THE MOLECULAR-WEIGHT DISTRIBUTION AND CONFORMATION OF CITRUS PECTINS IN SOLUTION STUDIED BY HYDRODYNAMICS

The molecular weight distribution of a citrus pectin has been analysed by a combined approach using gel-permeation chromatography with low-speed sedimentation equilibrium. (1) A pectin preparation from citrus fruit was fractionated on Sepharose CL-2B/Sepharose CL-4B. (2) Weight average molecular weights of the fractions were determined by low speed sedimentation equilibrium in multichannel cells. (3) An absolute calibration for the column for this material was thereby defined. (4) The (log-normal) molecular weight distribution thus obtained is consistent with a weight average of (90 000 +/- 10 000) g/mol, obtained separately on unfractionated material, and consistent with a distribution obtained on the same material but using light scattering as the molecular weight probe. The conformation of the pectin fractions in solution was studied in terms of: (1) the Wales-Van Holde parameter, k(s)/[eta]; (2) Mark-Houwink-Kuhn-Sakurada plots of sedimentation coefficient and intrinsic viscosity data versus molecular weight; (3) rod models and (4) worm-like-coil models. The sedimentation data is consistent with a rod model (or a worm-like-coil with a large persistence length) with mass per unit length approximately 430 g mol-1 nm-1. The intrinsic viscosity data is also consistent with a rod model but shows some anomalous features which may be suggestive of worm-like-coil behaviour at higher molecular weight, although it is not possible to fit this data with a realistic value of the mass per unit length.