Photodegradation and photosensitization of mycosporine-like amino acids

The photodegradation and photosensitization of several mycosporine-like amino acids (MAAs) were investigated. The photodegradation of the MAA, palythine, was tested with three photosensitizers: riboflavin, rose bengal and natural seawater. For comparison of degradation rates, the riboflavin-mediated photosensitization of six other MAAs was also examined. When riboflavin was used as a photosensitizer in distilled water, MAAs were undetectable after 1.5 h. Palythine showed little photodegradation when rose bengal was added as the photosensitizer (k = 0.12 × 10−3 m2 kJ−1). Palythine dissolved in natural seawater containing high nitrate concentrations also showed slow photodegradation rate constants (k = 0.26 × 10−3 m2 kJ−1) over a 24-h period of constant irradiation. Similar experiments in deep seawater with porphyra-334 and shinorine resulted in 75% of the initial MAA remaining after 4 h of irradiation and rates of 0.018 and 0.026 × 10−3 m2 kJ−1, respectively. Experiments conducted in deep seawater with riboflavin additions resulted in photodegradation rate constants between 0.77 × 10−3 and 1.19 × 10−3 m2 kJ−1 for shinorine and porphyra-334, respectively. Photoproduct formation appeared to be minimal with the presence of a dehydration product of the cycloheximine ring structure indicated as well as the presence of amino acids. Evidence continues to build for the role of MAAs as potent and stable UV absorbers. This study further highlights the photostability of several MAAs in both distilled and seawater in the presence of photosensitizers.