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

Abstract

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.

Unravelling the Photoprotective Mechanisms of Nature-Inspired Ultraviolet Filters Using Ultrafast Spectroscopy

“…After initial excitation to the S 1 , relaxation along the S 1 PES leads to a S 1 /S 0 CI with non-planar geometry (primarily at the CN groups at carbons one and three), this is in contrast to the planar geometry found for the optimized S 0 structure. This deformation results in rapid deactivation of the electronic excited state [ 90 ], similar to that reported for protonated palythine [ 86 ] and the core components of MAAs [ 89 ]. Hatakeyama et al [ 90 ] associate this geometry change with a π electron shift from the cyclohexene ring towards the protonated CN group of the glycine or threonine arm, which triggers the sp 3 hybridisation of the CN groups in the S 1 state.…”

“…Within the literature, there have been computational studies on the natural MAAs palythine [ 86 ] and porphyra-334 [ 88 , 90 ]; see Figure 14 for their respective structures. The MEPs of these MAAs are not reproduced here due to the similarity between those that have been presented throughout Section 2.2 of this review.…”

“…Two complementary computational studies have been conducted on porphyra-334 to unravel its photoprotective mechanism [ 88 , 90 ]. The work by Koizumi et al [ 88 ] used first-principles molecular dynamics [ 133 ] on hydrated porphyra-334 to investigate the mechanism that photoexcited porphyra-334 undergoes to dissipate excess energy to surrounding water.…”

“…The later work by Hatakeyama et al [ 90 ] investigated the electronic states of porphyra-334 using quantum chemical simulations, in order to complete the understanding of the relaxation mechanism it undergoes to return to its electronic ground state. In this work, the zwitterionic and less polar neutral forms of porphyra-334 were studied.…”

“…However, Losantos et al [ 84 ] and Woolley et al [ 85 ] have conducted such experiments on MAA motifs. In addition to these studies, complementary theoretical work has modelled the minimum energy relaxation pathways of MAAs and molecules alike, giving insight into their photoprotective mechanisms [ 84 , 86 , 87 , 88 , 89 , 90 ]. Section 2.2 of this review will primarily cover ultrafast spectroscopy and theoretical work conducted to the present date on MAAs and MAA motifs.…”