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The focus of this review is the application of mass spectrometry to the structural characterization of bacterial lipopolysaccharides (LPSs), also referred to as "endotoxins," because they elicit the strong immune response in infected organisms. Recently, a wide variety of MS-based applications have been implemented to the structure elucidation of LPS. Methodological improvements, as well as on- and off-line separation procedures, proved the versatility of mass spectrometry to study complex LPS mixtures. Special attention is given in the review to the tandem mass spectrometric methods and protocols for the analyses of lipid A, the endotoxic principle of LPS. We compare and evaluate the different ionization techniques (MALDI, ESI) in view of their use in intact R- and S-type LPS and lipid A studies. Methods for sample preparation of LPS prior to mass spectrometric analysis are also described. The direct identification of intrinsic heterogeneities of most intact LPS and lipid A preparations is a particular challenge, for which separation techniques (e.g., TLC, slab-PAGE, CE, GC, HPLC) combined with mass spectrometry are often necessary. A brief summary of these combined methodologies to profile LPS molecular species is provided.

The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H 2 pyr 2 en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H 2 pyr 2 en with NaBH 4 yielded the reduced Schiff base N,N'-ethylenebis-(pyridoxylaminato) (H 2 Rpyr 2 en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H 6 L 4 + , and all protonation constants were determined by pH-potentiometric and Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. Some additional X-ray data for 1, 2, 4, and 9 (SI-1); further discussion of IR spectra (

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