Co‐operative Non‐Enzymic

The absorbance and the optical rotatory dispersion of solutions of GpGpGpC and GpCpCpC individually and of the mixture of these two were measured in 0.1 M phosphate buffer (pH 7.2) as a function of temperature and concentration. The data indicate that below room temperature GpGpGpC self‐associates into large aggregates which presumably involve both G · C and G · G base pairs. It interacts as well with GpGpCpC to form a 1:1 GpGpGpC: GpCpCpC complex containing 4 G · C base pairs. The kinetics of formation of the 1:1 complex, as well as of self‐associated complexes, were studied by temperature jump relaxation method. The transition temperature T m of the 1:1 complex was determined from the variation of the relaxation amplitude with temperature. From the concentration dependence of T m , the values of – 21.6 kcal/mole and –50 entropy units were calculated for ΔH and ΔS respectively for the formation of the 1:1 complex. Temperature‐jump relaxation experiments on the individual components over the temperature range 20–40°C indicate a single process which was too rapid to follow ( t 0.5 ≪ 5 μsec), corresponding to the melting of a single stranded helix. In experiments with mixed solutions a slower measurable process was observed in the millisecond time range, which was attributed to 1:1 complex formation with a recombination rate constant k r = 5.4 10 6 M −1 × sec −1 and a dissociation rate constant k d = 40 sec −1 (at 21°C), Δ E ‡ r = 4.5 kcal/mole and Δ E ‡ d =+ 28.7 kcal/mole. Analysis of thermodynamic and kinetic data in terms of the “all or none” model suggests that the formation of the second base pair is the rate limiting step in the G · C helix formation. A single base pair adjacent to a pre‐existing helical sequence can be formed in 0.1 μsec. In contrast, the apparent formation and dissociation rate constants of GpGpGpC aggregates are smaller than those of the 1:1 complex by several orders of magnitude. The existence of a syn‐anti type equilibrium below room temperature is postulated to explain this observation.