Self-assembly of the sodium salts of fatty acids into limpid hydrogels through non-covalent interactions with peptides

It is crucial for hydrogel preparation to find appropriate gelators. Fatty acids are widely distributed in nature and thus their utilization has received much attention. In this study we demonstrated that poly(α,L-lysine) with a polymerization degree of 5 (PLL5) can bind to the sodium salts of fatty acids (SFA) with intermediate sizes, such as sodium laurate (SL), through non-covalent interactions. Such bindings resulted in SL polymerization. TEM analyses revealed that long, thin fibers initially formed upon mixing PLL5 with SL, becoming thick and entangled after 12 h, and finally leading to the formation of limpid hydrogels. In contrast, other SFA could not form such hydrogels under the same experimental conditions. Except for PLL5, PLL7 also induced SL self-assembly into similar limpid hydrogels, while its other analogues with smaller or larger sizes did not. Thus, the formation of the limpid hydrogel presented here exhibited high selectivity for the size of both SFA and poly(α,L-lysine). These findings might be beneficial for the application of fatty acids in industry.