Ordering of agarose near the macroscopic gelation point

Gel formation and spatial structure is an important area of study in polymer physics and in macromolecular and cellular biophysics. Agarose has a sufficiently complex gelation mechanism to make it an interesting prototype for many other gelling systems, including those involved in amyloid fibrillogenesis. Static (over a scattering vector range of 0.1-30 microm(-1)) and dynamic light scattering and rheology methods were used to follow the gelation kinetics of agarose at 0.5% in water or in the presence of 25 mM NaCl and quenched to temperatures of 20-43 degrees C. Light scattering results on gelling samples are fully described by a fractal aggregate model with four physically meaningful parameters. In all cases aggregates, with fractal dimensions at or near 3, form more rapidly and are smaller in characteristic size at lower quench temperatures. A region three to four times larger than the aggregate becomes depleted of agarose as the gelation proceeds. Below about 30 degrees C the aggregation process freezes spatial ordering rapidly, resulting in fragile macroscopic gels as determined by rheology. Salt effects are seen to be minimal and not important in the fundamental aggregation mechanism.