Phase behaviour of cord factor and related bacterial glycolipid toxins. A monolayer study.

C-6 esters of methyl alpha-D-glucoside and C-6, C-6' 'diesters of alpha, alpha'-D-trehalose with C18 and C32 threo and erythro mycolic acids (from chemical source) and of C80-erythro-mycolic acid (from natural source) have been synthesized. Esters of a C32 deoxy analogue were prepared as well. Throughout a monolayer study at the air-water interface, these glycolipids are shown to form well organized phases in which the two hydrocarbon chains of mycoloyl residues must be in interaction. Compression isotherms of C32 esters suggested a transition between liquid-expanded and liquid-condensed states. Latent heats Qc and entropy changes delta S associated with these phase transitions as well as the critical temperature at which they occur have been measured. Within the monolayer, the molecular packing of these glycolipids depends on the presence of the hydroxyl group of mycoloyl residues and on its stereochemistry. In particular intermolecular hydrogen bonds between these groups are postulated in the case of the bis(C32-erythro-mycoloyl)-trehalose. On the other hand, short chain C18 esters form fluid phases (t greater than 10 degrees C) whereas very long chain C80 mycoloyl esters of trehalose exist in a condensed state (t = 20 degrees C). These glycolipids were found to interact strongly with dipalmitoylphosphatidylcholine and egg yolk lecithins (3-sn-phosphatidylcholine). Their phase behaviours are discussed in connection with hypotheses concerning the way they can interact with mitochondrial membranes.