Kinetic and equilibrium processes in the formation and melting of agarose gels

Abstract

The formation and melting of agarose gels has been monitored by small-deformation oscillatory measurements of storage and loss moduli ( G' and G’). The onset of network formation on cooling at 1 °C min −1 moved to progressively higher temperature with increasing polymer concentration, from ~23 °C at 0.1 wt% to ~33 °C at 2.0 wt%. Isothermal measurements showed comparatively rapid gelation of concentrated (2.0 wt%) solutions at temperatures where the rate of gelation of more dilute samples (0.25 wt%) had dropped to zero. Both phenomena are interpreted in terms of kinetically determined helix-helix aggregation displacing the thermodynamic equilibrium between disordered coils and double helixes. Gel melting occurred as an equilibrium process, with moduli reaching constant, stable values after incremental increase in temperature. Rheological equilibration, however, was substantially slower than conformational disordering, as monitored by optical rotation, suggesting diffusion-controlled separation of strands after dissociation of ordered helices. Gels formed by rapid quenching to 5 °C melted at substantially (12–13 °C) lower temperature than those formed by cooling at 1 °C min −1. It is suggested that the helix-length required for stable association increases with increasing temperature, with slow cooling therefore giving longer helices than rapid quenching, and that increasing helix length promotes helix-helix aggregation. Consistent with this interpretation, the gels formed by slow cooling were stronger, and more turbid, than those obtained on quenching. Re-examination of a previous report that final melting of agarose gels occurs at a temperature well above completion of conformational disordering revealed a gross discrepancy in experimental evidence. Gel setting and melting temperatures obtained under identical conditions (1.0 wt% agarose in 50 wt% of dimethylsulphoxide) in both investigations were in close agreement, but the order-disorder transition temperature (from optical rotation) determined in the present work was ~25 °C higher, and indicated a finite residual helix-fraction on completion of gel melting.