FRUSTRATED CRYSTAL-STRUCTURE OF POLY(L-HYDROXYPROLINE)

The crystal structure of poly(L-hydroxyproline) (PHypro) with trans pl anar conformation of the peptide group is reexamined. The electron dif fraction pattern obtained from solution-grown triangular single crysta ls confirms the unit-cell geometry determined by Sasisekharan (Act a C rystallogr. 1959, 12, 903) (trigonal unit cell with parameters a = b = 12.3 Angstrom and c = 9.15 Angstrom, three left-handed 3-fold helices per unit cell). Analysis of the diffraction data and the single-cryst al morphology indicates that PHypro crystallizes in a frustrated struc ture characterized by the fact that the azimuthal orientation of one o f the helices differs significantly from that of the other two. Such a n arrangement is, however, compatible with hydrogen bonding between th e hydroxyl groups and the carbonyl groups of the neighbor helices. It further accounts for the highly unusual triangular morphology of solut ion-grown single crystals.