The effect of the water-soluble UV-absorbing substance (UVAS) extracted from the marine red alga Porphyra yezoensis Ueda on UV-dependent thymine photodimer production was investigated. The T<>T pyrimidine-pyrimidone 6-4 dimer and the cyclobutane cis-syn T<>T 5-6 dimer produced by UV irradiation with a xenon lamp were analyzed by reverse-phase high-performance liquid chromatography. Although the dimer production was reduced when the irradiation was filtered through a UVAS solution, it decreased more when thymine was mixed with UVAS. Furthermore, UVAS inhibited the degradation of UV-irradiated thymine. The inhibitory effect of UVAS was significantly greater than that of exogenously added adenine or guanine, which forms complementary base pairs with thymine. These data suggest that in addition to its filtering effect against UV radiation, UVAS also protects thymine by a direct molecule-to-molecule energy transfer process. The protective function of UVAS against UV irradiation is advantageous for this alga under strong UV irradiation.

Quantum chemistry based simulations were used to examine the excited state of porphyra-334, one of the fundamental mycosporine-like amino acids present in a wide variety of aqueous organisms. Our calculations reveal three characteristic aspects of porphyra-334 related to either its ground or excited state. Specifically, (i) the ground state (S 0 ) structure consists of a planar geometry in which three units can be identified, the central cyclohexene ring, the glycine branch, and the threonine branch, reflecting the π conjugation of the system; (ii) the first singlet excited state (S 1 ) shows a large oscillator strength and a typical ππ* excitation character; and (iii) upon relaxation at S 1 , the originally ground state planar structure undergoes a relaxation to a nonplanar one, S 1 , especially at the carbon−nitrogen (CN) groups linking the cyclohexene ring to the glycine or threonine arm. The induced nonplanarity can be ascribed to the fact that the carbon atoms of the CN groups prefer an sp 3 hybridization in the S 1 state. At the singlet state, these processes are unlikely to be trapped by singlet−triplet intersystem crossing especially when these occur in the hydrophilic zwitter-ion forms of porphyra-334. These results provide the missing information for thorough interpretation of the stability of porphyra-334 upon UV irradiation.

We report an atomistic insight into the mechanism regulating the energy released by a porphyra-334 molecule, the ubiquitous photosensitive component of marine algae, in a liquid water environment upon an electron excitation. To quantify this rapidly occurring process, we resort to the Fourier analysis of the mass-weighted auto-correlation function, providing evidence for a remarkable dynamic change in the number of hydrogen bonds among water molecules and between the porphyra-334 and its surrounding hydrating water. Hydrogen bonds between the porphyra-334 and close by water molecules can act directly and rather easily to promote an efficient transfer of the excess kinetic energies of the porphyra-334 to the surrounding solvating water molecules via an activation of the collective modes identified as hydrogen-bond stretching modes in liquid water which eventually results in a disruption of the hydrogen bond network. Since porphyra-334 is present in seaweeds, aquatic cyanobacteria (blue-green algae) and red algae, our findings allow addressing the question how algae in oceans or lakes, upon sunlight absorption, can release large amounts of energy into surrounding water without destabilizing neither their own nor the HO molecular structure.