The Use of Novel Supporting Films for the Electron Microscopy of Deoxyribosenucleic Acid and Other Macromolecules

Earlier attempts at studying with the electron microscope the long chain polar polymers, of which deoxyribonucleic acid (DNA) is probably the most important example, appear to have been frustrated for want of a suitable substrate. On cast films of the conventional materials, formvar and nitro-cellulose, the greater part of the DNA is gathered together in a mass at the centre of the drop as it dries. The effect is probably due to the hydro-phobic character of the supporting film and attention has therefore been directed to the use of more hydrophilic surfaces. Hall (1) has avoided this difficulty by spraying the DNA solution on to the very hydrophilic surface of mica which is subsequently shadowed and then replicated by a stripping technique. The method we wish to report consists of the use of a specially prepared polymer supporting film which, though it can be cast from chloroform, is both hydrophilic and basic. Therefore, the drop of DNA solution wets the film and the basic charge of the film attracts the negatively charged DNA macromolecules in the drop. The particular polymer used for the supporting film is a co-polymer of styrene and 2-vinyl pyridine. The pyridine group is slightly ionised in water and confers a small degree of basicity to the film while still being soluble in chloroform. The styrene is used as a diluent because pure polyvinyl pyridine is too water-soluble and its films adhere to glass too strongly to be stripped. Since this pair of monomers have very similar reactivity ratios, the polymer formed has practically the same composition as the monomer mixture, and can therefore be readily made in a wide range of compositions. The co-polymerisation is carried out at 40°C., with 0.1 per cent benzoyl peroxide as initiator. Because the polymerisation of vinyl pyridine is very sensitive to oxygen, the monomer must be freshly redistilled and oxygen must be rigourously excluded during polymerisation (2). A solvent is unnecessary during polymerisation, although tolu-ene may be used. The product is dissolved in chloroform and purified by precipitation with ether. A co-polymer containing 40 per cent vinyl pyridine has been found to have the most useful properties for studying proteins and nucleic acids. Supporting films are prepared by casting on glass from a 0.1 per cent solution of the polymer in chloroform. Although the film adheres more strongly and is mechanically less strong than formvar, it is possible to float …