Abstract

With the growing concern regarding commercially available ultraviolet (UV) filters damaging the environment, there is an urgent need to discover new UV filters. A family of molecules called mycosporines and mycosporine-like amino acids (referred to as MAAs collectively) are synthesized by cyanobacteria, fungi and algae and act as the natural UV filters for these organisms. Mycosporines are formed of a cyclohexenone core structure while mycosporine-like amino acids are formed of a cyclohexenimine core structure. To better understand the photoprotection properties of MAAs, we implement a bottom-up approach by first studying a simple analog of an MAA, 3-aminocyclohex-2-en-1-one (ACyO). Previous experimental studies on ACyO using transient electronic absorption spectroscopy (TEAS) suggest that upon photoexcitation, ACyO becomes trapped in the minimum of an S1 state, which persists for extended time delays (>2.5 ns). However, these studies were unable to establish the extent of electronic ground state recovery of ACyO within 2.5 ns due to experimental constraints. In the present studies, we have implemented transient vibrational absorption spectroscopy (as well as complementary TEAS) with Fourier transform infrared spectroscopy and density functional theory to establish the extent of electronic ground state recovery of ACyO within this time window. We show that by 1.8 ns, there is >75% electronic ground state recovery of ACyO, with the remaining percentage likely persisting in the electronic excited state. Long-term irradiation studies on ACyO have shown that a small percentage degrades after 2 h of irradiation, plausibly due to some of the aforementioned trapped ACyO going on to form a photoproduct. Collectively, these studies imply that a base building block of MAAs already displays characteristics of an effective UV filter.

Highlights

  • Ultraviolet (UV) radiation, in particular UVA (315–400 nm) and UVB (280–315 nm), that reaches the Earth’s surface has both positive and negative effects on living organisms (Holick, 2004; Lucas et al, 2006; Humble, 2010)

  • We first take a look at the transient electronic absorption (TEA) spectra acquired for ACyO in EtOH and dimethyl sulfoxide (DMSO) which are displayed as false color maps in Figure 2; for lineouts of the TEA spectra see Supplementary Figure 6

  • We note here that our TEA spectra strongly correlate with the findings of Woolley et al (2018); the ACyO excited-state dynamics in EtOH are very similar to those observed in methanol, and the ACyO excited state dynamics in DMSO are very similar to those observed in acetonitrile

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Summary

Introduction

Ultraviolet (UV) radiation, in particular UVA (315–400 nm) and UVB (280–315 nm), that reaches the Earth’s surface has both positive and negative effects on living organisms (Holick, 2004; Lucas et al, 2006; Humble, 2010). Nature has developed its own photoprotection to safeguard from harmful DNA damage. In humans, this natural photoprotection is achieved by producing melanin pigments that can absorb UVA and UVB radiation before it reaches DNA (Kollias et al, 1991). As melanin production is a delayed process and does not absorb all UVA and UVB radiation that reaches the skin, a more immediate form of protection is required, i.e., sunscreens (Eller and Gilchrest, 2000; Brenner and Hearing, 2008; Wang and Lim, 2016). One approach is to base candidate UV filters off structures observed in plants and microorganisms (Bandaranayake, 1998; Dean et al, 2014; Baker et al, 2016, 2018; Horbury et al, 2017, 2018, 2019; Luo et al, 2017; Zhao et al, 2019a,b)

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