CONFORMATION OF THE CYCLIC PENTAPEPTIDE GLY-L-PRO-L-SER-D-ALA-L-PRO IN THE CRYSTALLINE STATE AND AN EXAMPLE OF ROTATIONAL-ISOMERISM BETWEEN ANALOGS

Cyclic Gly1-L-Pro2-L-Ser3-D-Ala4-L-Pro5 (C18H27N5O6) contains all trans peptide units with significant deviations from planarity in three of them where ω3 = -165ω, ω4 = 160°, and ω5 = 169°. If the side groups are disregarded, the conformation of the cyclic backbone is almost identical with that found in cyclic Gly1-L-Pro2-Gly3-D-Ala4-L-Pro5. previously reported, with the exception that the backbones in the two molecules are related by a mirror. If the peptide backbones of both molecules are held in the same orientation, the side groups of the Ser-containing molecule have progressed by one peptide unit along the backbone as compared to the Gly3 molecule. One transannular 4→1 hydrogen bond (type 11') is formed encompassing Gly1-L-Pro2 with ø1 = 58°, Ψ1 = -128°, ø2 = -75°, and Ψ2 = 20°. This is a first observation, at least in crystals of relatively small peptides, for the second corner of a 0 turn to contain a Pro group. The space group is P21 with a = 12.392 (7) Å, b = 9.149 (8) Å, c = 10.428 (4) Å, β = 97.3°, and Z = 2. The peptide molecules are stacked in the crystal with NH…O=C and 0H…O=C hydrogen bonds between molecules in a stack. The stacks of peptide molecules are separated by layers of solvent molecules, CH2CI2. © 1979, American Chemical Society. All rights reserved.