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

Differential scanning calorimetry (DSC) was used to investigate the th ermal 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 (dep olymerization) and between - 13 and - 36 degrees C (cooling phase tran sition). Both effects were related to the relative strength of the lin kage types in the O-chain structure and to their capacity for intermol ecular hydrogen bonding. DSC curves of lipids A show endotherm peaks b etween 40 and 24 degrees C, around 15 degrees C and between - 23 and - 4 degrees C. Based on these effects, strong differences in thermal be haviour can be observed between that of Brucella and Vibrio cholerae o n the one hand, and that of Escherichia coli and Shigella flexneri on the other. Fluidity of the acyl chains and lyotropism, which are impor tant parameters in expression of biological activities, are discussed using the above data. To explain some properties. fluidity could be re lated to the temperature of the gel <----> liquid crystalline (beta <- ---> alpha) phase transition. which occurs at physiological temperatur e. Nevertheless. fluidity could be related to temperature of the previ ous thermal effect (between 6 and 20 degrees C), for which a softening or partial melting of the sample has been evidenced. The thermal effe ct measured between - 23 degrees C and - 4 degrees C indicates a great ly reduced water concentration of lipid A from Brucella, thus explaini ng its early fusion process and its activity by means of hydrophobic i nteractions.