EFFECTS OF DIFFERENT CATIONS ON THE HYDRODYNAMIC RADIUS OF DNA

The effects of different cations on the hydrodynamic radius (R(H)) of a 48-bp synthetic DNA are measured by time-resolved fluorescence polar ization anisotropy of intercalated ethidium. Relative statistical erro rs in R(H) are only similar to 1%. With increasing cation concentratio n, Na+ causes a small decrease in R(H), Cs+ causes a somewhat larger d ecrease by up to 0.5 Angstrom at 100 mM, and (CH3CH2N+ causes an incre ase in R(H) by similar to 0.5 Angstrom at 100 mM. The qualitatively di fferent effects of these monovalent cations indicates that the changes in R(H) with cation concentration do not arise primarily from electro lyte friction. Divalent cations cause much larger increases in R(H) wi th increasing cation concentration. Mg2+ causes an increase in R(H) by up to 1.0 Angstrom at 24.4 mM, and Mn2+ causes an increase in R(H) by up to 1.6 Angstrom at 24.4 mM. These effects are independent of DNA c oncentration. There is some positive correlation between the order of effects of the different cations on R(H) and the order of their effect s on interhelical hydration forces. It is suggested that these differe nt ions affect R(H) either by altering the hydration layer or possibly by some effect on DNA structure, such as stabilizing bends.