Aminoisobutyric acid (Aib) oligomers are known to form racemic mixtures of enantiomeric left-and right-handed structures. The introduction of a chiral cap converts the enantiomeric structures into diastereomers that, in principle, afford spectroscopic differentiation. Here, we screen different C-terminal caps based on a model Aib dipeptide using double resonance laser spectroscopy in the gas phase to record IR and UV spectra of individual conformations present in the supersonic expansion: NH-benzyl (NHBn) as a reference structure because of its common use as a fluorophore in similar studies, NH-pfluorobenzyl (NHBn-F), and α-methylbenzylamine (AMBA). For both the NHBn and NHBn-F caps, a single conformer is observed, with infrared spectra assignable to an enantiomeric pair of type II/II′ β-turns in these molecules lacking a chiral center. The higher oscillator strength of the NHBn-F cap enabled UV−UV hole burning, not readily accomplished with the NHBn cap. The AMBA-capped structure, with its chiral center, produced two unique conformers, one of which was a nearly identical lefthanded type II β-turn, while the minor conformer is assigned to a C7−C7 sequential double ring, which is an emergent form of a 2 7 -ribbon. Although not observed, the type II′ β-turn diastereomer, with opposite handedness, is calculated to be 11 kJ/mol higher in energy, a surprisingly large difference. This destabilization is attributed primarily to steric interference between the Cterminal acyl oxygen of the peptide and the chirality-inducing methyl of the AMBA group. Last, computational evidence indicates that the use of an N-terminal aromatic cap hinders the formation of a 3 10 -helix in Ac-Aib 2 dipeptides.