New patterns of hydrogen bonded interactions between polypeptide chains. Crystal and molecular structure of glycylglycylglycine.

Abstract

Crystals of glycylglycylglycine (C6H-11N3O4), grown from an aqueous methanol solution, are triclinic, space group P1, with the unit cell dimensions (at 22 +/- 3 degrees) a = 11.656(3), b = 14.817(3), c = 4.823(2) A, alpha = 88.45(3), beta = 95.96(3), gamma = 105.42(3) degrees, Z = 4 (with two molecules in the asymmetric unit) with a density of Dobs = 1.58 g X cm -3 and Dcalc = 1.572 g X cm -3. The crystal structure was solved by a combination of multisolution and trial and error methods and refined with full-matrix least-squares method to a final R value of 0.036 for the observed 3021 reflections (I greater than or equal to 2 sigma). The conformation of the two molecules I and II in the asymmetric unit is very similar (except around the N-terminal end); they have the fully extended trans-planar conformation, and have omega values ranging from 2 to 4 degrees. The peptide chain repeating distances (C1 alpha - C3 alpha) are 7.27 A and 7.18 A in the two molecules as compared with the value of 6.68 A for extended beta-sheets with beta-carbons. There are four different interactions between these two molecules characterized by different hydrogen bonding. Molecule I is hydrogen bonded to a neighboring molecule I using four hydrogen bonds. Molecule II is hydrogen bonded to another II, using bifurcated interactions involving the peptide nitrogen. Molecule I is hydrogen bonded to two different molecules II forming distinctly different hydrogen bonding patterns from the two mentioned above. The molecules are packed in rows, in a head-to-tail fashion (C-terminal opposite N-terminal) and are held together in sheets by hydrogen bonds between carbonyl and amide groups, corresponding to the very familiar anti-parallel pleated sheet arrangement for polypeptides. The hydrogen bonds involving the amino nitrogens as donors are significantly longer and presumably weaker compared to those involving the NH+3 group. The C=O distances show variations that correlated with hydrogen bonding. The N-H...O angle varies from 152 to 174 degrees and the bent N-H...O hydrogen bonds show bifurcated interactions.