Abstract
In plants, sinapate esters offer crucial protection from the deleterious effects of ultraviolet radiation exposure. These esters are a promising foundation for designing UV filters, particularly for the UVA region (400 – 315 nm), where adequate photoprotection is currently lacking. Whilst sinapate esters are highly photostable due to a cis-trans (and vice versa) photoisomerization, the cis-isomer can display increased genotoxicity; an alarming concern for current cinnamate ester-based human sunscreens. To eliminate this potentiality, here we synthesize a sinapate ester with equivalent cis- and trans-isomers. We investigate its photostability through innovative ultrafast spectroscopy on a skin mimic, thus modelling the as close to true environment of sunscreen formulas. These studies are complemented by assessing endocrine disruption activity and antioxidant potential. We contest, from our results, that symmetrically functionalized sinapate esters may show exceptional promise as nature-inspired UV filters in next generation sunscreen formulations.
Highlights
In plants, sinapate esters offer crucial protection from the deleterious effects of ultraviolet radiation exposure
The spectroscopic measurements, both time-resolved and steadystate, have demonstrated that the addition of the second ester moiety has not impeded the high UV photostability that the sinapate backbone possesses[4]. This has had the opposite effect, highlighted by the minimal drop (≤3.3%) in the absorbance of DES after 2 h of UV irradiation at solar intensities, see Fig. 3. Alongside this increase in photostability, the critical wavelength of DES has significantly red-shifted compared to ethyl sinapate (ES), cf. 364 nm for DES compared to 352 nm for ES
This resulted in four dynamical processes, described by rate constants k1,2,3,4, being recovered; we note that k5 is used to describe the long-lived ground state bleach, which does not recover within the time-window of our experiment
Summary
Sinapate esters offer crucial protection from the deleterious effects of ultraviolet radiation exposure. 1234567890():,; Naturally occurring sinapate esters[1] have shown promise as starting points for a generation of ultraviolet (UV) filters that offer exemplary photoprotection They exhibit high levels of photostability under UV exposure, due to an efficient trans-cis and cis-trans photoisomerization resulting in a photoequilibrium between these isomers. Do these sinapate esters display high UV photostability, they demonstrate potent antioxidant capabilities[8] They are not without their issues, which must be taken into consideration if to be included in any future sunscreen formulation: firstly, their absorption does not completely span the UVA region (400–315 nm), lacking optimum UVA photoprotection; secondly, their UVA λmax is close to the UVB (315–280 nm), of which there are already a plethora of effective UVB filters; thirdly, the two isomers have differing absorption profiles, with the cis-isomer having (in general) the weaker absorption[4,5,9]; the genotoxicity of the cis-isomer has been shown to be significantly higher in a related cinnamate[10]. While the addition of identical ester moieties across the double bond spectrally shifts (to lower energy) in the absorbance (a positive attribute towards UVA filter design), this alone does not indicate whether such an approach maintains the desired properties (vide supra) that may facilitate symmetry driven sinapate esters being promising UV filters
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