title: High resolution 13C-n.m.r. spectroscopy of 'mixed linkage' xylans.
abstract: Three xylan fractions, obtained by stepwise precipitation with ethanol, were analysed by 75-MHz 13C-n.m.r. spectroscopy. Diad frequencies, determined from the C-2 resonances, show that the (1----3)-linkages are interspersed throughout the chain rather than grouped contiguously. This type of distribution is in agreement with a random coil conformation and with the constancy of the optical rotation in solvents of different ionic strength and chaotropic power. These diad frequencies were compared with the theoretical values calculated for a random distribution from the ratio of (1----4)-:(1----3)-linkages in the 1H-n.m.r. spectra, and from the methylation analysis for one of the fractions.
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina.
authors.forename: M C
authors.initials: MC
authors.lastname: Matulewicz
authors.forename: A S
authors.initials: AS
authors.lastname: Cerezo
authors.forename: R M
authors.initials: RM
authors.lastname: Jarret
authors.forename: N
authors.initials: N
authors.lastname: Syn
mesh_terms: D002236:Carbohydrate Conformation; D002247:Carbon Isotopes; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D009904:Optical Rotation; D012624:Seaweed; D014990:Xylans / Q000737:chemistry* / Q000302:isolation & purification
chemical_list: D002247:Carbon Isotopes; D014990:Xylans