The photoprotector mechanism of mycosporine-like amino acids. Excited-state properties and photostability of porphyra-334 in aqueous solution

The photostability and photophysical parameters of an aqueous solution of the mycosporine-like amino acid (MAA) porphyra-334 have been determined. The excited-singlet state lifetime, measured by time-correlated single photon counting, was 0.4 ns. Laser flash photolysis experiments at 355 nm did not show any transient species. The triplet state of porphyra-334 was sensitized by triplet–triplet energy transfer. The T–T absorption spectrum was determined and the maximal absorption coefficient at 440 nm was estimated to be 1×104 M−1 cm−1. In this way an upper limit for the intersystem crossing quantum yield was determined. The very low quantum yield of fluorescence (ΦF=0.0016) and triplet formation (ΦT<0.05) together with a photodecomposition quantum yield of 2–4×10−4, in the absence and the presence of oxygen respectively, can be explained by a very fast internal conversion process. These results support the photoprotective role assigned to this MAA in living systems.