Sol–sol structural transition of aqueous agarose systems

Abstract

SynopsisA structural transition is reported to occur in aqueous sols of agarose, an electrically uncharged biostructural polysaccharide. The transition has no measurable effect on size dispersity on the shape of the solute polysaccharide as observed by precision photon correlation spectroscopy. It originates a low-angle pattern of scattered light similar to that which monitors phase separations in polymer blends. Thus, it must be due to some extent to spatially modulated polymer clustering, typical of spinodal decomposition. In the interval of temperatures studied, it precedes very distinctly in time the thermoreversible sol-gel transition, which is known to be promoted a t higher concentrations. It also anticipates to an appreciable extent the spatial density modulation observed in the gel. Although reported here for the first time, a spinodal decomposition of the sol that precedes and possibly triggers the processes leading to gelation does not come unexpectedly in terms of site-bond correlated-percolation theory. In general, this occurrence raises the question as t o whether the spontaneous onset of regions of higher and lower polymer concentration (spinodal separation) may be regarded as a novel path for biomolecuiar interactions and the self-assembly of order in biomolecular systems.

Molecular interactions in aqueous solutions: The role of random fluctuations of external parameters and of thermodynamic instabilities

“…The gelation of aqueous agarose sols, its reversibility, and the molecular and supramolecular structure of the polysaccharide, as well as the structure of the surrounding water, have been the object of many studies [24,25,. These have shown that the underlying physics includes two concentration-dependent processes, both implying molecular recognition, i.e., a conformational transition from single strands to double helices and aggregation of double helical segments to form bundles.…”

“…Recent experiments have shown for the first time the role of spinodal decomposition in the promotion of functional encounters of solute biomolecules [24,25]. The specific case is that of agarose, a biopolysaccharide capable of self-assembling in a supramolecularly ordered gel.…”

“…In short, these experiments have shown that chaotic fluctuations occurring under conditions of thermodynamic instability are capable of promoting two ordering processes, each of which is concentration-dependent and a necessary step to gelation: the coil-to-double helix transition, at a molecular level, and the following aggregation of helices and formation of the gel, at a supramolecular level. The order parameter and the overall structural pattern in the gel are determined by the chaotic fluctuations that originate the spinodal decomposition and progress in a band of wavelengths narrowed by the competing processes that we have described [ 13,24,25].…”

“…The possibility of percolation is offered by rare kinks existing in the repetitive polymer strand, which allow the occurrence of branching when double helices are formed. Experiments of low-angle light scattering, microviscosity [24,3 11, macroviscosity, viscoelastic behavior [32,33], and high-sensitivity turbidity [ 13,251 on agarosewater systems quenched from 80°C to a temperature in the range 40-50°C have shown the occurrence of a spinodal decomposition [24,31], followed by the gelation process. The order parameter (Ac') and the correlation length of the final isothermally formed gels have been determined by turbidity measurements, and the lowangle scattering of the gel has been compared with that of the decomposed sol, thus evidencing similar correlation lengths [25].…”

Fabrication of translucent tetragonal zirconia by gelcasting of thin zirconia nano-slurry

“…25) The average pore diameter in agarose hydrogel depends on the amount of the geling agent, and the reported pore diameter ranges 200 to 500 nm. 26),27) The strength of the agarose hydrogel depends on the macromolecular network in the hydrogel.…”