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On the cross-section Structure of deoxyribonucleic acid in solution

1971; Elsevier BV; Volume: 55; Issue: 3 Linguagem: Inglês

10.1016/0022-2836(71)90320-2

1089-8638

Stanley Bram, W. W. Beeman,

DNA and Nucleic Acid Chemistry

We present X-ray scattering data on dilute solutions of calf thymus DNA in a number of monovalent chlorides, particularly NaCl and CsCl. Salt concentrations are from 0.05 to 1.0 m. The data include a range of small angles where the scattered flux is sensitive to the cross-section parameters of the molecule. Our measured mass per unit length for NaDNA is within 2 or 3% of the theoretical value for the B form. For CsDNA our measurement is about 9% below theory but within experimental error. At larger angles, where internal periodicities dominate the scattering, the data on NaDNA are in much better agreement with the B than with the A form. We observe two cross-section radii of gyration, a result characteristic of a cylindrical core of high electron density surrounded by a shell of much lower electron density. The larger radius of gyration is that of the entire structure, the smaller characterizes the core. Our results imply that the core is ionized or partially ionized DNA and that the shell is a Debye-Hückel layer of counterions and perhaps electrostricted solvent about 20Åthick. The experimental data and the model are not accurate enough to decide what fraction of the counterions should be considered site bound to the DNA although there is some evidence to indicate that site binding, if it exists, is not large.