Determination of fatty acid positions in native lipid A by positive and negative electrospray ionization mass spectrometry

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Abstract
Lipid A is the endotoxic principle of the lipopolysaccharide fraction from Gram‐negative bacteria. It is involved in the elicitation of cytokine production that leads to massive inflammation and to septic shock as a lethal consequence. For this reason, the structural elucidation of lipopolysaccharides from toxic Gram‐negative bacteria is an important and complicated task, mainly owing to its natural heterogeneity. Here, a new methodology to infer the distribution of the primary and secondary acyl residues is described, based on electrospray ionization mass spectrometry (ESI‐MS) of intact lipid A under high cone voltage in order to achieve in‐source fragmentation. Under these conditions, acyl fragmentation is induced and a different regioselective cleavage of secondary fatty acids is observed in positive and negative ESI‐MS, allowing the rapid identification of the lipid A structure. Copyright © 2004 John Wiley & Sons, Ltd.