The folding preferences of two capped, constrained beta/gamma-dipeptide isomers, Ac-beta(ACPC)-gamma(ACHC)-NHBn and Ac-gamma(ACHC)-beta(ACPC)-NHBn, (designated beta gamma and gamma beta, respectively), have been investigated using single- and double-resonance ultraviolet and infrared spectroscopy in the gas phase. These capped beta/gamma-dipeptides have the same number of backbone atoms between their N- and C-termini as a capped a-tripeptide and thus serve as a minimal structural unit on which to test their ability to mimic the formation of the first turn of an alpha-helix. Resonant two-photon ionization and UV-UV hole-burning spectroscopy were performed in the S-0-S-1 region, revealing the presence of three unique conformations of beta gamma and a single conformation of gamma beta. Resonant ion-dip infrared spectra were obtained in the NH stretch region from 3300 to 3500 cm(-1) and in both the amide I and amide II regions from 1400 to 1800 cm(-1). These infrared spectra were compared to computational predictions from density functional theory calculations at the M05-2X/6-31+G(d) level, leading to assignments for the observed conformations. Two unique bifurcated C8/C13 H-bonded ring structures for beta gamma and a single bifurcated C9/C13 H-bonded ring structure for gamma beta were observed. In all cases, the H-bonding patterns faithfully mimic the an alpha-helix, most notably by containing a 13-membered H-bonded cycle but also by orienting the interior amide group so that it is poised to engage in a second C13 H-bond as the beta/gamma-peptide lengthens in size. The structural characteristics of the beta/gamma-peptide version of the 13-helix turn are compared with the a-helix counterpart and with a reported crystal structure for a longer beta/gamma-peptide oligomer.
