Intrachain reactions of supercoiled DNA simulated by Brownian dynamics.

We considered an irreversible biochemical intrachain reaction of supercoiled DNA as a random event that occurs, with certain probability, at the instant of collision between two reactive groups bound to distant DNA sites. Using the Brownian dynamics technique, we modeled this process for a supercoiled DNA molecule of 2.5 kb length in dilute aqueous solution at an NaCl concentration of 0.1 M. We calculated the mean reaction time tau(Sigma) as a function of the intrinsic second-order rate constant k(I), the reaction radius R, and the contour separation S of the reactive groups. At the diffusion-controlled limit (k(I) --> infinity), the kinetics of reaction are determined by the mean time tau(F) of the first collision. The dependence of tau(F) on R is close to inversely proportional, implying that the main contribution to the productive collisions is made by bending of the superhelix axis. At sufficiently small k(I), the mean reaction time can be satisfactory approximated by tau(Sigma) = tau(F)(app) + 1/(k(I)c(L)), where c(L) is the local concentration of one reactive group around the other, and tau is an adjustable parameter, which we called the apparent time of the first collision. The value of tau depends on R very weakly and is approximately equal to the mean time of the first collision caused by mutual reptation of two DNA strands forming the superhelix. The quasi-one-dimensional reptation process provides the majority of productive collisions at small k(I) values.