HYALURONIC-ACID - MOLECULAR-CONFORMATION AND INTERACTIONS IN THE TETRAGONAL FORM OF THE POTASSIUM-SALT CONTAINING EXTENDED CHAINS

[摘要] X-ray diffraction data were used to determine the detailed crystal structure of a tetragonal form of potassium hyaluronate containing relatively extended 4-fold helical chains (rise per disaccharide h = 0.95 nm). The polysaccharides chains are left-handed (43) helices. Two antiparallel chains pack in a tetragonal unit cell with P43212 space group symmetry. The chain conformations are stabilized intramolecularly by O4-O5 H-bonds across the .beta.(1 .fwdarw. 3) linkage and by a pair of intermolecular H-bonds per disaccharide between adjacent antiparallel chains. Fourier difference synthesis revealed 1 K+ ion and 2 water molecules per disaccharide. Six polyanion O atoms from 3 neighboring chains together with one of these water molecules form the co-ordination polyhedra. Further stability is brought about through inter- and intrachain water bridges involving both water molecules. The probable reason for the stability of this extended allomorph is discussed in terms of the preferred co-ordination geometry of the K+ ion.