Chain conformation in concentrated pectic gels: evidence from 13C NMR

13C NMR spectroscopy is a sensitive method of exploring the conformation of polysaccharides in both solid and gel states, because the positions of the peaks for the two carbons on either side of the glycosidic linkage, C-1 and C-4 for pectins, depend on the chain conformation. This approach was used to determine the conformation of calcium pectate at gel concentrations of ca. 0.3 g cm−3, comparable with those found in vivo. The solid acid, methyl-esterified, and sodium forms of pectate, which are known to have a right-handed, threefold helical (31) conformation, were used to define the spectral features of this conformation. These features were also present in both gel and solid calcium pectate, and were more conspicuous in the solid form. When dilute calcium pectate gels are dried they are known from CD and EXAFS evidence to undergo a conformational transition from the “egg-box” twofold helical (21) conformation to the 31 conformation. Calcium guluronate does not undergo this change, nor does calcium pectate in the presence of excess of monovalent cations. Changes in the 13C NMR spectrum marking this transition were observed on drying calcium pectate gels, were prevented by excess of K+, and were not found for guluronate. This enabled the spectral features corresponding to the 21 conformation to be identified. The spectral assignments were in agreement with the stereoelectronic theory of conformation-dependent chemical shifts, which also allowed a third set of peaks to be assigned to conformations dispersed between the 31 and 21 helices. About 70% of the pectate was visible in the gel spectra, the remaining chains being too mobile to be detected. It was concluded that the egg-box form, as dimers or larger aggregates, was the largest single component of the gels studied here, but that substantial quantities of 31 and intermediate helical aggregated forms were also present. Intact, pectin-containing plant cell walls also show spectral features characteristic of these components.