Unique Structural Relaxations and Molecular Conformations of Porphyra-334 at the Excited State.

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 pi conjugation of the system; (ii) the first singlet excited state (S-1) shows a large oscillator strength and a typical pi pi* 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.