Breakage by hydrodynamic shear of the bonds between cohered ends of λ‐DNA molecules

The rate of breakage by hydrodynamic shear of the cohered ends of λ‐DNA molecules has been observed for the circular monomers, joined half molecules, and joined quarter molecules, in a capillary apparatus with known flow parameters. The rate constant for breakage has been measured as a function of shear stress, temperature, ionic strength, and molecular length. There is a large temperature coefficient, with an activation energy of 120 ± 20 kcal./mole. The values of d ln k/dG , where k is the rate constant for breaking and G is shear gradient, in aqueous solution at 25°C. are about 3.8 ± 0.3 × 10 −4 see. The shear stresses needed for breakage of joined quarter molecules and of circular monomers, respectively, are about equal, and about half that needed for breakage of joined half molecules. The rate of breakage at a given shear stress increases with decreasing ionic strength, approximately as [Na + ] −1.6 . Self‐protection effects are not observed for opening of circular monomers at a DNA concentration of 5 μg./ml. but are observed for breakage of joined half molecules at concentrations down to 0.5 μg./ml. The large temperature coefficient which is approximately equal to that of the thermal dissociation of the cohered ends is interpreted to mean that shear breakage is a mechanically assisted thermal reaction in which the thermal fluctuations provide most of the free energy of activation for breakage. A detailed model for this interpretation is presented. The self‐protection effect implies that those molecules which break are not average molecules but exceptional ones which, due to some fluctuation, are more fully extended in the flow field.