Plant Sunscreens in the UV-B: Ultraviolet Spectroscopy of Jet-Cooled Sinapoyl Malate, Sinapic Acid, and Sinapate Ester Derivatives

Abstract

Ultraviolet spectroscopy of sinapoyl malate, an essential UV-B screening agent in plants, was carried out in the cold, isolated environment of a supersonic expansion to explore its intrinsic UV spectral properties in detail. Despite these conditions, sinapoyl malate displays anomalous spectral broadening extending well over 1000 cm(-1) in the UV-B region, presenting the tantalizing prospect that nature's selection of UV-B sunscreen is based in part on the inherent quantum mechanical features of its excited states. Jet-cooling provides an ideal setting in which to explore this topic, where complications from intermolecular interactions are eliminated. In order to better understand the structural causes of this behavior, the UV spectroscopy of a series of sinapate esters was undertaken and compared with ab initio calculations, starting with the simplest sinapate chromophore sinapic acid, and building up the ester side chain to sinapoyl malate. This "deconstruction" approach provided insight into the active mechanism intrinsic to sinapoyl malate, which is tentatively attributed to mixing of the bright V ((1)ππ*) state with an adiabatically lower (1)nπ* state which, according to calculations, shows unique charge-transfer characteristics brought on by the electron-rich malate side chain. All members of the series absorb strongly in the UV-B region, but significant differences emerge in the appearance of the spectrum among the series, with derivatives most closely associated with sinapoyl malate showing characteristic broadening even under jet-cooled conditions. The long vibronic progressions, conformational distribution, and large oscillator strength of the V (ππ*) transition in sinapates makes them ideal candidates for their role as UV-B screening agents in plants.

Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents

“…Unlike the others, however, the absorption spectrum of sinapoyl malate is broad and featureless, even under jet-cooled conditions. 158 In each case, the UV absorptions are attributed to p* ' p excitations, involving orbitals that are delocalised across the ring and the conjugated side-chain. IR-UV double resonance studies serve to exclude spectral congestion (e.g.…”

“…Evidence in support of this conjecture is provided by TD-DFT calculations, which suggest that the dark state is a 1 np* state with substantial charge-transfer character. 158 The steady-state fluorescence spectrum following excitation of an aqueous solution of sinapoyl malate at its UV absorption maximum shows a large Stokes shift and a low fluorescence quantum yield, f fl B 3 Â 10 À3 -suggesting that population in this 1 np* state also undergoes efficient non-radiative decay.…”

“…the CT state suggested by Zwier and coworkers). 158 In either scenario, the excited molecule is assumed to distortmost plausibly (by analogy with the documented photophysics of other cinnamates) -along the E-Z isomerisation coordinate towards a region of CI with the S 0 PES. Vibrational energy transfer to the surrounding solvent cools the resulting S 0 molecules.…”

Photophysics of sunscreen molecules in the gas phase: a stepwise approach towards understanding and developing next-generation sunscreens

“…Guided by the previous literature, which suggests that the photochemistry of carbonyl compounds is closely linked with the existence of triplet states [54,127,142–149], Rodrigues et al. used computational methods to search for such states in EF.…”

Chemo-enzymatic Synthesis, Derivatizations, and Polymerizations of Renewable Phenolic Monomers Derived from Ferulic Acid and Biobased Polyols: An Access to Sustainable Copolyesters, Poly(ester-urethane)s, and Poly(ester-alkenamer)s

“…Having developed expertise in lignin related phenol compounds chemistry (42)(43)(44)(45)(46)(47), we recently put a great deal of effort into developing new valorizations of ferulic acid. In this context, we first devised and designed a new class of bio-based polyfunctional molecules incorporating ferulic acid moieties through the development of an enzymatic process involving polystyrene supported Candida antarctica lipase B (aka CAL-B or Novozyme ® 435) (48).…”