Ultraviolet and Infrared Spectroscopy of Synthetic Foldamers, Aib Homopeptides, and Solvated 1,2-Diphenylethane in the Gas Phase
The work presented here implements a supersonic jet expansion source to funnel the population of model peptides and solvated-bichromophore clusters into their low lying structural minima and to collisionally cool these minima to their respective zero-point vibrational levels. Single-conformation ultraviolet and infrared spectroscopy techniques are then used to probe these systems and investigate their electronic properties and uncover their intrinsic conformational preferences in the gas phase. Model β/γ-peptides known as synthetic foldamers and aminoisobutyric acid (Aib) homopeptides incorporate structural constraints that are designed/known to impose particular structural motifs. Here the ability of a β/γ-dipeptide to replicate the backbone length of an α-tripeptide and subsequently form the first portion of an α-helix is presented. Additionally, tests of the propensity for (Aib)n homopeptides to form 310-helices, in spite of accumulation of a macrodipole moment are shown. In fact, Aib is a strong 310-helix former, but there appears to be a point around n = 6 where competing forces funnel the population into a competing conformational family. 1,2-Diphenylethane (DPE) is a model, flexible bichromophore comprising two phenyl rings bound by an ethane bridge. By complexing DPE with (H 2O)n (n = 1-3) the effects of step-wise solvation on the electronic spectroscopy (i.e. the impact of water on vibronically coupled chromophores) and the conformational preferences of both the DPE monomer and the (H2O)n structures were investigated. In three of the four resultant clusters, the water molecule(s) were found to bind symmetrically to the DPE host, and an S0-S2 origin transition was not observed. However, in the fourth case, in which the anti conformation of the DPE monomer serves as host, localization is observed, and the S0-S2 origin is detected. Also of note is that the water-trimer, which almost always adopts a cycle geometry, was found to exclusively adopt a chain-geometry in the presence of DPE.
Zwier, Purdue University.
Chemistry|Analytical chemistry|Physical chemistry
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