanding the evidence on the assigned UV-photoprotective role of the molecules in vivo. The experiments on shinorine confirmed a high photostability and a poor fluorescence quantum yield, in concordance with previous results on porphyra-334. The estimation of triplet production quantum yields for both MAAs was achieved by laser-flash photolysis measurements. In particular, photosensitization experiments on porphyra-334 support the participation of the triplet state in the photodecomposition mechanism yielding a more precise value of phi(T). As well, photoacoustic calorimetry experiments allowed the first direct quantification of the nonradiative relaxation pathways of the excited MAAs in solution, corroborating that the vast majority (ca. 97%) of the absorbed energy is promptly delivered to the surroundings as heat, consistently with the low photodecomposition and emission yields observed.