Structural studies on the phosphate-free lipid A of Rhodomicrobium vannielii ATCC 17100

The structure of the free lipid A from Rhodomicrobium vannielii ATCC 17100 was elucidated. It consists of a central beta-1',6-linked glucosamine disaccharide which is not substituted by phosphate. About 30% of the disaccharide molecules are substituted with mannopyranose in beta-1,4'-linkage to the non-reducing glucosamine. The reducing glucosamine can be directly reduced with NaBH4, indicating either that this glucosamine is not substituted at C1 or its substituent has been removed during the preparation of free lipid A or is removed during reduction with NaBH4. The following formula shows the 'backbone' structure of the free lipid A from Rm. vannielii ATCC 17100: beta-Manp(1-- leads to 4)-beta-GlcpN(1 leads to 6)GlcpN. 3-(R)-Hydroxyhexadecanoic acid is linked to the amino group of the reducing glucosamine. The residue at the amino group of the non-reducing glucosamine has not been identified. The hydroxyl groups of the central disaccharide are acylated with 3-(tetradecanoyloxy)-tetradecanoic acid, 3-hydroxytetradecanoic acid, delta 14-docosenoic acid (delta 14-C22:1) and acetyl groups. The hydroxyl groups of the mannose are not substituted.