Evidence from light-scattering studies for a dimeric structure for deoxyribonucleic acid in solution

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Abstract
Molecular wt. and radius of gyration of deoxyribonucleic acid prepared from herring sperm was measured by light scattering under a variety of conditions. In neutral soln. the molecular wt. of the best samples is near to 6 x 106, in good agreement with the results of Doty et al (1954) for calf thymus nucleic acid. On the addition of acid to pH 2.8 the coil decreased in size, although the molecular wt. remained the same. Very nearly the original radius of gyration was recovered on neutralization. On reducing the pH to 2.6 and below higher apparent molecular wts. indicating considerable aggregation were obtained. On reneutralization it was found that an irreversible decrease in size had occurred. The molecular wt. of nucleic acid exposed to pH 2.2 for 3 minutes before reneutralizing was reduced to 2.5 x 106. It is unlikely that hydrolysis has contributed much to this decrease. The addition of urea reduced the molecular weight to about 1/2 of its original value although the radius of gyration is slightly increased. On removal of the urea by dialysis there is only a small further drop in molecular weight but a marked increase in degree of coiling. Urea has only a small additional effect on the molecular weight of nucleic acid re-neutralized from pH 2.2. Results are consistent with the view that DNA is present in solution as a 2-stranded structure held together by hydrogen bonds which is dissociated when these are broken by ionization of amino groups or by addition of urea.