The possibilities and prospects of obtaining high-resolution information (below 30 A) on biological material using the electron microscope. Some comments and reports inspired by an EMBO workshop held at Gais, Switzerland, October 1973

Commercially available electron microscopes routinely provide resolution of some 2–4 Å, as determined on the spacing of crystalline lattices of certain stable, small-molecular substances. On biological material either macromolecules or macromolecular assemblies— ‘biologically significant’ details below some 20 Å have hitherto not been observed.we consider as ‘biologically significant’ those structural details observed or contained in electronmicrographs which are consistent with, or confirmed by, other data obtained from biochemical or functional experiments or by other physical methods (optical, magnetic, electric).

Chemical studies on an acriflavine-phosphotungstate complex; staining of phage deoxyribonucleic acid molecules for the electron microscope

Electrophoretic studies showed that the acriflavine-phosphotungstate (A-PTA) complex, which was formed by the reaction of acriflavine with phosphotungstic acid (PTA), contained a lower net positive charge than neutral acriflavine alone. Spectral studies from the titrations of specified aliquots of acriflavine with increasing volumes of PTA showed that the formation of A-PTA was characterized by a shift of the absorption peak of acriflavine from 452 mµ to a lower wavelength, at 446 mµ. The saturation of PTA binding sites on the acriflavine occurred when 3 molecules of dye reacted with 1 molecule of PTA. Binding curves of both neutral acriflavine and of A-PTA with "native" and with heat-denatured DNA showed shifts of λmax to longer wavelengths. A-PTA bound less native and less denatured DNA than the dye alone. The coupling of acriflavine with PTA caused a decrease in the net ability of the A-PTA complex to bind DNA. Double stranded DNA molecules from T3 bacteriophage were stained with the A-PTA complex for the electron microscope.

Autocorrelation functions of noisy electron micrographs of stained polynucleotide chains

Polyuridylic acid was deposited on carbon films in the presence of the thallous salt of mercury-p-dihydroquinone-O,O-diacetic acid under conditions leading to attachment of one of these molecules to each nucleotide. In electron micrographs of the stained polynucleotide chain, detail was obscured by the noise resulting from the substrate. Optically obtained autocorrelation functions of the distribution of density along the strands revealed peaks suggesting a structural regularity of 8 angstroms; this was interpreted as the internucleotide spacing.