Dissociation of Deoxyribonucleic Acid by Urea

WE found 1 that the molecular weight of deoxyribonucleic acid from herring sperm heads, measured by light-scattering in dilute aqueous solution, was reduced from 6 × 10 6 to 3 × 10 6 on adding a concentrated urea solution to make the final urea concentration 4 M . This halving of the molecular weight was accompanied by a 20 per cent increase in the radius of gyration of the coil. This behaviour is consistent with the twin spiral model of Crick and Watson 2 if it is assumed that the urea splits the hydrogen bonds holding the two molecules together. After removal of the urea by dialysis the ‘split’ molecule coils up into a much more compact shape 1 ; if the dialysis is conducted from solutions containing less than 0.003 per cent deoxyribonucleic acid, the molecular weight remains unchanged at 3 × 10 6 ; in more concentrated solutions aggregation occurs and molecular weights ranging up to 2 × 10 7 have been measured.