Abstract

AbstractThe adsorption at the air‐water interface of calf thymus H3‐DNA labeled in the dry state by the Wilzbach method was studied by measuring surface concentration, surface tension, and surface potential. It was found that, correspondingly to the behavior of E. coli H3‐DNA labeled by incorporation of thymine H3 and described in another paper, both the rate of adsorption and the amount of material adsorbed increased with increase in DNA concentration, salt concentration, or in the valency of the counterion. Surface pressure and potential did not change in the course of adsorption, and this is also in accordance with the properties found for E. coli DNA. However, while the surface concentration of the E. coli DNA corresponds approximately to monomolecular layer adsorption, the radiation from the adsorbed layer of the calf thymus H3‐DNA indicated apparently multilayer adsorption. On comparing the physical properties of the H3‐DNA labeled by the Wilzbach method to those of nonlabeled DNA, it is found that while the chemical composition and the bihelical structure is essentially maintained in the labeled material, exposure to tritium gas results in a reduction in molecular weight and produces random breaks in the strands of the bihelix. The H3‐DNA produced by Wilzbach's method is not labeled homogeneously. The more the molecule is exposed to the gaseous tritium, the more efficient is the isotopic, exchange and the greater the alteration in physical properties. The defects in the labeled H3‐DNA molecule make it more surface active, thus H3‐DNA of higher specific radioactivity concentrates at the interface, conveying the impression of multilayer formation although actually the adsorbed layer is approximately monomolecular.

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