Pyrolysis field ionization mass spectrometry of cell wall components and bacterial cell walls

Abstract

Bacterial cell walls and their building blocks have been investigated using thermally programmed pyrolysis at 50–500°C, followed by field ionization mass spectrometry of their pyrolysates. Spectra of selected model compounds, namely N-acetylglucosamine, N-acetylmuramic acid, N-acetylmuramyl- l-alanyl- d-isoglutamine and murein (peptidoglycan), isolated from the cell walls of Bacillus subtilis var. nig. WM were compared with the spectrum of the cell walls of this microorganism. The cell wall building blocks N-acetylmuramic acid and N-acetylmuramyl- l-alanyl- d-isoglutamine show pyrolysis products identical with N-acetylglucosamine, as the O-glycosidic bonds of the amino sugar derivatives are thermally very labile. In general, products at m/z 203, 185 and 167, generated by water elimination, as well as the characteristic peak at m/z 125 are observed. In addition, some peaks that originate from the O-glycosidic side-chain are registered. The murein, however, does not show any large, significant pyrolysis fragments from the peptide side-chains, owing to their strong cross-linkages. The pyrolysis of this biopolymer yields, in principle, only peaks characteristic of aminopolysaccharides: m/z 83, 85, 95, 97, 109, 111, 123, 125, 137, 139, 151, 167, 185 and 203. The peaks in the lower mass region are generally difficult to assign to a particular structure, such as an amino sugar. For a first analytical evaluation of the method, for instance for taxonomy problems, spectra obtained from the well described cell walls of Bacillus subtilis were also checked with the spectrum of cell walls of a yet unknown composition ( Propioni shermanii). The pyrolysis spectrum of cell walls of Bacillus subtilis is very similar to that of murein and shows that it consists mainly of amino sugars. In contrast, the spectrum of cell walls from Propioni shermanii shows strong signals indicating neutral sugars: m/z 98, 110, 112, 126, 144 and 162.