Anatomy of a gel. Amino acid derivatives that rigidify water at submillimolar concentrations

On the basis of suggestive X-ray data, 14 aroyl L-cystine derivatives were designed, synthesized, and examined for their ability to gelate water. Seve ral members of this amino acid family are remarkably effective aqueous gela tors (the best being one that can rigidify aqueous solutions at 0.25 mM, ca . 0.01%, in less than 30 s!). A few of the analogues separate from water as crystals, indicating a close relationship between gelation and crystalliza tion. All effective gelators self-assemble into fibrous structures that ent rain the solvent in the capillary spaces among them. Hydrogen-bonding sites on the compounds that might stabilize the fibers were identified from spec ific substitutions that replace a hydrogen donor with a methyl group, enhan ce the hydrogen-accepting ability of a carbonyl oxygen, or promote the hydr ogen-donating, ability of an amide proton. The structural variations were c haracterized via minimal gelation concentrations and times, X-ray crystallo graphy, light and electron microscopy, rheology, and calorimetry. The multi ple techniques, applied to the diverse compounds, allowed an extensive sear ch into the basis of gelation. It was learned, for example, that the compou nd with the lowest minimum gelator concentration and time also has one of t he weakest gels (i.e., it has a low elastic modulus). This is attributed to kinetic effects that perturb the length of the fibers. It was also argued that pi/pi stacking, the carboxyl carbonyl (but not the carboxyl proton), a nd solubility factors all contribute to the stability of a fiber. Polymorph ism also plays a role. Rheological studies at different temperatures show t hat certain gels are stable to a 1-Hz, 3-Pa oscillating shear stress at tem peratures as high as 90 degreesC. Other gels have a "catastrophic" break at lower temperatures. Calorimetric data indicate a smooth transition from ge l to sol as the temperature is increased. These and other issues are discus sed in this "anatomy" of a gel.