LASER TEMPERATURE-JUMP STUDY OF STACKING IN ADENYLIC ACID POLYMERS

The kinetics of the single-strand stacking of adenylic acid polymers have been investigated by using the laser temperature-jump method. This technique allowed the determination of rate constants for polyriboadenylic acid [poly(A)], the riboadenylic acid heptamer, and polydeoxy-riboadenylic acid [poly(dA)] at 0.05 M ionic strength. At this low ionic strength a single, wavelength-independent relaxation is observed for the ribopolymers. The thermodynamics were determined from an analysis of the melting curves measured by ultraviolet absorption. By assumption of a two-state model, the rate constants for formation of the stacked state at 25 °C for poly(A) and poly(dA) are 0.7ËŸ107 and 2.7ËŸ107 s-1, respectively. The reverse rate constants are 3.2ËŸ106 s-1 for poly (A) and 4.3ËŸ106 s-1 for poly(dA). The activation energy for poly(A) was 4.0 kcal/mol for the forward rate and 15.2 kcal/mol for the reverse rate. Poly(dA) has a similar activation energy of 3.2 kcal/mol for the forward rate and a lower reverse rate activation energy of 12.3 kcal/mol. The ribo heptamer results are similar to poly (A). The forward rate constants are relatively small considering the low activation barriers. This is interpreted as a conformational effect on the activation entropy. At 0.2 M ionic strength with 0, 2, or 5 mM Mg2+, poly(A) exhibits a wavelength dependence to the relaxation time. This indicates the presence of more than two states. At these high salt concentrations, the relaxation times are longer than at 0.05 M ionic strength. This is interpreted as a decrease in the rate of unstacking. Thus, high salt appears to stabilize the stacked state. © 1979, American Chemical Society. All rights reserved.