Structure and metal-binding properties of lipopolysaccharides from heptoseless mutants of Escherichia coli studied by ¹³C and ³¹P nuclear magnetic resonance

The structure and metal-binding properties of lipopolysaccharides (LPS) from heptoseless mutants of Escherichia coli were studied by ¹³C and ³¹P NMR techniques. Carbon-13 NMR spectra were used to determine the linkages and configurations of the saccharide backbone and the types and locations of fatty acyl groups in E. coli LPS. Resonance assignments for native LPS were made by chemical shift correlation with model compounds, deacylated LPS, lipid A, deacylated lipid A, and fatty acids released from LPS by mild alkaline hydrolysis. The 3-deoxy-D-manno-octulosonate (KDO) dissacharide was tentatively assigned the structure KDOα2→5KDOα2→. The presence of amide- and ester-linked 3-hydroxy and 3-acyloxy fatty acids in native LPS was confirmed directly from the ¹³C spectrum and evidence is presented for a labile acyl ester at C-3' (GlcN(II)) of the lipid A moiety. A significant finding was that the KDO disaccharide is linked to the C-6' position of the lipid A moiety, rather than C-3', as previously reported. The effects of binding Ca²⁺, Cd²⁺, Yb³⁺, Gd³⁺, and La³⁺ on the ³¹P NMR spectrum of LPS indicated that the glycosidic diphosphate moiety participates in a high affinity metal-binding site.