Extended x-ray absorption fine structure investigation of solid and gel forms of calcium poly(+.alpha.-D-galacturonate)

The Ca K-edge transmission spectra of calcium poly(?-D-galacturonate) in solid and gel forms, of calcium oligo(?-L-guluronate) in solid form, and of calcium 2-keto-D-gluconate trihydrate (of known crystal structure) have been measured and interpreted. Detailed analysis of the extended X-ray absorption fine structure of the model compound shows that Ca-O bond distance distributions can be reliably partitioned. With the parameters extracted from the model compound, it is found that the distribution of Ca-O distances is different in the solid poly(galacturonate) and the solid oligo(guluronate). In the former, the Ca2+ is ten coordinate, with a 2:6:1:1 (or 2:5:2:1) partitioning of the Ca-O distances and having Ca-O = 2.40 (2), 2.55 (2), 2.81 (2), and 2.95 (2) Å (or 2.39 (2), 2.54 (2), 2.78 (2), and 2.95 (2) Å). By way of contrast, the solid oligo(guluronate) has a more compact first shell of Ca-O distances, with further Ca-O distances well separated from this, to give a 3:3:2 Ca-O bond distance distribution with Ca-O = 2.33 (2), 2.45 (2), and 2.76 (2) Å. The gel form of calcium poly(?-D-galacturonate) gives results very similar to those of the oligo(guluronate). These differences in Ca-O coordination are ascribed to differences in the availability of oxygen atoms in the coordinating cavity caused by differences in polysaccharide chain conformation. Geometrical calculations assuming 31 and 21 conformations for the saccharide chains have been performed and compared with the distribution of Ca-O and Ca?C distances found from the EXAFS analysis. It is concluded that both oligo(guluronate) and the gel form of poly(galacturonate) have a 21 chain conformation, whereas the solid form of poly(galacturonate) has a 31 chain conformation. This provides direct structural evidence that a polymorphic phase transition occurs in calcium poly(?-D-galacturonate) on drying. © 1986 American Chemical Society.