DIFFUSION COEFFICIENT OF DNA IN SOLUTION AT ZERO CONCENTRATION AS MEASURED BY ELECTRON MICROSCOPY

A theoretical analysis is given, supported by a model experiment, of diffusioncontrolled adsorption of DNA from solution onto a protein monolayer as a function of diffusion time. The diffusion coefficients of intact DNA molecules from bacteriophages T3 and χ were calculated from adsorption kinetics involving electron microscope count of individual DNA molecules adsorbed from 0.20 m-ammonium acetate onto a cytochrome c surface film. The diffusion coefficients were found to be independent of concentration between 3 × 10-9 and 2 × 10-7 g/cm3, concentrations where DNA molecules of this size do not interact significantly. Hence, an extrapolation to zero concentration is unnecessary. The type of kinetics observed is evidence that the adsorption is irreversible. It also provides an independent estimate of the average coil diameter of T3 DNA (3 μ), in agreement with previous electron microscope measurements (2.8 μ). The diffusion coefficients found are, respectively, 19 and 38% smaller than values calculated from the Svedberg equation, combined with an empirical formula (Crothers & Zimm, 1965) for the sedimentation coefficient of DNA extrapolated to zero concentration. Assuming that this combination gives true diffusion coefficients, then deviations from it found by us are explained by a linear density of DNA, bound to a cytochrome c film, which is about 15% higher as compared to the B-configuration in solution. © 1968.