Synthesis of mycolic acid biosurfactants and their physical and surface-active properties

Abstract

Five mycolic acids [2-alkyl-3-hydroxy FA: R1C*H(OH)C*HR2COOH] were synthesized using acyl chlorides with alkyl chains of different lengths (total carbon numbers of mycolic acids, 12, 16, 20, 24, 36). The relationship between the chemical structures of the mycolic acids and their surface-active properties was determined. The acids were synthesized in three steps: (i) dimerization of acyl chloride into alkyl ketene dimer, (ii) selective reduction of C=C to C-C by hydrogenation, and (iii) beta-lactone ring cleavage under alkaline conditions. The yields of C-12-, C-16-, C-20-, C-24-, and C-36-mycolic acid were 72, 73, 73, 73, and 73%, respectively. The critical micelle concentrations (CMC) of C-12-, C-16-, and C-20-mycolic acid were 2.2 x 10(-4), 1.36 x 10(-4), and 7.4 x 10(-5) M, respectively. As the carbon number increased, the surface tension at the CMC value was also lower; the values for C-12-, C-16-, and C-20-mycolic acid were 46.54, 43.59, and 41.57 dyn/cm, respectively. The emulsifying activities of mycolic acids were determined for n-tetradecane, n-hexadecane, cyclohexane, and diesel oil. The results showed that C-12-mycolic acid was the best emulsifier for diesel oil, C-16-mycolic acid was the best emulsifier for n-tetradecane and n-hexadecane, and C-20-mycolic acid was the best emulsifier for cyclohexane. This study showed that mycolic acids having surface-active properties can be chemically synthesized for potential applications in the detergent/cleaning material industries, for example, in oil spill cleanup, oil recovery, textiles, pharmaceuticals, and cosmetics.