Binding Water Clusters to an Aromatic-Rich Hydrophobic Pocket: [2.2.2]Paracyclophane–(H₂O)_<i>n</i>, <i>n</i> = 1–5

[2.2.2]­Paracylcophane (tricyclophane, TCP) is a macrocycle with three phenyl substituents linked by ethyl bridges (−CH₂CH₂−) in the para-position, forming an aromatic-rich pocket capable of binding various substituents, including nature’s solvent, water. Building on previous work [Buchanan, E. G.; et al. <i>J. Chem. Phys.</i> <b>2013</b>, <i>138</i>, 064308] that reported on the ground state conformational preferences of TCP, the focus of the present study is on the infrared and ultraviolet spectroscopy of TCP–(H₂O)_<i>n</i> clusters with <i>n</i> = 1–5. Resonant two-photon ionization (R2PI) was used to interrogate the mass selected electronic spectrum of the clusters, reporting on the perturbations imposed on the electronic states of TCP as the size of the water clusters bound to it vary in size from <i>n</i> = 1–5. The TCP–(H₂O)_<i>n</i> S₀–S₁ origins are shifted to lower frequency from the monomer, indicating an increased binding energy of the water or water network in the excited state. Ground state resonant ion-dip infrared (RIDIR) spectra of TCP–(H₂O)_<i>n</i> (<i>n</i> = 1–5) clusters were recorded in the OH stretch region, which probes the H-bonded water networks present and the perturbations imposed on them by TCP. The experimental frequencies are compared with harmonic vibrational frequencies calculated using density functional theory (DFT) with the dispersion-corrected functional ωB97X-D and a 6-311+g­(d,p) basis set, providing firm assignments for their H-bonding structures. The H₂O molecule in TCP–(H₂O)₁ sits on top of the binding pocket, donating both of its hydrogen atoms to the aromatic-rich interior of the monomer. The antisymmetric stretch fundamental of H₂O in the complex is composed of a closely spaced set of transitions that likely reflect contributions from both para- and ortho-forms of H₂O due to internal rotation of the H₂O in the binding pocket. TCP–(H₂O)₂ also exists in a single conformational isomer that retains the same double-donor binding motif for the first water molecule, with the second H₂O acting as a donor to the first, thereby forming a water dimer. The OH stretch infrared spectrum reflects a cooperative strengthening of both π-bound and OH···O H-bonds due to binding to TCP. The TCP–(H₂O)_<i>n</i>, <i>n</i> = 3–5 clusters all form H-bonded cycles, retaining their preferred structures in the absence of TCP, but distorted significantly by the presence of the TCP molecule. TCP–(H₂O)₃ divides its population between two conformational isomers that differ in the direction of the H-bonds in the cycle, either clockwise or counterclockwise, which are distinguishable by virtue of the <i>C</i>₂ symmetry of the TCP monomer. TCP–(H₂O)₄ and TCP–(H₂O)₅ have OH stretch IR spectra that are close analogues of their benzene–(H₂O)_<i>n</i> counterparts in the H-bonded OH stretch region, but differ somewhat in the free and π OH stretch regions as the tetramer and pentamer cycles begin to spill out of the pocket interior. Lastly, excited state RIDIR spectroscopy in the OH stretch region is used to probe the response of water cluster to ultraviolet excitation, showing how the proximity of a given water molecule to the aromatic-rich π clouds affects the infrared spectrum of the water network

Solvent Effects on Vibronic Coupling in a Flexible Bichromophore: Electronic Localization and Energy Transfer induced by a Single Water Molecule

Size and conformation-specific ultraviolet and infrared spectra are used to probe the effects of binding a single water molecule on the close-lying excited states present in a model flexible bichromophore, 1,2-diphenoxyethane (DPOE). The water molecule binds to DPOE asymmetrically, thereby localizing the two electronically excited states on one or the other ring, producing a S₁/S₂ splitting of 190 cm^–1. Electronic localization is reflected clearly in the OH stretch transitions in the excited states. Since the S₂ origin is imbedded in vibronic levels of the S₁ manifold, its OH stretch spectrum reflects the vibronic coupling between these levels, producing four OH stretch transitions that are a sum of contributions from S₂-localized and S₁-localized excited states. The single solvent water molecule thus plays multiple roles, localizing the electronic excitation in the bichromophore, inducing electronic energy transfer between the two rings, and reporting on the state mixing via its OH stretch absorptions

Mixed 14/16 Helices in the Gas Phase: Conformation-Specific Spectroscopy of Z‑(Gly)_<i>n</i>, <i>n</i> = 1, 3, 5

Single-conformation ultraviolet and infrared spectroscopy has been carried out on the neutral peptide series, Z-(Gly)_<i>n</i>-OH, <i>n</i> = 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(<i>g</i>−) 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

Cyclic Constraints on Conformational Flexibility in γ‑Peptides: Conformation Specific IR and UV Spectroscopy

Single-conformation spectroscopy has been used to study two cyclically constrained and capped γ-peptides: Ac-γ_ACHC-NHBn (hereafter γ_ACHC, Figure 1a), and Ac-γ_ACHC-γ_ACHC-NHBn (γγ_ACHC, 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 γ_ACHC and γγ_ACHC, 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. γ_ACHC forms a single nearest-neighbor C9 hydrogen-bonded ring whereas γγ_ACHC 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 γ_ACHC subunit [Guo, L., et al. Angew. Chem. Int. Ed. 2011, 50, 5843−5846]. In the gas phase, the γ_ACHC-H₂O complex was also observed and spectroscopically interrogated in the molecular beam. Here, the monosolvated γ_ACHC 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