The presence of multiple α,α-dialkyl amino acids such as α-methylalanine (α-aminoisobutyric acid, Aib) leads to predominantly helical structures, either with α-helical or 310-helical hydrogen bonding patterns. The crystal structure of emerimicin-(1-9) benzyl ester (Ac-Phe-Aib-Aib-Aib-Val-Gly-Leu-Aib-Aib-OBzl) reported here shows essentially pure α-helical character, whereas other similar compounds shows predominantly 310- helical structures. The factors that govern helical preference include the inherent relative stability of the α-helix compared with the 310-helix, the extra hydrogen bond seen with s10-helix when packed in a crystalline lattice. The balance of these forces, when combined with the steric requirements of the amino acid side chains, determines the relative stability of the two helical conformations under a given set of experimental conditions.