Abstract
Mixtures of dimycolates of alpha-D-trehalose (cord factor) and monomycolates have been isolated from Mycobacterium phlei and separated as trimethylsilyl derivatives according to the polarity of the fatty acid residues. The free glycolipids can be recovered by mild hydrolysis. Silylation and disilylation reactions did not induce any isomerisation. The structure of these trehalose esters has been determined by using a series of reactions elaborated on synthetic acyl-sugar models. Free hydroxyl groups are transformed into tetrahydropyran ethers, deacylated by dimsyl sodium, methylated and the sugar derivatives are hydrolyzed. The O-methyl-sugars obtained contain a methyl ether group located at the position where the acyl group was present initially. Identification by gas chromatography of the O-methyl-sugars thus allows the location of the fatty acid residues in the glycolipid. It has been demonstrated that no migration occurs. Two types of 6-monomycoloyl-alpha-D-trehalose have been isolated, differing by the nature of the mycolic acid residues. One of them, called MA, contains "alpha-phlei mycolic acid". The other one, called MB, contains "gamma-phlei mycolic acid" which is the ester of 2-eicosanol with the omega-carboxyl function of a dicarboxylic mycolic acid. Three types of 6,6'-dimycoloyl-alpha-D-trehalose (cord factor) have been obtained. Two of them are symmetrical diesters, containing either two residues of alpha-mycolic acid, or two residues of gamma-mycolic acid. The last one is an unsymmetrical diester, the hydrolysis of which gives one mole of alpha-mycolic acid and one mole of gamma-mycolic acid. The ratio of the different diesters in the cord factor fraction might be explained by some equilibrium between the different 6-ester groups or by a transformation in situ of one species into another one by biochemical modification of the mycolic acid residues.
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