Experimental study of the excited-state properties and photostability of the mycosporine-like amino acid palythine in aqueous solution

Abstract

Characterization of the excited states of the mycosporine-like amino acid palythine (λ_max = 320 nm) in aqueous solutions was achieved experimentally. The low value for the photodegradation quantum yield, (1.2 ± 0.2) × 10⁻⁵, confirms that palythine is highly photostable in air saturated-aqueous solutions. Laser flash photolysis of acetone in the presence of palythine allowed for the observation of a transient spectrum which is consistent with the triplet–triplet absorption of palythine. Kinetic treatment of the transient signals yields a lifetime of the triplet state of ca. 9 µs and a triplet energy around 330 kJ mol⁻¹. The photoacoustic calorimetry results are consistent with non-radiative decay as the major fate of excited palythine. A comparison of the photodegradation quantum yields and photophysical properties of palythine with those previously determined for the other mycosporine-like amino acids, shinorine and porphyra-334, suggests that geometrical isomerization around the CN bond may contribute to the rapid deactivation of this group of molecules.