Bioengineering of emulsifier structure: emulsan analogs

Strategies were investigated to modulate the side chain structure of emulsans formed by Acinetobacter calcoaceticus RAG-1. Analysis of emulsan fatty acid side chain groups by gas chromatography--mass spectrometry (GC-MS) revealed that by provoking the exogenous n-alkanoic fatty acids 15:0, 16:0, and 17:0, emulsan analogs were formed with 53, 46, and 44 mol%, respectively, of fatty acid substituents with chain lengths equal to that of the carbon source. In contrast, the increase in emulsan fatty acids of chain lengths less than 15 or greater than 17 by providing corresponding shorter and longer chain length fatty acids as carbon sources was not substantial. When [1-13C]-labeled (99% enriched) palmitic acid was used as a carbon source along with acetate, analysis of M/z 75/74 and 87/88 isotopomer ratios by GC-MS indicated that 84 and 86% of the 16:0 (9-cis) side groups, respectively, were incorporated intact from the 16:0 carbon source. The percentage of 14- 15-, 16-, 17-, and 18-carbon chain length fatty acid esters that were monounsaturated were 11, 26, 50, 70, and 85% respectively. Based on the observed percentage of unsaturated chain length dependence and almost identical enrichment at C-1 of 16:0 and 16:1 (9-cis) side groups from [1-13 C]-labeled experiments, it was concluded that desaturation of preformed n-alkanoic acids was the predominant mechanism of their formation. Further work established correlations between side chain structure and product emulsification specificity/activity, so that bioengineered emulsans with improved selectivity can now be formed.