Adsorption of mono- and multivalent cat- and anions on DNA molecules

Adsorption of monovalent and multivalent cat- and anions on a deoxyribose nucleic acid (DNA) molecule from a salt solution is investigated by computer simulation. The ions are modelled as charged hard spheres, the DNA molecule as a point charge pattern following the double-helical phosphate strands. The geometrical shape of the DNA molecules is modelled on different levels ranging from a simple cylindrical shape to structured models which include the major and minor grooves between the phosphate strands. The densities of the ions adsorbed on the phosphate strands, in the major and in the minor grooves are calculated. First, we find that the adsorption pattern on the DNA surface depends strongly on its geometrical shape: counterions adsorb preferentially along the phosphate strands for a cylindrical model shape, but in the minor groove for a geometrically structured model. Second, we find that an addition of monovalent salt ions results in an increase of the charge density in the minor groove while the total charge density of ions adsorbed in the major groove stays unchanged. The adsorbed ion densities are highly structured along the minor groove while they are almost smeared along the major groove. Furthermore, for a fixed amount of added salt, the major groove cationic charge is independent on the counterion valency. For increasing salt concentration the major groove is neutralized while the total charge adsorbed in the minor groove is constant. DNA overcharging is detected for multivalent salt. Simulations for a larger ion radii, which mimic the effect of the ion hydration, indicate an increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure

Reevaluation of ENDL of sigma(n,f) and anti nu/sub p/ for /sup 235/U and /sup 239/Pu from 100 keV to 20 MeV

Reevaluations of the neutron-induced fission cross sections from 100 keV to 20 MeV and anti nu/sub p/(E) from 0 to 20 MeV for /sup 235/U and /sup 239/Pu have been completed and entered into the Livermore Evaluated Nuclear Data Library (ENDL). For /sup 235/U(n,f) the recent evaluation of W.P. Poenitz of Argonne National Laboratory has been adopted as the /sup 235/U(n,f) ENDL standard. For /sup 239/Pu(n,f) the /sup 239/Pu//sup 235/U fission cross section ratio measurement of Carlson and Behrens of LLNL, which has served as the basis for the /sup 239/Pu(n,f) ENDL evaluation to the present, is compared with several new ratio measurements for the new evaluation. For anti nu/sub p/(E) ratio values of Soleihac, et al. were renormalized to the currently accepted value of /sup 252/Cf anti nu/sub sf/ and used as the basis for the present ENDL evaluation. Experimental anti nu/sub p/(E) values measured by Gwin, et al. in 1978 from 0.5 keV to 10 MeV are reasonably consistent with the values of Soleihac, et al. Results of criticality calculations using the new evaluated cross sections and anti nu/sub p/(E) are compared for various critical mass assemblies. Also, Monte Carlo calculations using these new evaluations are compared with experimental data from the LLNL 14-MeV pulsed sphere measurements