The title peptide, N-benzyloxycarbonyl-alpha-aminoisobutyryl-alpha-aminoisobutyryl-alpha-aminoisobutyryl-l-alanine tert-butyl ester or Z-Aib-Aib-Aib-l-Ala-OtBu (Aib is alpha-aminoisobutyric acid, Z is benzyloxycarbonyl and OtBu indicates the tert-butyl ester), C27H42N4O7, is a left-handed helix with a right-handed conformation in the fourth ... residue, which is the only chiral residue. There are two 4->1 intramolecular hydrogen bonds in the structure. In the lattice, molecules are hydrogen bonded to form columns along the c axis.