(265b) Formation and Microstructure of Self-Assembled Molecular Gels

Molecular gels are associated with the formation of strongly anisotropic space-spanning structures produced by self-assembly of low molecular weight gelator molecules that associate through hydrogen bonding, π-π stacking, acid-base or van der Waals interactions. One specific class of gel forming molecules is based on aromatic short peptide derivatives. Gelation in these systems is associated with π-π stacking of antiparallel peptide ß-sheets, resulting in the formation of fibrils that branch out to fill space. We explore structural changes taking place in solution during and after gelation of a hydrophobic dipeptide molecule, N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-FF). When water is added to a solution of Fmoc-FF in dimethyl sulfoxide (DMSO), the molecules self-assemble into rigid (G’ > 10⁵ Pa) gels at Fmoc-FF concentrations as low as 0.1 wt%, creating space-spanning fibrous networks (as confirmed by transmission electron microscopy). We investigate the changes in the system’s microstructure at compositions below, at and above the gel point, along lines of increasing Fmoc-FF volume fractions and increasing water concentrations, using small-angle and wide angle X-ray scattering. We find that at compositions right below the critical gel point concentration, precursors with distinctive spacing are being formed. As the concentration is increased to and above the gel point, the scattering curves exhibit branched fibrous structure features and an absence of the pre-gel aggregates. Evidence of characteristic spacing of ß-sheet π-π stacking can be seen as well. Furthermore, scattering studies of the temporal changes in the microstructure during the gel transition, as well as the effects of temperature on the gel structure, are presented.