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

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