Quantitative analyses of the isoforms of surfactin produced by Bacillus subtilis HSO 121 using GC-MS

Lipopeptide is one of the most important biosurfactants. The content of each isoform of a lipopeptide is crucial to the study of the properties of metabolites as well as the biological and biochemical characters of microbes. However, this information has not been accurately provided by the current analysis method. A new method for the quantitative determination of each isoform in the surfactin family has been established. The surfactin was firstly hydrolyzed in an acid solution at 90°C for 20 h, dried and then treated with bis(trimethylsilyl)trifluoroacetamide at 60°C for 20 min. The derived hydrolysates were then analyzed by GC-MS for a quantitative determination via comparison with working curves made with amino acids. GC-MS analyses show that the nine isoforms with an amount of 4.80 × 10(-7) mol (493 µg) in a 500 µg surfactin sample were detected, in which the mole fractions of surfactin isoforms with different fatty acid chains were 0.32% (n C(12)), 4.89% (iso C(13)), 6.27% (anteiso C(13)), 23.05% (iso C(14)), 8.95% (n C(14)), 17.69% (iso C(15)), 38.69% (anteiso C(15)), 0.07% (iso C(16)), and 0.07% (n C(16)), respectively. This approach can be applied to quantitative analyses for other families of lipopeptides as long as the sequence of amino acid residues in the peptide is determined.