Macrocyclic lactone synthesis by lipases in water-in-oil microemulsions

Five microbial lipases from Chromobacterium viscosum, Candida cylindracea, Pseudomonas (source Fluka), Pseudomonas (source Genzyme) and lipoprotein lipase ex Microbial (Genzyme) have been screened for lactonisation activity towards 16-hydroxyhexadecanoic acid (HHA) in a variety of different w/o microemulsion systems. With the exception of Candida cylindracea (CC), all the lipases exhibited lactonisation activity although they were inherently more active in microemulsion systems based on the anionic surfactant sodium bis(2-ethylhexyl)sulphosuccinate (AOT) than in those based on the cationic surfactant cetyltrimethylammonium bromide (CTAB). Lactone yields are typically 50-60% and are markedly better than those reported previously using microemulsions in combination with chemical catalysts. Lipase stability is superior in the CTAB microemulsion systems, while lipase stability in the low water content AOT microemulsion systems was still good with the exception of CC lipase, which is rapidly inactivated. Buffering the water pools of AOT microemulsions using diglycine buffer at pH 8.0 improved biocatalyst stability. The lactonisation activity of lipases in CTAB w/o microemulsion systems compares favourably with that obtained using the same preparations as a solid suspension in the corresponding water-saturated organic solvent. In addition, the unusual solubility properties of microemulsions allowed the use of considerably higher concentrations of substrate in the microemulsion systems as compared to water-saturated organic solvents such as n-heptane. Lactone yields obtained at equivalent concentrations in the corresponding organic solvents containing conventional condensation catalysts were consistently measured at approx. 10%.