Speed versus stability-structure-activity effects on the assembly of two-component gels

This paper reports the structural modification of a two-component gelation system comprising a 1:1 complex formed between a peptide carboxylic acid and phenylethylamine. Changing amino acids has a profound effect on the speed of gel formation and the minimum gelation concentration (MGC) yet the thermal stability of the gel remains unchanged. Variable temperature NMR studies demonstrate that at room temperature, the speed at which the gel forms is controlled by the solubility of the acid-amine complexes, which mediates the initial nucleation step required for gel assembly. On increasing the temperature, however, a thermodynamic enthalpy-entropy balance means all of the gels break down at around the same temperature. Those gels which are more favourably and rapidly formed at room temperature on enthalpic grounds are also more temperature sensitive as a consequence of the greater entropic cost of efficient packing within the gel fibres. This constitutes a rare example in which the time required for gelation can be structurally controlled, with NMR providing unique insight into the dynamics of these gel-phase materials. We suggest that in the future, combining solvent and solute (gelator) solubility parameters may provide further insight into these materials.