KINETIC-STUDY OF DOUBLE-HELIX FORMATION AND DOUBLE-HELIX DISSOCIATIONOF POLYADENYLIC-ACID

The changes in the secondary structure of polyadenylic acid [poly(A)], caused by variations of pH at constant temperature, have been investi gated. The position of the equilibrium 2 single-strands half arrow rig ht over half arrow left double-strand is shifted towards the left-hand side by an increase in the ionic strength of the solution, and for co nstant values of the latter, by the addition of small amounts of dival ent transition metal ions. The kinetics of the conformational changes have been investigated by T-Jump, stopped-flow and classical spectroph otometric methods. With the latter two techniques the reactions were i nitiated by changes in solution pH. The process of double-helix dissoc iation was found to involve at least three first-order steps. The proc ess of double-helix formation begins with a second-order step leading, via a labile steady-state intermediate, to a multitude of mismatched double-strands. These, in turn, evolve to the final, completely formed double-helical form according to a series of first-order steps. The v alue of the second-order rate constant (10(5) dm3 mol-1 s-1) is simila r to those found for the double-helix formation reactions of oligonucl eotides. Increases in pH from 4 to 5.9 were used to study the conversi on of a metastable double-stranded form of poly(A) into a form that is to be considered the most stable under the experimental conditions. T his rearrangement process occurs through a series of three first-order steps, like the (much faster) process of double-helix dissociation.