Single-conformation ultraviolet and infrared spectra of a series of model γ-peptides are reported, with the goal of providing new tests of amide stacking as an amide–amide binding motif. The data also serve to illustrate the power and challenges of carrying out single-conformation spectroscopy of neutral molecules of this size in the gas phase under jet-cooled conditions. Building on recent work on Ac-γ2-hPhe-NHMe (James et al., J. Am. Chem. Soc., 2009, 131, 14243), the effects of derivatization and H2O complexation on amide stacking are studied. Ac-γ2-hPhe-N(Me)2 shows only amide stacked structures, blocking the competing position for formation of an amide-amide H-bond. The Ac-γ2-hPhe-NHMe-H2O complex includes structures in which the H2O molecule forms a bridge between the two stacked amide planes, retaining and enhancing amide stacking. IR population transfer methods are also employed to study the dynamics of photodissociation of the amide stacked-H2O complex. Finally, IR ion-gain spectroscopy is introduced as a means of recording infrared spectra containing contributions from all conformers present, based on IR-induced broadening of the UV absorptions. Its role in estimating fractional abundances is tested on Ac-γ2-hPhe-NHMe.
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