Frustrated Crystal Structure of Poly( l -hydroxyproline)

The crystal structure of poly( l -hydroxyproline) (PHypro) with trans planar conformation of the peptide group is reexamined. The electron diffraction pattern obtained from solution-grown triangular single crystals confirms the unit-cell geometry determined by Sasisekharan ( Acta Crystallogr . 1959, 12, 903) (trigonal unit cell with parameters a = b = 12.3 Å and c = 9.15 Å, three left-handed 3-fold helices per unit cell). Analysis of the diffraction data and the single-crystal morphology indicates that PHypro crystallizes in a frustrated structure characterized by the fact that the azimuthal orientation of one of the helices differs significantly from that of the other two. Such an arrangement is, however, compatible with hydrogen bonding between the hydroxyl groups and the carbonyl groups of the neighbor helices. It further accounts for the highly unusual triangular morphology of solution-grown single crystals.