New structural variants of homoserine lactones in bacteria.

N-Acylhomoserine lactones (AHLs) are used by a wide variety of bacteria for cell-cell communication in "quorum-sensing". These compounds are derived from L-homoserine lactone and a fatty acid, which varies in chain-length, degree of saturation, and the presence or absence of an oxygen atom at C-3. In this study we describe for the first time the occurrence of acyl chains carrying a methyl branch, and present a GC-MS-based method that can be used to distinguish these compounds from unbranched isomers. The bacterium Aeromonas culicicola produces several methyl branched AHLs. In Jannaschia helgolandensis--a marine bacterium of the Roseobacter clade--a doubly unsaturated AHL, (2E,9Z)-N-(2,9-hexadecadienoyl)-L-homoserine lactone, occurs. The location and configuration of the double bonds was proven by spectrometric investigation and synthesis. Finally, a method was developed to establish the absolute configuration of 3-hydroxyalkanoyl-HSLs by mild cleavage and chiral gas chromatography. The AHLs synthesized during this study were tested in sensor systems specific for certain AHL types. The results show that these compounds display varying responses to the respective sensors; this underlines the importance of determining the whole bouquet of AHLs and its function to fully understand their importance for regulatory functions in bacteria.