Conformational characteristics of the -Pro-Ala- dipeptide sequence 1

Abstract

Abstract The molecular and crystal structures of N-acetyl-L-prolyl-L-alanine tert-butylamide (1) monohydrate, N-isobutyryl-D-prolyl-D-alanyl-L-alanine tert-butylester (2) monohydrate, and N-benzyloxycarbonyl-D-prolyl-L-alanine tert-butylamide (3) have been determined by X-ray diffraction. Crystal parameters: (1) orthorhombic, space group P212121, a = 8.950(2) Å, b = 27.772(3) Å, c = 6.887(2) Å, and Z = 4; (2) monoclinic, space group P21, a = 6.241(1) Å, b = 9.657(2) Å, c = 19.031 (2) Å, β = 93.5(2)°, and Z = 2; (3) monoclinic, space group P21, a = 10.747(2) Å, b = 8.414(1) Å, c = 12.195(2) Å, β = 97.9(1)°, and Z = 2. The structures were solved by direct methods. The least-squares refinements led to conventional R factors of 0.043, 0.040, and 0.042 for (1), (2), and (3), respectively. While the -L-Pro-L-Ala sequence of (1) is of the poly-(Pro)n II type (semi-extended conformation), the -D-Pro-D-Ala- and -D-Pro-L-Ala- sequences of (2) and (3), respectively, are folded in a β-bend conformation (type-I′ for the former, while type-II′ for the latter).