KINETIC AND EQUILIBRIUM PROCESSES IN THE FORMATION AND MELTING OF AGAROSE GELS

The formation and melting of agarose gels has been monitored by small- deformation oscillatory measurements of storage and loss moduli (G' an d G ''). The onset of network formation on cooling at 1 degrees C min( -1) moved to progressively higher temperature with increasing polymer concentration, from similar to 23 degrees C at 0.1 wt% to similar to 3 3 degrees C at 2.0 wt%. Isothermal measurements showed comparatively r apid gelation of concentrated (2.0 wt%) solutions at temperatures wher e the rate of gelation of more dilute samples (0.25 wt%) had dropped t o zero. Both phenomena are interpreted in terms of kinetically determi ned helix-helix aggregation displacing the thermodynamic equilibrium b etween disordered coils and double helixes. Gel melting occurred as an equilibrium process, with moduli reaching constant, stable values aft er incremental increase in temperature. Rheological equilibration, how ever, was substantially slower than conformational disordering, as mon itored by optical rotation, suggesting diffusion-controlled separation of strands after dissociation of ordered helices. Gels formed by rapi d quenching to 5 degrees C melted at substantially (12-13 degrees C) l ower temperature than those formed by cooling at 1 degrees C min(-1). It is suggested that the helix-length required for stable association increases with increasing temperature, with slow cooling therefore giv ing longer helices than rapid quenching, and that increasing helix len gth promotes helix-helix aggregation. Consistent with this interpretat ion, the gels formed by slow cooling were stronger, and more turbid, t han those obtained on quenching. Re-examination of a previous report t hat final melting of agarose gels occurs at a temperature well above c ompletion of conformational disordering revealed a gross discrepancy i n experimental evidence. Gel setting and melting temperatures obtained under identical conditions (1.0 wt% agarose in 50 wt% of dimethylsulp hoxide) in both investigations were in close agreement, but the order- disorder transition temperature (from optical rotation) determined in the present work was similar to 25 degrees C higher, and indicated a f inite residual helix-fraction on completion of gel melting. (C) 1998 E lsevier Science Ltd. All rights reserved.