The crystal and molecular structure of tert-butoxycarbonyl -alpha-aminoisobutyryl-isoleucyl-methylester

The crystal structure of the synthetic peptide Boc - Aib - Ile - OMe (C-16 H-30 N-2 O-5) has been determined from three-dimensional X - ray diffractio n data. The peptide crystallizes in triclinic space group pi with a = 9.570 (9), b = 10.261(7), c = 10.610(2) Angstrom, alpha = 101.9(0), beta = 91.7(0 ), gamma = 98.6(0)degrees V = 1006.1(12) Angstrom(3), Z = 2, D-calc = 1.09 Mg m(-3). The structure was solved by direct methods and refined by full-ma trix least-squares method to an R value of 0.072 (lambda = 1.5418 Angstrom) . The conformation of Aib residue in molecule A is a, and in molecule B is alpha(R). The lie residue in molecule A adopts folded conformation, while i n molecule B it is in the extended region. The peptide units are trans and show significant deviations from planarity.