Structure and conformation of linear peptides. V. Structure of L-prolyl-glycyl-glycine

The tripeptide, L-prolyl-glycyl-glycine, crystallizes in the trigonal space group P3(2), with a = b = 8.682(2) A, c = 12.008(2) and Z = 3. The structure was solved by direct methods and refined to an R-value of 0.07 for 727 reflections (I greater than 1.0 sigma). The molecule exists as a zwitterion in the crystal. The peptide units are trans and show significant deviations from planarity (omega 1 = 169.7 degrees, omega 2 = -170.1 degrees). The peptide backbone adopts a left-handed helical conformation similar to that of polyglycine II and polyproline II.