THE AQUEOUS-SOLUTION STRUCTURE OF A LIPOTEICHOIC ACID FROM STREPTOCOCCUS-PNEUMONIAE STRAIN R6 CONTAINING 2,4-DIAMINO-2,4,6-TRIDEOXY-GALACTOSE - EVIDENCE FOR CONFORMATIONAL MOBILITY OF THE GALACTOPYRANOSE RING

The 2D-NOESY spectra for the per-N-acetylated and the native lipoteich oic acid (LTA) oligomer from Streptococcus pneumoniae strain R6 clearl y indicate a difference in conformation of the 2,4,6-trideoxy-galactop yranose ring. Whereas the 2,4-N-acetylated Gal24N adopts the usual C-4 (1) chair conformation, the native 2-N-acetyl-4-amino Gal24N exhibits conformational mobility with comparable populations in the C-4(1) chai r and S-5(1) skew conformations, as determined using MD simulation for the partial trisaccharide Me-beta-D-Glc6P-(1 --> 3)-alpha-D-Gal24N-[6 -PC]-(1 --> 4)-alpha-D-GalNAc and from the intra-ring NOE effects. P-3 1-NMR spectra point to a strong electrostatic or hydrogen-bonding inte raction between the free 4-NH2 group on the Gal24N and the negatively charged diester phosphate group between adjacent pentasaccharide repea ting-units [Ribitol-(5 --> 6)-beta-D-Glc6P]. Molecular modelling and M D simulation experiments confirmed that such an interaction was feasib le with the Gal24N galactopyranose ring in the inverted B-1,B-4 or ske w S-5(1) conformation.