H-bonded helices in conventional peptides (containing exclusively homochiral alpha-amino acid residues) feature a uniform H-bonding directionality, N-terminal side C=O to C-terminal side NH. In contrast, heterochiral alpha-peptides can form helices in which the H-bond directionality alternates along the backbone because neighboring amide groups are oriented in opposite directions. Alternating H-bond directions are seen also in helices formed by unnatural peptidic backbones, e.g., those containing beta- or gamma-amino acid residues. In the present study, we used NMR spectroscopy and crystallography to evaluate the conformational preferences of the novel gamma-amino acid (1R,2R,3S)-2-(1-aminopropyl)-cyclohexanecarboxylic acid (APCH), which is constrained by a six-membered ring across its C alpha C beta bond. These studies were made possible by the development of a stereoselective synthesis of N-protected APCH. APCH strongly enforces the alpha/gamma-peptide 12/10-helical secondary structure, which features alternating H-bond directionality. Thus, APCH residues appear to have a conformational propensity distinct from those of other cyclically constrained gamma-amino acid residues.