Synthesis and self-assembly of new quinuclidinolium-based surfactants: enhanced activity of cytochrome c and efficient antimicrobial activity.

Abstract

Bio-based surfactants are a great alternative to conventional surfactants owing to their low toxicity and minimal environmental impact. Herein, four new 3-quinuclidinol-based surfactants, namely, [C₁₆Qui][Br], [C₁₂Qui][Br], [C₁₂AQui][Br] and [C₈Qui][Br], have been synthesized and characterized for the first time. The thermal stability of all the surfactants at higher than 250 °C makes them suitable for high-temperature applications. The self-assembly behavior of these surfactants is thoroughly characterized using various techniques. The interactions between relatively hydrophobic bulky head groups, along with other hydrophobic interactions, drive micelle formation. Small-angle neutron scattering (SANS) measurements provide insights into the micellar structures of all surfactants: [C₁₆Qui][Br] assembles into spherical micelles, [C₁₂Qui][Br] and [C₁₂AQui][Br] form prolate ellipsoidal structures, and [C₈Qui][Br] arranges into loosely packed, random polymer-like micelles. The micellar structure of short-chain surfactants is solely driven by interactions between head groups, while more compact micellar structures are observed for long-chain surfactants. The presence of an amide functionality near the quinuclidinolium head group further governs the characteristics and properties of the micelles. The interlocking of head groups in the self-assembled structure and distinct structural arrangement of surfactant ions are studied through ¹H-NMR spectroscopy and 2D ¹H-¹H correlation and nuclear Overhauser effect spectroscopies (COSY and NOESY). The aqueous solutions of these surfactants are employed as a catalytic reactor to enhance the enzymatic activity of cytochrome c. The synergistic effect of hydrophobic and electrostatic interactions enhances the enzymatic activity in both monomeric and micellar forms. Interestingly, these surfactants exhibit good microbial inhibition depending upon their structural features and net charge. The present study provides additional understanding of the synthesis of bio-based surfactants and their utilization in diverse biocatalytic and medical applications.