title: 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.
authors.affiliation: Department of Chemistry, Purdue University , West Lafayette, Indiana 47907-2084, United States.
authors.forename: Jacob C
authors.initials: JC
authors.lastname: Dean
authors.forename: Ryoji
authors.initials: R
authors.lastname: Kusaka
authors.forename: Patrick S
authors.initials: PS
authors.lastname: Walsh
authors.forename: Florent
authors.initials: F
authors.lastname: Allais
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Zwier
mesh_terms: D003373:Coumaric Acids / Q000737:chemistry*; D004952:Esters; D008293:Malates / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D010666:Phenylpropionates / Q000737:chemistry*; D010936:Plant Extracts / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013473:Sunscreening Agents / Q000737:chemistry*
chemical_list: C079164:2-O-sinapoylmalate; D003373:Coumaric Acids; D004952:Esters; D008293:Malates; D010666:Phenylpropionates; D010936:Plant Extracts; D013473:Sunscreening Agents; C073734:sinapinic acid