Molecular and crystal structure of a terminally protected, folded tripeptide

The molecular and crystal structure of the terminally protected tripeptide N-benzyloxycarbonyl-alpha-aminoisobutyryl-L-phenylalanyl-L-alanine methyl ester has been determined by X-ray diffraction. Crystal parameters: orthorhombic, space group P2(1)2(1)2(1), a = 11.542(2) Angstrom, b = 23.806(3) Angstrom, c = 8.987(2) Angstrom, and Z = 4. The crystal structure was solved by direct methods. The least-squares refinement led to R(1) = 0.039 [on F greater than or equal to 4 sigma(F)] and wR(2) = 0.127 (on F-2, all data). The -Aib-L-Phe- sequence of the tripeptide is folded into a type-I beta-bend conformation.