We explore the ultrafast photoprotective properties of a series of sinapic acid derivatives in a range of solvents, utilizing femtosecond transient electronic absorption spectroscopy. We find that a primary relaxation mechanism displayed by the plant sunscreen sinapoyl malate and other related molecular species may be understood as a multistep process involving internal conversion of the initially photoexcited 1(1)ππ* state along a trans-cis photoisomerization coordinate, leading to the repopulation of the original trans ground-state isomer or the formation of a stable cis isomer.

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.

The conformational preferences of a series of short, aromatic-capped, glutamine-containing peptides have been studied under jet-cooled conditions in the gas phase. This work seeks a bottom-up understanding of the role played by glutamine residues in directing peptide structures that lead to neurodegenerative diseases. Resonant ion-dip infrared (RIDIR) spectroscopy is used to record single-conformation infrared spectra in the NH stretch, amide I and amide II regions. Comparison of the experimental spectra with the predictions of calculations carried out at the DFT M05-2X/6-31+G(d) level of theory lead to firm assignments for the H-bonding architectures of a total of eight conformers of four molecules, including three in Z-Gln-OH, one in Z-Gln-NHMe, three in Ac-Gln-NHBn, and one in Ac-Ala-Gln-NHBn. The Gln side chain engages actively in forming H-bonds with nearest-neighbor amide groups, forming C8 H-bonds to the C-terminal side, C9 H-bonds to the N-terminal side, and an amide-stacked geometry, all with an extended (C5) peptide backbone about the Gln residue. The Gln side chain also stabilizes an inverse γ-turn in the peptide backbone by forming a pair of H-bonds that bridge the γ-turn and stabilize it. Finally, the entire conformer population of Ac-Ala-Gln-NHBn is funneled into a single structure that incorporates the peptide backbone in a type I β-turn, stabilized by the Gln side chain forming a C7 H-bond to the central amide group in the β-turn not otherwise involved in a hydrogen bond. This β-turn backbone structure is nearly identical to that observed in a series of X-(AQ)-Y β-turns in the protein data bank, demonstrating that the gas-phase structure is robust to perturbations imposed by the crystalline protein environment.

Single-conformation spectroscopy has been used to study two cyclically constrained and capped γ-peptides: Ac-γACHC-NHBn (hereafter γACHC, Figure 1a), and Ac-γACHC-γACHC-NHBn (γγACHC, Figure 1b), under jet-cooled conditions in the gas phase. The γ-peptide backbone in both molecules contains a cyclohexane ring incorporated across each Cβ-Cγ bond and an ethyl group at each Cα. This substitution pattern was designed to stabilize a (g+, g+) torsion angle sequence across the Cα-Cβ-Cγ segment of each γ-amino acid residue. Resonant two-photon ionization (R2PI), infrared-ultraviolet hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been used to probe the single-conformation spectroscopy of these molecules. In both γACHC and γγACHC, all population is funneled into a single conformation. With RIDIR spectra in the NH stretch (3200-3500 cm(-1)) and amide I/II regions (1400-1800 cm(-1)), in conjunction with theoretical predictions, assignments have been made for the conformations observed in the molecular beam. γACHC forms a single nearest-neighbor C9 hydrogen-bonded ring whereas γγACHC takes up a next-nearest-neighbor C14 hydrogen-bonded structure. The gas-phase C14 conformation represents the beginning of a 2.614-helix, suggesting that the constraints imposed on the γ-peptide backbone by the ACHC and ethyl groups already impose this preference in the gas-phase di-γ-peptide, in which only a single C14 H-bond is possible, constituting one full turn of the helix. A similar conformational preference was previously documented in crystal structures and NMR analysis of longer γ-peptide oligomers containing the γACHC subunit [Guo, L., et al. Angew. Chem. Int. Ed. 2011, 50, 5843-5846]. In the gas phase, the γACHC-H2O complex was also observed and spectroscopically interrogated in the molecular beam. Here, the monosolvated γACHC retains the C9 hydrogen bond observed in the bare molecule, with the water acting as a bridge between the C-terminal carbonyl and the π-cloud of the UV chromophore. This is in contrast to the unconstrained γ-peptide-H2O complex, which incorporates H2O into both C9 and amide-stacked conformations.

The capped α/γ-peptide foldamers Ac-γACHC-Ala-NH-benzyl (γα) and Ac-Ala-γACHC-NH-benzyl (αγ) were studied in the gas phase under jet-cooled conditions using single-conformation spectroscopy. These molecules serve as models for local segments of larger heterogeneous 1:1 α/γ-peptides that have recently been synthesized and shown to form a 12-helix composed of repeating C12 H-bonded rings both in crystalline form and in solution [Guo, L.; et al. J. Am. Chem. Soc. 2009, 131, 16018]. The γα and αγ peptide subunits are structurally constrained at the Cβ-Cγ bond of the γ-residue with a cis-cyclohexyl ring and by an ethyl group at the Cα position. These triamides are the minimum length necessary for the formation of the C12 H-bond. Resonant two-photon ionization (R2PI) provides ultraviolet spectra that have contributions from all conformational isomers, while IR-UV hole-burning (IR-UV HB) and resonant ion-dip infrared (RIDIR) spectroscopies are used to record single-conformation UV and IR spectra, respectively. Four and six conformers are identified in the R2PI spectra of the γα and αγ peptides, respectively. RIDIR spectra in the NH stretch, amide I (C═O stretch), and amide II (NH bend) regions are compared with the predictions of density functional theory (DFT) calculations at the M05-2X/6-31+G* level, leading to definite assignments for the H-bonding architectures of the conformers. While the C12 H-bond is present in both γα and αγ, C9 rings are more prevalent, with seven of ten conformers incorporating a C9 H-bond involving in the γ-residue. Nevertheless, comparison of the assigned structures of gas-phase γα and αγ with the crystal structures for γα and larger α/γ-peptides reveals that the constrained γ-peptide backbone formed by the C9 ring is structurally similar to that formed by the larger C12 ring present in the 12-helix. These results confirm that the ACHC/ethyl constrained γ-residue is structurally preorganized to play a significant role in promoting C12 H-bond formation in larger α/γ-peptides.