Investigating the Ultrafast Dynamics and Long-Term Photostability of an Isomer Pair, Usujirene and Palythene, from the Mycosporine-like Amino Acid Family

Abstract

Mycosporine-like amino acids are a prevalent form of photoprotection in micro- and macro-organisms. Using a combination of natural product extraction/purification and femtosecond transient absorption spectroscopy, we studied the relaxation pathway for a common mycosporine-like amino acid pair, usujirene and its geometric isomer palythene, in the first few nanoseconds following photoexcitation. Our studies show that the electronic excited state lifetimes of these molecules persist for only a few hundred femtoseconds before the excited state population is funneled through an energetically accessible conical intersection with subsequent vibrational energy transfer to the solvent. We found that a minor portion of the isomer pair did not recover to their original state within 3 ns after photoexcitation. We investigated the long-term photostability using continuous irradiation at a single wavelength and with a solar simulator to mimic a more real-life environment; high levels of photostability were observed in both experiments. Finally, we employed computational methods to elucidate the photochemical and photophysical properties of usujirene and palythene as well as to reconcile the photoprotective mechanism.

Examining the substituent effect on mycosporine-inspired ultraviolet filters

“…This would suggest that there is also a radical cation for each molecule generated which subsequently will contribute to the persistent GSB (when observed). In line with our previous studies on natural MAAs, 40,41 this solvated electron absorption for all stable molecules was determined to be the result of multiphoton induced dynamics implying that it is unlikely to be initiated by the sun (Figure S13 ESI).…”

“…40,41 We previously assigned the fast vibrational cooling in the electronic ground state to the large number of solute-solvent hydrogen bonds combined with the zwitterionic nature of MAAs. 40,41,47 As this is not the case here given the neutral form of 1-4 and the smaller number of hydrogen donating and accepting sites (most likely resulting in fewer solute-solvent hydrogen bonds), we reconcile this through water being a highly interactive solvent that facilitates very efficient energy dissipation. This is complemented by studies in ethanol which show longer extracted lifetimes (Figures S17-18 and Table S3 ESI).…”

“…[29][30][31][33][34][35][37][38][39] We elected to use RI-CC2 due to the good levels of accuracy achieved in predicting the UV-visible spectra of these molecules as has also been found in our previous work for natural mycosporine-like amino acids. 40,41 Transient electronic absorption spectroscopy…”

Spectroscopic insight on impact of environment on natural photoprotectants

“…† This absorption is assigned to an S 1 ) S 0 transition with pp* character for all MAAs based on previous literature. 14,15,17,18 Table 1 presents the l max for each environment presented in this work.…”

“…4a) that has been proposed computationally in gas-phase or implicit solvent environments is also followed when MAAs are on a surface. [15][16][17][18][19] In Fig. 4b-g, the transient absorption spectra (TAS) are presented for (1) the crude extract in glycerol, (2) skin I and (3) skin II (Fig.…”