The control of the configuration of nucleic acid molecules deposited for electron microscopy, by ionic bombardment of carbon films

Surface activation of carbon film supports for biological electron microscopy

The effect of glow-discharging on naked carbon-filmed grids has been evaluated. The effect of different locations of the grids within a DC discharge, operated in the normal glow, was analyzed by applying various biological specimens to the grids. Two locations were found to give consistent results: (a) in Crookes dark space, particulate specimens, negatively stained, spread evenly--suggesting a new negative surface charge of the support; (b) below the anode, nucleic acids, selectively (positively) stained, appeared as well spread filaments, indicating a net positive surface charge.

Adhesion of particulate specimens to support films for electron microscopy: a model system for assessing the surface properties of support films, and its application to chromatin particles

Silica microspheres bearing a known surface charge were used to test the adhesive properties of support films and support film treatments commonly used in the electron microscopy of particulate specimens. Adhesion was strongly correlated with surface charge, negatively charged microspheres binding well only to positively charged support films and vice versa in solutions of low ionic strength. This charge dependency could be overcome by increasing the ionic strength to about 100 mmol with monovalent cations; under these conditions, it was not necessary to provide an oppositely charged film surface to obtain adhesion. Chromatin particles (nucleosomes) which have a net negative charge, behaved very much like the negatively charged silica with respect to adhesion, confirming that the microspheres provided an accurate indication of support film surface properties. The chromatin particles showed dramatic structural changes under conditions when adhesion was either poor, or very strong, indicating the need for careful selection of binding conditions for delicate biological specimens. A new and simple method for pretreating carbon films to improve adhesion was developed, and a preliminary account of this technique is presented.

Localization and role of calcium in the erythrocyte coat: effects of enzymes and storage

The effects of various treatments on erythrocyte shape, surface, cell coat and calcium binding sites have been investigated by means of high voltage electron microscopy (HVM), scanning electron microscopy (SEM) and conventional electron microscopy (TEM). Papain caused the formation of small blisters within the cellular surface as well as crenation and 'budding' of the erythrocytes. After neuraminidase treatment, long filaments were observed to radiate from the surface of the erythrocyte. The other enzymes investigated, RNA'se DNA'se, phospholipase, protease and trypsin, produced no demonstrable effect on the cellular structure, nor (with the possible exception of trypsin) on the cell coat as seen by subsequent staining with ruthenium red. Putative calcium binding sites on and in the erythrocyte membrane were demonstrated. Following incubation with radioactive calcium, activity was found in the erythrocyte membranes. Calcium binding could be reduced by prior treatment of the erythrocyte with EDTA, neuraminidase, and to a lesser extent, by papain and trypsin. Other enzymes had no demonstrable effect. Stored erythrocytes showed a progressive diminution in calcium binding over a period of up to 4 weeks.

Adsorption of DNA molecules to different support films

Protein-free adsorption of the DNA of the Escherichia coli bacteriophage T7 to carbon, collodion, aluminium-beryllium and aluminium films was studied. It was found that the appearance of DNA strands depended greatly upon the kind of support film used. Direct adsorption of DNA to aluminium-beryllium or aluminium films yielded specimens with 'thin and long' and 'thick and short' regions along the strand. Well extended, uncoiled and unaggregated DNA molecules were obtained only when DNA was adsorbed to carbon, collodion or mica in the presence of intercalating dyes such as ethidium bromide. Adsorption properties of the different films are well correlated with their surface charge. Aluminium-beryllium films carry a strong positive surface charge, aluminium films a weak positive charge and carbon films a weak negative charge. It is suggested that for the preparation of specimens by spontaneous adsorption of well extended and unaggregated strands it is necessary that the DNA molecule is stiffened by a ligand such as an intercalating dye, and that the charge on the surface of the support film is opposite to the charge of the macromolecule.