Experimental Correlation between Apparent pKa and Gelation Propensity in Amphiphilic Hydrogelators Derived from l-Dopa

We report the gelation propensity of three gelators derived from l-dihydroxyphenylalanine (l-Dopa), where the amino group is derivatized with three different fatty acids (lauric acid, palmitic acid, and azelaic acid). The long aliphatic side chains should introduce additional van der Waals interactions among the molecules, contributing to the self-assembly process. The hydrogels have been prepared with the pH change method, and both the hydrogels and the corresponding aerogels have been analyzed using several techniques. In any case, Lau-Dopa provides stronger hydrogels compared with the other gelators. This property may be ascribed to its tendency to efficiently form supramolecular beta-sheet structures, as outlined by the ECD, IR, and SEM analyses. Moreover, the preliminary measurement of the apparent pK(a) displays for Lau-Dopa two plateaux, as previously observed for, one at about pH 12 and a second one at pH 7.5. Thus, its pK(a) results in two apparent pK(a) shifts of similar to 8.5 and similar to 4 pH units above the theoretical pK(a), as a consequence of a multistep self-assembly pathway that correlates, in the final beta-sheet-based hydrogel, with a high degree of order and stability.