Structure and conformation of linear peptides

The tripeptide, L-prolyl-glycyl-glycine, crystallizes in the trigonal space group P3₂, with a = b = 8.682(2)Å, 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 > 1.0s?). The molecule exists as a zwitterion in the crystal. The peptide units and trans and show significant deviations from planarity (ω₁ = 169.7^o, ω₂=-170.1^o). The peptide backbone adopts a left-handed helical conformation similar to that of polyglycine II and polyproline II.