Conformational characteristics of the -Pro-Ala- dipeptide sequence

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 P2(1)2(1)2(1), a = 8.950(2) Angstrom, b = 27.772(3) Angstrom, c = 6.887(2) Angstrom, and Z = 4; (2) monoclinic, space group P2(1), a = 6.241(1) Angstrom, b = 9.657(2) Angstrom, c = 19.031(2) Angstrom, beta = 93.5(2)degrees, and Z = 2; (3) monoclinic, space group P2(1), a = 10.747(2) Angstrom, b = 8.414(1) Angstrom, c = 12.195(2) Angstrom, beta = 97.9(1)degrees, 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), 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 beta-bend conformation (type-I' for the former, while type-II' for the latter).