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 (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.
Molecules with several flexible coordinates have potential energy surfaces with a large number of minima and many transition states separating them. A general experimental protocol is described that ...is capable of studying conformational isomerization in such circumstances, measuring the product quantum yields following conformation-specific infrared excitation, and measuring energy thresholds for isomerization of specific X → Y reactant-product isomer pairs following excitation via stimulated emission pumping (SEP). These methods have been applied to a series of molecules of varying size and conformational complexity, including 3-indolepropionic acid (IPA), meta-ethynylstyrene, N-acetyltryptophan methyl amide (NATMA), N-acetyltryptophan amide (NATA), and melatonin. Studies of isomerization in solute−solvent complexes are also described, including a measurement of the barrier to isomerization in the IPA−H2O complex, and a unique isomerization reaction in which a single water molecule is shuttled between H-bonding sites on the trans-formanilide (TFA) molecule.
Two-color infrared multiphoton dissociation (2C-IRMPD) spectroscopy is a technique that mitigates spectral distortions due to nonlinear absorption that is inherent to one-color IRMPD. We use a ...2C-IRMPD scheme that incorporates two independently tunable IR sources, providing considerable control over the internal energy content and type of spectrum obtained by varying the trap temperature, the time delays and fluences of the two infrared lasers, and whether the first or second laser wavelength is scanned. In this work, we describe the application of this variant of 2C-IRMPD to conformationally complex peptide ions. The 2C-IRMPD technique is used to record near-linear action spectra of both cations and anions with temperatures ranging from 10 to 300 K. We also determine the conditions under which it is possible to record IR spectra of single conformers in a conformational mixture. Furthermore, we demonstrate the capability of the technique to explore conformational unfolding by recording IR spectra with widely varying internal energy in the ion. The protonated peptide ions YGGFL (NH3 +-Tyr-Gly-Gly-Phe-Leu, Leu-enkephalin) and YGPAA (NH3 +-Tyr-Gly-Pro-Ala-Ala) are used as model systems for exploring the advantages and disadvantages of the method when applied to conformationally complex ions.
UV photofragment spectroscopy and IR–UV double resonance methods are used to determine the structure and spectroscopic responses of a three-dimensional 2.2.2-benzocryptand cage to the incorporation ...of a single K+ or Ba2+ imbedded inside it (labeled as K+-BzCrypt, Ba2+-BzCrypt). We studied the isolated ion-cryptand complex under cryo-cooled conditions, brought into the gas phase by nano-electrospray ionization. Incorporation of a phenyl ring in place of the central ethyl group in one of the three N-CH2-CH2-O-CH2-CH2-O-CH2-CH2-N chains provides a UV chromophore whose S0–S1 transition we probe. K+-BzCrypt and Ba2+-BzCrypt have their S0–S1 origin transitions at 35,925 and 36,446 cm–1, respectively, blue-shifted by 174 and 695 cm–1 from that of 1,2-dimethoxybenzene. These origins are used to excite a single conformation of each complex selectively and record their IR spectra using IR–UV dip spectroscopy. The alkyl CH stretch region (2800–3000 cm–1) is surprisingly sensitive to the presence and nature of the encapsulated ion. We carried out an exhaustive conformational search of cage conformations for K+-BzCrypt and Ba2+-BzCrypt, identifying two conformations (A and B) that lie below all others in energy. We extend our local mode anharmonic model of the CH stretch region to these strongly bound ion-cage complexes to predict conformation-specific alkyl CH stretch spectra, obtaining quantitative agreement with experiment for conformer A, the gas-phase global minimum. The large electrostatic effect of the charge on the O- and N-lone pairs affects the local mode frequencies of the CH2 groups adjacent to these atoms. The localized CH2 scissors modes are pushed up in frequency by the adjacent O/N-atoms so that their overtones have little effect on the alkyl CH stretch region. However, the localized CH2 wags are nearly degenerate and strongly coupled to one another, producing an array of delocalized wag normal modes, whose highest frequency members reach up above 1400 cm–1. As such, their overtones mix significantly with the CH stretch modes, most notably involving the CH2 symmetric stretch fundamentals of the central ethyl groups in the all-alkyl chains and the CH stretches adjacent to the N-atoms and antiperiplanar to the nitrogen lone pair.
