Characterization of galactomannans isolated from legume endosperms of Caesalpinioideae and Faboideae subfamilies by multidetection aqueous SEC

Abstract

Galactomannans isolated from legume seed endosperms, including those of commercial interest, have been characterized by multidetection aqueous SEC. Galactomannans derived from seeds of the Faboideae subfamily had substantially higher M w than those from Caesalpinioideae seeds ( M w,Fab = 2.4–3.1 × 10 6 g/mol, M w,Caes. = 0.86–2.1 × 10 6 g/mol) and within the latter botanical subfamily, an apparent correlation between M w and the degree of galactose substitution DG was found. The molar mass distributions were unimodal and differed primarily by a scale factor, with distributional widths narrower than a true Flory ‘most-probable distribution’; good fits to Schulz–Zimm model were obtained. Across subfamilies no differences were found in the exponents of [ η]– M and R v– M relationships (0.61 ± 0.02, 0.54 ± 0.01, respectively), the Flory chain stiffness ratio ( C ∞ = 20 ± 1 (BSF analysis)), or the persistence length ( L p = 5.5 ± 0.2 nm) obtained from SEC fraction data. However, it was found that prefactors in the [ η]– M and R v– M relationships as well as the unperturbed parameter K Θ decrease in proportion to DG and therefore chain density. Generalized relationships incorporating galactose-dependent prefactors were therefore developed to model SEC fraction data of native galactomannans ([ η] GM = (1800 ± 200) × M o −1.61 × M 0.61±0.02, R v,GM = 0.63 ± 0.05 × M o −0.54 × M 0.54±0.01) as well as lower- M fractions obtained by ultrasonication ([ η] GM = (730 ± 100) × M o −1.71 × M w 0.71±0.02, R v,GM = 0.49 ± 0.05 × M o −0.57 × M w 0.57±0.01, M ≈ 1 × 10 5-native). As a consequence of this dependence and the observed patterns in molar mass variation, [ η] varies within a narrow range for galactomannans as a whole despite substantial M w differences.