SOLUTION CONFORMATIONS OF PECTIN POLYSACCHARIDES - DETERMINATION OF CHAIN CHARACTERISTICS BY SMALL-ANGLE NEUTRON-SCATTERING, VISCOMETRY, AND MOLECULAR MODELING

The solution behavior of pectin polysaccharides has been investigated by small angle neutron scattering (SANS), viscosimetric, and molecular modeling studies. The samples used in the experimental study were obt ained from apple and citrus and had degrees of methylation ranging fro m 28 to 73%, with a rhamnose content lying between 0.6 and 2.2%. Persi stence lengths, derived from intrinsic viscosity measurements, ranged from 59 to 126 Angstrom, whereas those derived by SANS were between 45 and 75 Angstrom. These values correspond to 10-17 monomer units. The modeling simulations were performed for both homogalacturonan itself a nd homogalacturonan carrying various degrees of rhamnose inserts (rham nogalacturonan). This required the evaluation of the accessible confor mational space for the eight disaccharides that represent the constitu ent repeating segments of the homogalacturonan and rhamnogalacturonan polysaccharides. For each dimer; complete conformational analysis was accomplished using the flexible residue method of the MM3 molecular me chanics procedure and the results used to access the configurational s tatistics of representative pectic polysaccharide chains. For homogala cturonan, an extended chain conformation having a persistence length o f 135 Angstrom (corresponding to 30 monomers) was predicted The inclus ion of varying amounts of rhamnose units (5-25%) in the model in stric t alternating sequence with galacturonate residues (equivalent to the rhamnogalacturonan ''hairy region'' chains) only slightly reduced the calculated persistence length. The extended overall chain conformation remained relatively unchanged as a consequence of the self-cancellati on of the kinking effects of successive paired rhamnose units. (C) 199 6 John Wiley & Sons, Inc.