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

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 <i>ab initio</i> 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 (¹ππ*) state with an adiabatically lower ¹nπ* 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

Local Mode Approach to OH Stretch Spectra of Benzene–(H₂O)_<i>n</i> Clusters, <i>n</i> = 2–7

Isomer-specific resonant ion-dip infrared spectra are presented for benzene–water (Bz–(H₂O)_<i>n</i>) clusters with two to seven water molecules. Local mode Hamiltonians based on scaled M06-2X/6-311++G­(2d,p) density functional calculations are presented that accurately model the spectra across the entire OH stretch region (3000–3750 cm^–1). The model Hamiltonians include the contribution from the water bend overtone and an empirical parameter for the local OH stretch–bend Fermi coupling. The inclusion of this coupling is necessary for accurate modeling of the infrared spectra of clusters with more than three water molecules. For the cyclic water clusters (<i>n</i> = 3–5), the benzene molecule perturbs the system in a characteristic way, distorting the cycle, splitting degeneracies, and turning on previously forbidden transitions. The local OH stretch site frequencies and H···OH hydrogen bond lengths follow a pattern based on the each water monomer’s proximity to benzene. The patterns observed for these cyclic water clusters provide insight into benzene’s effects on the three-dimensional hydrogen-bonded networks present in water hexamer and heptamer structures, which also have their spectra dramatically altered from their pure water counterparts

Mimicking the First Turn of an α‑Helix with an Unnatural Backbone: Conformation-Specific IR and UV Spectroscopy of Cyclically Constrained β/γ-Peptides

The folding preferences of two capped, constrained β/γ-dipeptide isomers, Ac-β_ACPC-γ_ACHC-NHBn and Ac-γ_ACHC-β_ACPC-NHBn, (designated βγ and γβ, respectively), have been investigated using single- and double-resonance ultraviolet and infrared spectroscopy in the gas phase. These capped β/γ-dipeptides have the same number of backbone atoms between their N- and C-termini as a capped α-tripeptide and thus serve as a minimal structural unit on which to test their ability to mimic the formation of the first turn of an α-helix. Resonant two-photon ionization and UV–UV hole-burning spectroscopy were performed in the S₀–S₁ region, revealing the presence of three unique conformations of βγ and a single conformation of γβ. Resonant ion-dip infrared spectra were obtained in the NH stretch region from 3300 to 3500 cm^–1 and in both the amide I and amide II regions from 1400 to 1800 cm^–1. These infrared spectra were compared to computational predictions from density functional theory calculations at the M05-2X/6-31+G­(d) level, leading to assignments for the observed conformations. Two unique bifurcated C8/C13 H-bonded ring structures for βγ and a single bifurcated C9/C13 H-bonded ring structure for γβ were observed. In all cases, the H-bonding patterns faithfully mimic the first full turn of an α-helix, most notably by containing a 13-membered H-bonded cycle but also by orienting the interior amide group so that it is poised to engage in a second C13 H-bond as the β/γ-peptide lengthens in size. The structural characteristics of the β/γ-peptide version of the 13-helix turn are compared with the α-helix counterpart and with a reported crystal structure for a longer β/γ-peptide oligomer

Conformer-Specific and Diastereomer-Specific Spectroscopy of <i>αβα</i> Synthetic Foldamers: Ac–Ala−β_ACHC–Ala–NHBn

The folding propensities of a capped, cyclically constrained, mixed <i>α/β</i> diastereomer pair, (<i>SRSS)</i> Ac–Ala−β_ACHC–Ala–NHBn (hereafter <i>RS</i>) and (<i>SSRS)</i> Ac–Ala−β_ACHC–Ala–NHBn (<i>SR</i>), have been studied in a molecular beam using single-conformation spectroscopic techniques. These <i>α/β</i>-tripeptides contain a cyclohexane ring across each C_α<i>–</i>C_β bond, at which positions their stereochemistries differ. This cyclic constraint requires any stable species to adopt one of two ACHC configurations: equatorial CO/axial NH or equatorial NH/axial CO. Resonant two-photon ionization (R2PI) and infrared–ultraviolet hole-burning (IR–UV HB) spectroscopy were used in the S₀–S₁ region of the UV chromophore, revealing the presence of three unique conformational isomers of <i>RS</i> and two of <i>SR</i>. Resonant ion-dip infrared spectra were recorded in both the NH stretch (3200–3500 cm^–1) and the amide I (1600–1800 cm^–1) regions. These experimental vibrational frequencies were compared with the scaled calculated normal-mode frequencies from density functional theory at the M05-2X/6-31+G­(d) level of theory, leading to structural assignments of the observed conformations. The <i>RS</i> diastereomer is known in crystalline form to preferentially form a C11/C9 mixed helix, in which alternating hydrogen bonds are arranged in near antiparallel orientation. This structure is preserved in one of the main conformers observed in the gas phase but is in competition with both a tightly folded C7_eq/C12/C8/C7_eq structure, in which all four amide NH groups and four CO groups are engaged in hydrogen bonding, as well as a cap influenced C7_eq/NH···π/C11 structure. The <i>SR</i> diastereomer is destabilized by inducing backbone dihedral angles that lie outside the typical Ramachandran angles. This diastereomer also forms a C11/C9 mixed helix as well as a cap influenced bifurcated C7_ax–C11/NH···π/C7_eq structure as the global energy minimum. Assigned structures are compared with the reported crystal structure of analogous <i>α/β</i>-tripeptides, and disconnectivity graphs are presented to give an overview of the complicated potential energy surface of this tripeptide diastereomer pair

Role of Ring-Constrained γ‑Amino Acid Residues in α/γ-Peptide Folding: Single-Conformation UV and IR Spectroscopy

The capped α/γ-peptide foldamers Ac-γ_ACHC-Ala-NH-benzyl (γα) and Ac-Ala-γ_ACHC-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. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 16018]. The γα and αγ peptide subunits are structurally constrained at the Cβ–Cγ bond of the γ-residue with a <i>cis</i>-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