A Chemoenzymatic Approach to Stereospecific Green Synthesis of Rhamnolipid Biosurfactants

Abstract

Surfactants are an integral component of the modern industrial and consumer economies. They perform indispensable functions and yet their production and disposal pose grave environmental concerns. To address these concerns, work has been undertaken for several decades in the burgeoning field of biosurfactants – surfactants derived from biological sources. Of the biosurfactants, rhamnolipids are perhaps one of the most promising. They boast superb surfactant performance, are structurally simple, and are, most importantly, environmentally benign. Chemical approaches to the production of biosurfactants have been undertaken for a number of years with limited industrial applicability. Recent developments from the Pemberton lab, however, open new, industrially relevant synthetic pathways to rhamnolipids. Until this point, even these state-of-the-art production methodologies have lacked the ability to control for rhamnolipid stereochemistry. Natural products are given in high diastereomeric purity from their correspondent biological sources. The lack of stereochemical control results in either impure products or sacrifices yield, efficiency, and green indices if diastereomeric purity is achieved. To address this issue, the present work reports a chemoenzymatic approach to the stereospecific and green synthesis of rhamnolipids. Immobilized lipase B of Candida antarctica was used to provide enantiomerically pure starting materials for rhamnolipid synthesis. Optimal enzyme performance parameters such as time, temperature, conversion efficiency, etc. were quantitatively determined, primarily using spectroscopic methods. Diastereomerically pure rhamnolipid products were subsequently synthesized and characterized by standard methods. In addition, their interfacial properties were investigated using surface tensiometry with a du Noüy ring, and key parameters such as CMC, γCMC, and Γm were determined and/or calculated. The reported work demonstrates a novel advance in the field of biosurfactant production by chemical means, and a controlled synthesis that maintains green indices and is industrially relevant can now be achieved.