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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 [capital 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.

The photophysics and photochemical behavior of the phenoxazin‐3‐one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N‐oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (ΦF= 0.75). At lower pH the fluorescence is strongly reduced. The N‐oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N‐oxide in the presence of amines occurs from the triplet state.

The polymerization of 2-hydroxyethyl methacrylate photoinitiated by riboflavin in the presence of triethanolamine was investigated. The polymerization was also studied using as photoinitiator lumichrome, the major product obtained in the anaerobic photoreduction of riboflavin. Photopolymerization rates were measured as a function of amine concentration in the UV (366 nm) and visible (>450 nm) regions. The quenching of the excited states of the dyes by triethanolamine was investigated by fluorescence lifetime and laser flash photolysis experiments. Quenching rate constants were determined in the absence and the presence of monomer. These rate constants and singlet and triplet lifetimes were used to fit the polymerization rate vs amine concentration curves. From the fitting it was concluded that the interaction of both singlet and triplet excited states with the amine led to the 2-hydroxyethyl methacrylate polymerization.

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The quenching of excited singlet and triplet states of riboflavin, lumiflavin and lumichrome was investigated in methanol. The quenchers were aromatic electron donors and aliphatic amines. Bimolecular quenching rate constants were determined from static and dynamic fluorescence measurements. Triplet quenching was studied by laser flash photolysis. Transient absorption spectra showed the presence of semireduced flavins and lumichrome, and the radical cation of quenchers. The results confirm that the quenching rate constants for aliphatic donors are lower than those of aromatic donors of similar oxidation potential. Plots of the quenching rate constants vs. the free energy for the electron transfer reaction, DG , were fitted by the Rehm-Weller model of electron transfer quenching. The aliphatic quenchers needed a higher intrinsic barrier for the fitting that was ascribed to the internal reorganisation of the amines. For the aromatic donors the singlet quenching rate constants reach the diffusional limit at highly negative DG . However, for the triplet quenching the limiting value of the rate constants is lower than the plateau of the singlet quenching. This is explained in terms of the non-adiabaticity of the triplet quenching process.

Characterization of the excited states of the mycosporine-like amino acid palythine (lambda(max) = 320 nm) in aqueous solutions was achieved experimentally. The low value for the photodegradation quantum yield, (1.2 +/- 0.2) x 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 micros 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 C=N bond may contribute to the rapid deactivation of this group of molecules.

The photophysics and photochemical behavior of the phenoxazin-3-one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N-oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (phiF = 0.75). At lower pH the fluorescence is strongly reduced. The N-oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N-oxide in the presence of amines occurs from the triplet state.