Structure of the Escherichia coli K2 capsular antigen. Stereochemical configuration of the glycerophosphate and distribution of galactopyranosyl and galactofuranosyl residues.

The Escherichia coli K2 capsular antigen is known to be composed of alpha-D-galactopyranosyl(1--2)glycerophosphate and alpha-D-galactofuranosyl(1--2)glycerophosphate units which are connected by phosphodiester bonds to C-4 of the galactopyranosyl and C-5 or C-6 of the galactofuranosyl moieties. In the present study the glycerophosphates were released by two different procedures and shown to have the sn-glycero-3-phosphate stereochemical configuration. In the first, the chain was fragmented by Smith degradation to glycerophosphothreitol from which the glycerophosphate was released by alkali hydrolysis. The structure-dependent low recovery of alpha-glycerophosphate (less than 10%) initiated the development of another degradative sequence which consisted of periodate oxidation, beta elimination, hydrazinolysis, and alkaline treatment. This way, approximately 90% of the glycerophosphate was released as sn-glycero-3-phosphate. beta elimination revealed in addition that most of the galactofuranosyl residues carry the phosphodiester bond at position 5. Separation by gel permeation chromatography and analysis of the fragments obtained by beta elimination showed that pyranosidic and furanosidic galactosyl residues alternate in the same chain and suggested the sequences Galf(p)GroP-(GalpGroP)n-Galf- and -GalfGroP-(GalpGroP)n-Galf-, where n is 6, 4, and 3, respectively.