Structure and conformations of two cycloisomeric hexapeptides: cyclo(L-Leu-L-Phe-Gly-D-Phe-L-Leu-Gly-) trihydrate and cyclo(L-Phe-L-Leu-Gly-D-Leu-L-Phe-Gly-) trihydrate.

cyclo(L-Leucyl-L-phenylalanyl-glycyl-D-phenylalanyl-L-leucyl-glycyl-) trihydrate (IV), C34H46N6O6.-3H2O, Mr = 688.8, monoclinic, P2(1), a = 11.720 (2), b = 36.354 (4), c = 8.888 (1) A, beta = 103.88 (1) degree, V = 3676.3 A3, Z = 4, Dx = 1.244 g cm-3, lambda (Cu K alpha) = 1.54178 A, mu = 7.6 cm-1, F(000) = 1480, T = 138 K, final R = 0.052 for 7661 unique reflections. cyclo(L-Phenylalanyl-L-leucyl-glycyl-D-leucyl-L-phenylalanyl-glycyl-) trihydrate (V), C34H46N6O6.-3H2O, Mr = 688.8, triclinic, P1, a = 101.54 (2), b = 19.111 (5), c = 8.527 (1) A, alpha = 101.54 (2), beta = 93.42 (2), gamma = 94.27 (2) degree, V = 1852.4 A3, Z = 2, Dx = 1.235 g cm-3, lambda (Cu K alpha) = 1.54178 A, mu = 7.5 cm-1, F(000) = 740, T = 138 K, final R = 0.063 for 7574 unique reflections. Peptides IV and V both have two independent conformers (molecules A and B). The peptide ring in each case contains one beta (I) turn and one beta (II') turn. A molecules in both structures have two transannular N--H...O hydrogen bonds, while B molecules form only one strong transannular hydrogen bond. The conformational differences between the two independent molecules (A and B) are much larger than the differences between the corresponding molecules of the two structures (A and A, and B and B). The crystal structures of the two peptides are very similar and consist of parallel bands of hydrophobic side chains and polar peptide regions. In each structure, molecules are stacked one over another with the hexapeptide ring lying perpendicular to the axis of the stack. The water molecules form well delimited solvent channels sandwiched between the layers of peptide molecules, and bridge the peptides through extensive hydrogen bonding.