Equilibrium spatial distribution of aqueous pullulan: Small-angle X-ray scattering and realistic computer modeling

A rotational isomeric state model has been developed for the polysaccharide pullulan dissolved in water. The model is consistent with the mean-square radius of gyration and Debye scattering function as measured for pullulan o ligomers containing 3, 6, 9, and 12 glucose residues by small-angle X-ray s cattering. It is also consistent with the unperturbed chain dimensions of h igh molecular weight aqueous pullulan and the temperature coefficient of th e unperturbed dimensions. The model is based on identification of an initia l set of rotational isomeric states using the AMBER* molecular mechanics fo rce field in conjunction with a continuum model for aqueous solvation. The relative weights of the rotational isomeric states were subsequently adjust ed to achieve an optimized fit to the data. Three important rotational isom eric states (encompassing together more than 77% of the probability distrib ution) are identified for the (1-->6)-linkage while a single state is recog nized for the (1-->4)-linkage. One of the states for the (1-->6)-linkage ma y be stabilized by a hydrogen bond that spans the linkage and introduces a bend into the chain trajectory which serves to reduce the mean global dimen sions of the chain.