Exploring photoprotection in natural ultraviolet filters

Ultraviolet radiation causes a number of deleterious effects on humans and other living organisms. As such, photoprotection is crucial, and whilst humans produce melanin as a natural defence mechanism, this is not usually sufficient. Therefore, alternative photoprotection is required and one way is through the use of sunscreens. The ultraviolet filters currently used within sunscreens have a number of associated drawbacks. As a result, alternative ultraviolet filters are needed and one approach is taking inspiration from nature. This thesis does exactly that as the molecules of interest are a family of natural ultraviolet filters, mycosporines and mycosporine-like amino acids. In this thesis, a bottom-up approach has been adopted to glean insight into the simplest system first and then increasing the complexity of the studied system from a photochemical and photophysical standpoint. In Chapter 2, the cyclohexenone building block of mycosporines was investigated as an extension of previous work. Experimental and computational results determined that the majority of population recovers back to the electronic ground state on ultrafast timescales. As a result, the cyclohexenone core is not as poor of an ultraviolet filter as previously thought. In Chapters 3 and 4, a number of mycosporine-like amino acids were explored spectroscopically and computationally. The results linked the ultrafast dynamics to their unrivalled long-term photostability, thus highlighting the efficiency of the photoprotective mechanism in these molecules. Finally, Chapter 5 took the investigations further by examining a crude extract from Palmaria palmata containing mycosporinelike amino acids in more viscous and restricted surface environments. It was found that the photoprotective mechanism was retained in these environments. Overall, this thesis demonstrates the efficient photon-to-heat conversion capabilities of mycosporine-like amino acids making them ideal candidates as ultraviolet filters. Furthermore, such results can guide the future experiments and chemical design of not only ultraviolet filters in sunscreens but in any field where photon-to-heat conversion is desirable

Unravelling the photoprotective mechanisms of nature-inspired ultraviolet filters using ultrafast spectroscopy

There are several drawbacks with the current commercially available ultraviolet (UV) filters used in sunscreen formulations, namely deleterious human and ecotoxic effects. As a result of the drawbacks, a current research interest is in identifying and designing new UV filters. One approach that has been explored in recent years is to use nature as inspiration, which is the focus of this review. Both plants and microorganisms have adapted to synthesize their own photoprotective molecules to guard their DNA from potentially harmful UV radiation. The relaxation mechanism of a molecule after it has been photoexcited can be unravelled by several techniques, the ones of most interest for this review being ultrafast spectroscopy and computational methods. Within the literature, both techniques have been implemented on plant-, and microbial-inspired UV filters to better understand their photoprotective roles in nature. This review aims to explore these findings for both families of nature-inspired UV filters in the hope of guiding the future design of sunscreens

A perspective on femtosecond pump–probe spectroscopy in the development of future sunscreens

Given the negative impacts of overexposure to ultraviolet radiation (UVR) on humans, sunscreens have become a widely used product. Certain ingredients within sunscreens are responsible for photoprotection and these are known, collectively herein, as ultraviolet (UV) filters. Generally speaking, organic UV filters work by absorbing the potentially harmful UVR and dissipating this energy as harmless heat. This process happens on picosecond time scales and so femtosecond pump–probe spectroscopy (FPPS) is an ideal technique for tracking this energy conversion in real time. Coupling FPPS with complementary techniques, including steady-state spectroscopy and computational methods, can provide a detailed mechanistic picture of how UV filters provide photoprotection. As such, FPPS is crucial in aiding the future design of UV filters. This Perspective sheds light on the advancements made over the past two years on both approved and nature-inspired UV filters. Moreover, we suggest where FPPS can be further utilized within sunscreen applications for future considerations

Investigating the ultrafast dynamics and long-term photostability of an isomer pair, Usujirene and Palythene, from the Mycosporine-like amino acid family

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

Synthesis and characterisation of novel composite sunscreens containing both avobenzone and octocrylene motifs †

Avobenzone and octocrylene are popular sunscreen active ingredients. Experiments that probe the stability of avobenzone in binary mixtures with octocrylene are presented, together with the synthesis of a class of novel composite sunscreens that were designed by covalently linking avobenzone and octocrylene groups. Spectroscopy, both steady-state and time-resolved, of the fused molecules was performed to investigate the stability of the new molecules and their potential function as ultraviolet filters. Computational results are detailed for truncated versions of a subset of the molecules to reveal the energy states underlying the absorption processes of this new class of sunscreen. The results indicate that the combination of elements of the two sunscreen molecules into one molecule creates a derivative with good stability to UV light in ethanol and in which the main degradation pathway of the avobenzone component in acetonitrile is reduced. Derivatives containing p-chloro substituents are particularly stable to UV light

Exploring the blueprint of photoprotection in Mycosporine-like Amino Acids

Microorganisms require protection against the potentially damaging effects of ultraviolet radiation exposure. Photoprotection is, in part, provided by mycosporine-like amino acids (MAAs). Previous reports have proposed that nonradiative decay mediates the impressive photoprotection abilities of MAAs. In this letter, we present the first ultrafast dynamics study of two MAAs, shinorine and porphyra-334. We demonstrate that, in aqueous solution, these MAAs relax along their S1 coordinates toward the S1/S0 conical intersection within a few hundred femtoseconds after photoexcitation and then traverse the conical intersection and vibrationally cool in approximately 1 ps through heat transfer to the solvent. This new insight allows a quintessential component of microbial life to be unraveled and informs the development of molecular photon-to-heat converters for a myriad of applications

Spectroscopic insight on impact of environment on natural photoprotectants

Biomimicry has become a key player in researching new materials for a whole range of applications. In this study, we have taken a crude extract from the red algae Palmaria palmata containing mycosporine-like amino acids – a photoprotective family of molecules. We have applied the crude extract onto a surface to assess if photoprotection, and more broadly, light-to-heat conversion, is retained; we found it is. Considering sunscreens as a specific application, we have performed transmission and reflection terahertz spectroscopy of the extract and glycerol to demonstrate how one can monitor stability in real-world applications