UNCOILING TRANSITION FOR DNA IN SOLUTION

We study a simple DNA helix model, consisting of two infinite chains o f evenly spaced charges to represent the phosphate groups, wound in a helix which lies on an imaginary cylindrical surface. The change in th e free energy per helix charge between coiled and uncoiled conformatio ns of the helix in solution is studied as a function of the charge per unit length along the helix axis. This allows us to study the effects of the solution on the helix stability and coiling. The change in the foe energy is calculated from Soumpasis's pair potential of mean forc e, applied to all pairs of helix charges [D. M. Soumpasis, Proc. Natl. Acad. Sci. U.S.A. 81, 5116 (1984)]. The local counterion concentratio n is calculated from the counterion radial distribution that results f rom solving the Poisson-Boltzmann equation for an infinite uniformly c harged cylinder [R. M. Fuoss, A. Katchalsky, and S. Lifson, Proc. Natl . Acad. Sci. 37, 579 (1951)], whose linear charge density is equal to the charge per unit length along the helix axis. Our results show that the helix is less stable on decreasing bulk dielectric constant and m ore stable on increasing counterion radius. Experimental data are disc ussed on DNA in solutions with water, ethanol, and methanol as the sol vent.