Conformational analysis of galactomannans: from oligomeric segments to polymeric chains

Conformational features of the glycosidic bond linking two mannosyl units of four different oligomeric fragments of galactomannans have been calculated by means of adiabatic mapping of the glycosidic Φ, Ψ torsion angles using the MM3 force field. These fragments differed in their substitution pattern. The aim of this study was to ascertain the role played by the galactosyl side groups on the conformational flexibility of the galactomannan chain backbone. Although the overall features of all the potential energy surfaces created appear similar, these maps show that the position of the lowest energy minimum conformer and the lower energy region change significantly if one or both mannosyl residues are substituted by a galactosyl side group. Thus, these groups lead to significant differences in the accessible conformational space, when compared with that of the mannobiose molecule. Predicted homonuclear and heteronuclear coupling constants averaged over each entire map also reflect the conformational differences. Computed maps were used to predict polymeric unperturbed dimensions, C∞, a, 〈R〉 and 〈s2〉1/2 of idealized galactomannan chains by Monte Carlo methods. For low values of Man:Gal ratios, chain extension appears to be strongly dependent on the degree of substitution. For 2:1 and 3:1 Man:Gal ratios, random, alternate and block patterns of substitution have been investigated. It has also been shown that the spatial extension of the polymer chains is dependent on the scheme of substitution. Such studies provide a unique insight into the dependence of these two factors on the stiffness and flexibility of different galactomannan chains.