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

SUMMARY DNA molecules bind to carbon films placed on the surface of aqueous solutions and are extended and oriented by the flow forces on removal from the surface. The number of molecules bound increases with ionic strength suggesting that the bonding is hydrophobic. If carbon films are subjected to ionic bombardment in a flow discharge, they can be modified so that molecules bind strongly, and are deposited in a tangled, random‐coil configuration, uninfluenced by flow forces. The strength of binding is decreased by reducing the discharge time and current, and by addition of salt (NaCl), suggesting that the bonding is ionic. When weakly bound, the molecules are condensed by ethanol. The strength of binding can be adjusted so that the molecules are largely untangled by flow, and in a form suitable for the mapping of bound enzymes, or other identifiable markers. Aggregation and extension of molecules, that occur on unmodified films, and the binding of cytochrome c, that occurs in methods based on that of Kleinschmidt & Zahn (1959) are avoided. Both double and single‐stranded molecules can be deposited and seen after rotary shadowing with Pt. The discharge can be performed in a vacuum produced by a standard oil‐filled rotary pump.