DIFFERENTIAL SCANNING CALORIMETRY INVESTIGATIONS ON LPS AND FREE LIPIDS-A OF THE BACTERIAL-CELL WALL

Differential scanning calorimetry (DSC) was used to investigate the thermal stability and behaviour of the lipopolysaccharides (LPS) of the outer membrane of Gram-negative bacteria and their lipid portion. DSC curves of LPS show thermal features between 200 and 129 degrees C (depolymerization) and between - 13 and - 36 degrees C (cooling phase transition). Both effects were related to the relative strength of the linkage types in the O-chain structure and to their capacity for intermolecular hydrogen bonding. DSC curves of lipids A show endotherm peaks between 40 and 24 degrees C, around 15 degrees C and between - 23 and - 4 degrees C. Based on these effects, strong differences in thermal behaviour can be observed between that of Brucella and Vibrio cholerae on the one hand, and that of Escherichia coli and Shigella flexneri on the other. Fluidity of the acyl chains and lyotropism, which are important parameters in expression of biological activities, are discussed using the above data. To explain some properties. fluidity could be related to the temperature of the gel <----> liquid crystalline (beta <----> alpha) phase transition. which occurs at physiological temperature. Nevertheless. fluidity could be related to temperature of the previous thermal effect (between 6 and 20 degrees C), for which a softening or partial melting of the sample has been evidenced. The thermal effect measured between - 23 degrees C and - 4 degrees C indicates a greatly reduced water concentration of lipid A from Brucella, thus explaining its early fusion process and its activity by means of hydrophobic interactions.