The Phase Transition Behaviour Of Multibilayer Preparations Of Free Lipid A, The Endotoxic Principle Of The Outer Membrane Of Gram-Negative Bacteria: A FT-IR Study

FT-IR spectra of free lipid A, isolated from Escherichia coli containing C-14 and C-12 acyl chains have been recorded and tentative band assignments were undertaken. The highly complex phase transition behaviour of fully hydrated multibilayer preparations of lipid A has been investigated using temperature-induced frequency shifts, difference spectra and second derivatives of >CH2 asymmetric and symmetric stretching modes near 2918 and 2850 cm -1, respectively. Three "phase transitions" near 22, 34 and 41oC could be detected, the latter being the main transition and - most probably - the familiar B-u-transition. The high value of 41°C found for the main transition temperature may reflect the stability and permeation barrier function of this molecule within the outer membrane of gram-negative bacteria.