Binding kinetics of mercury(II) TO POLYRIBONUCLEOTIDES

Kinetic studies of the interaction of Hg(II) with polyribonucleotides have been used to investigate structural fluctuations of the bases in nucleic acids. The reaction of Hg(II) with poly(A)-poly(U) occurs in two phases which differ in time scale by a factor of about 100. The slow phase is first order and exhibits cooperativity or autocatalytic kinetics. The rate is found to increase as decreasing chain length of poly(U) is used to make the double helical complex. The reaction appears to initiate at the ends of poly(U) strands and may be associated with a molecular rearrangement which results in strand separation with Hg(II) being linked only to uridine. The fast reaction phase is second order ans shows little cooperative behavior. Protons are released at this stage indicating alteration of the double helix. The measured second-order rate constant is nearly three orders of magnitude smaller than that found for poly(U) alone. This rate difference suggests that the reactive sites are blocked by double helix formation, and become available for reaction with Hg(II) only through a structural fluctuation. The ratio of rate constants for the reaction of Hg(II) with poly(U) and poly(A)-poly(U) was used to place an upper limit on the equilibrium constant for the structural fluctuation of 2 times 10- minus 3 at 15 degrees and 0.5 M NaClO4. The heat of the "breathing" reaction can be estimated to be similar to 9 kcal/mol from comparison of the temperature coefficient of the reaction with poly(U) to that with poly(A)-poly(U).