Osmium-labeled polysaccharides for atomic microscopy

With the objective of localizing cell surface polysaccharides, the reaction of several osmium (VI)-ligand complexes with glycols has been applied to sugar residues in mono- and polysaccharides. The hydrophilic ligands 4,4′-dicarboxy-2,2′-bipyridine and N,N,N′,N′-tetramethylethylenediamine have been employed to produce water-soluble osmate esters of the sugar glycols. Methyl glycosides react with osmium (VI) reagents to give stable products containing one osmium atom per sugar. α-Cyclodextrins, with six glucose residues in a ring, and β-cyclodextrins with seven, can bind up to, but not exceeding three osmium ligand complexes per molecule, indicating possible nearest-neighbor exclusion of reaction among the residues. Reaction of as little as 1% of the sugar residues in amylose with the dicarboxybipyridyl osmate complex allows the amylose strands to be extended on a grid for electron microscopy. Further reaction with the tetramethylethylenediamine osmate complex rapidly saturates the amylose to a level of 0.39 osmium atom per sugar residue, consistent with the nearest-neighbor exclusion hypothesis. High resolution scanning transmission electron microscopy images reveal a row of osmium atoms along each amylose strand.