Static and dynamic light scattering by a thermoreversible gel from Rhizobium leguminosarum 8002 exopolysaccharide

Thermally reversible gelation of an exocellular polysaccharide from the bacterium Rhizobium leguminosarum phaseoli strain 8002 was studied by static and dynamic light scattering (LS) and by small angle X-ray scattering (SAXS). The very stiff double helical polysaccharide spontaneously forms a gel when salt (0.1 M NaCl) is added to aqueous solutions of the polymer with concentrations larger than 1.5% (w/v). The combination of static LS and SAXS revealed a heterogeneous structure that could be described by three characteristic correlation lengths. Large globular particles with a radius of gyration of R(G) = 308 +/- 10 nm appear embedded in a disordered medium with a correlation length of xi(1) = 120 +/- 20 nm and a fractal dimension of d(f) = 1.6 +/- 0.2. The matrix consists of entangled or associated chain sections with a cross-sectional radius of r(c) = 0.7 +/- 0.1 nm. The large clusters have an apparent fractal dimension of d(f) = 3.2 +/- 0.3. A third correlation length of 2.1 +/- 0.2 nm found in the system may be assigned to the fairly long side chains which form a disordered layer around the double helix. The heterogeneous structure was confirmed by dynamic LS, which revealed a very slow translational mode of motion in addition to the common fast translational motion. The time domain between these two relaxation processes appeared to be filled with a spectrum of further relaxation times but could not unambiguously be analyzed by Laplace inversion of the time correlation function.