CONFORMATIONAL FEATURES OF GALACTURONANS .1. STRUCTURE AND ENERGY MINIMIZATION OF CHARGED AND UNCHARGED GALACTURONAN DIMERIC UNITS

Structural parameters (e.g. the geometry, partial charges and dipole m oment) of the alpha-D-galacturonic residue have been calculated by usi ng semi-empirical quantum mechanical methods on several combinations o f either the uncharged or charged forms, GalAH and GalA(-), respective ly. Three residue types have been explored: (i) the isolated residue, termed GEO1 and GEO1C for GalAH and GalA(-), respectively; (ii) the re sidue with a methyl group attached to the O4 and O1 positions (GEO2 an d GEO2C); and (iii) the internal residue in a trimer, e.g. GalAH-GalAH -GalAH or the corresponding fully charged version (GEO3 and GEO3C). Th e presence of a charged group in the galacturonate residue and the dis tribution of the excess negative charge on the molecule lead to signif icant differences in the structural parameters in comparison with thos e of the uncharged galacturonic residue. These perceptible differences in internal coordinates of GalAH and GalA(-) residues appear to play a major role in the delimitation of the conformational space that is a ccessible to the dimers, as clearly seen by inspection of the conforma tional maps. Although the overall features seem alike, the maps show t hat the position of the minimum and the shape of the lower energy regi on significantly change if one or both residues in the dimer are charg ed. The relevance of these results for the conformational properties o f polygalacturonate chains is discussed elsewhere (Ruggiero et al. int . J. Biol. Macromol. 1995, 17, 213-218).