Whole cell matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and in situ structure analysis of streptocidins, a family of tyrocidine-like cyclic peptides

Streptocidins, a family of tyrocidine-like cyclic decapeptides, are an ideal demonstration object for the detection and in situ structure analysis of natural compounds directly in microbial cells using whole cell matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOFMS), an emerging technique that can be used for rapid sensitive metabolic profiling of microorganisms. Five main members of the streptocidin family (A-E) were detected in Brevibacillus cells picked from agar plates and identified by in situ structure analysis with post-source decay MALDI-TOFMS. This efficient modern method allows the precise detection of metabolites within minutes without the need to isolate and purify the target compounds. The generated mass spectra are of similar quality to those obtained for the purified peptides. In addition, surface extracts were prepared by treating Brevibacillus cells with 70% acetonitrile in the presence of 0.1% trifluoroacetic acid and fractionated by high-resolution reversed-phase high-performance liquid chromatography (HPLC). In this way ten minor streptocidins were detected demonstrating the full biosynthetic variety of streptocidin production on the cellular level. The streptocidins differ from the well-known tyrocidines essentially in position 3 of the decapeptide chain by replacement of the aromatic amino acid (F/W) found in tyrocidines by L-leucine or L-valine.