Identification of amino acid substitutions in the lipopeptide surfactin using 2D NMR spectroscopy

It is generally accepted that surfactin, being produced by various Bacillus subtilis strains, is a cyclic lipopeptide built from the heptapeptide L-Glu-L-Leu-D-Leu-L-Val-L-Asp-D-Leu-L-Leu and a β-hydroxy fatty acid with variable chain length of 13 – 15 carbon atoms. We investigated surfactin from Bacillus subtilis ATCC 21332 and OKB 105, dissolved in pyridine and methanol, with two-dimensional H NMR spectroscopy. In the NH-fingerprint region, 21 well resolved cross peaks are observed instead of the expected seven cross peaks for the given heptapeptide. We were able to assign all proton signals to 21 amino acids, to identify three heptapeptides, and thus to prove the existence of structural analogues of surfactin. In the major fraction A, the peptide sequence is as given above. In fractions B and C, the C-terminal leucine is replaced by valine and isoleucine, respectively.