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 t he 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 sensitize d by triplet-triplet energy transfer. The T-T absorption spectrum was deter mined and the maximal absorption coefficient at 440 nm was estimated to be 1X10(4) 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 ( Phi (F)=0.0016) and triplet formation (Phi (T)<0.05) together with a photod ecomposition quantum yield of 2-4X10(-4), in the absence and the presence o f oxygen respectively, can be explained by a very fast internal conversion process. These results support the photoprotective role assigned to this MA A in living systems. (C) 2000 Elsevier Science S.A. All rights reserved.