A study of positive staining for electron microscopy using collagen as a model system—II. Staining by uranyl ions

Abstract The staining behaviour of uranyl salts was studied using reconstituted periodic-banded collagen fibrils as a model system and comparing electron-optical data and collagen sequence data by a computer-aided correlation procedure. Because it is only weakly dissociated and a variety of ionic uranyl complexes (cationic, neutral and anionic) coexist in aqueous solution, uranyl acetate stains not only negatively-charged but also positively-charged side-chains on the collagen. The relative uptake on charged groups of opposite sign depends on several factors, including the concentration and pH of the staining solution and the presence of extraneous phosphate groups (from the buffer used to reconstitute the fibrils), but no conditions could be found which allowed uranyl acetate to stain negative charges only. When the stain was uranyl nitrate, known to dissociate strongly in aqueous solution, uptake of uranyl ions was predominantly (although not exclusively) on negative charges on the collagen, provided that extraneous phosphate had previously been removed by dialysis.