A COMMENT ON THE PREPARATION OF LIPOSOMES FROM AND ON THE BETA[--]ALPHA ACYL CHAIN MELTING BEHAVIOR OF ROUGH MUTANT LIPOPOLYSACCHARIDE

We would like to comment on the investigations of Vaara, M., Plachy, W.Z. and Nikaido, H. (Vaara, M. et al. (1990) Biochim. Biophys. Acta 1024, 152-158) on the partitioning of hydrophobic probes in lipopolysaccharide bilayers. These authors reported that they did not succeed in preparing closed vesicles (liposomes) from rough mutant lipopolysaccharide. We describe the conditions under which lipopolysaccharide liposomes are formed most readily. We, furthermore, summarize data which strongly support the existence of thermotropic phase transitions of lipopolysaccharides (with transition temperatures lying in the range of 30-36-degrees-C) contradictory to Vaara et al. who argue that such transitions are artefacts. Exemplary measurements of the beta <--> alpha acyl chain melting for lipopolysaccharide from Escherichia coli deep rough mutant (strain F515) as compared to synthetic and natural phospholipids are presented using fluorescence spectroscopy, Fourier-transform infrared spectroscopy and differential scanning calorimetry. These results unequivocally prove the necessity to perform experiments at 37-degrees-C for a determination of the outer membrane permeability under physiological conditions.