The techniques of laser-induced fluorescence (LIF), resonant two-photon ionization spectroscopy (R2PI), UV−UV hole-burning spectroscopy, fluorescence-dip infrared spectroscopy (FDIRS), and resonant ...ion-dip infrared spectroscopy (RIDIRS) have been used to study the ultraviolet and infrared spectra of individual conformations of small, flexible biomolecules cooled in a supersonic expansion. The water-containing clusters of these molecules and of other rigid molecules that possess multiple H-bonding sites are considered. The water molecules in many of the solute−(water) n clusters form hydrogen-bonded bridges between donor and acceptor sites on the solute molecule. The infrared spectroscopy of these bridges has been explored in some detail. Water bridges are also formed when one of the H-bonding sites is on a flexible side chain. These bridges have a profound influence on the conformational preferences of the flexible biomolecules.
Single-conformation ultraviolet and infrared spectroscopy has been carried out on the neutral peptide series, Z-(Gly) n -OH, n = 1,3,5 (ZGn) and Z-(Gly)5-NHMe (ZG5-NHMe) in the isolated environment ...of a supersonic expansion. The N-terminal Z-cap (carboxybenzyl) provides an ultraviolet chromophore for resonant two-photon ionization (R2PI) spectroscopy. Conformation-specific infrared spectra were recorded in double resonance using resonant ion-dip infrared spectroscopy (RIDIRS). By comparing the experimental spectra with the predictions of DFT M05-2X/6-31+G(d) calculations, the structures could be characterized in terms of the sequence of intramolecular H-bonded rings of varying size. Despite the enhanced flexibility of the glycine residues, a total of only six conformers were observed among the four molecules. Two conformers for ZG1 were found with the major conformation taking on an extended, planar β-strand conformation. Two conformers were observed for ZG3, with the majority of the population in a C11/C7/C7/π(g−) structure that forms a full loop of the glycine chain. Both ZG5 molecules had their population primarily in a single conformation, with structures characteristic of the first stages of a “mixed” β-helix. C14/C16 H-bonded rings in opposing directions (N → C and C → N) tie the helix together, with nearest-neighbor C7 rings turning the backbone so that it forms the helix. φ/ψ angles alternate in sign along the backbone, as is characteristic of the mixed, C14/C16 β-helix. The calculated conformational energies of these structures are unusually stable relative to all others, with energies significantly lower than the PGI/PGII conformations characteristic of polyglycine structures in solution and in the crystalline form, where intermolecular H-bonds play a role.
The 4H-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of ...electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T1(n,π*) ← S0 phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion. The jet cooling has eliminated congestion appearing in previous room-temperature measurements of the T1 ← S0 band system and has enabled us to determine precise fundamental frequencies for seven vibrational modes of the molecule in its T1(n,π*) state. We have also analyzed the rotational contour of the 00 0 band, obtaining experimental values for spin–spin and spin-rotation constants of the T1(n,π*) state. We used the experimental results to test predictions from two commonly used computational methods, equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles (EOM-EE-CCSD) and time-dependent density functional theory (TDDFT). We find that each method predicts harmonic frequencies within a few percent of observed fundamentals, for in-plane vibrational modes. However, for out-of-plane modes, each method has specific liabilities that result in frequency errors on the order of 20–30%. The calculations have helped to identify a perturbation from the T2(π,π*) state that leads to unexpected features observed in the T1(n,π*) ← S0 origin band rotational contour.
The 4
-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of ...electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T
(n,π*) ← S
phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion. The jet cooling has eliminated congestion appearing in previous room-temperature measurements of the T
← S
band system and has enabled us to determine precise fundamental frequencies for seven vibrational modes of the molecule in its T
(n,π*) state. We have also analyzed the rotational contour of the 0
band, obtaining experimental values for spin-spin and spin-rotation constants of the T
(n,π*) state. We used the experimental results to test predictions from two commonly used computational methods, equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles (EOM-EE-CCSD) and time-dependent density functional theory (TDDFT). We find that each method predicts harmonic frequencies within a few percent of observed fundamentals, for in-plane vibrational modes. However, for out-of-plane modes, each method has specific liabilities that result in frequency errors on the order of 20-30%. The calculations have helped to identify a perturbation from the T
(π,π*) state that leads to unexpected features observed in the T
(n,π*) ← S
origin band rotational contour.
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