Proton nuclear magnetic resonance investigations of the nucleation and propagation reactions associated with the helix-coil transition of d-ApTpGpCpApT in H2O solution.

The chemical shifts and line widths of the Watson-Crick ring NH resonances of the self-complementary duplex of d-ApTpGpCpApT have been monitored in the presence of 0.1 M phosphate at neutral pH in aqueous solution. While the resonance positions of the terminal and internal AT base pairs shift upfield and broaden as average resonances with increasing temperature (helix and coil exchange several times prior to exchange with water from the coil form), those of the central GC base pairs broaden in the absence of upfield shifts (exchange with water occurs each time helix converts to coil). The line-width changes at the AT base pairs monitor the lifetime of the coil state at these positions prior to exchange with water while the line-width changes at the GC base pairs monitor the lifetime of the helix prior to dissociation to strands. This permits the separation of the propagation reaction at the AT base pairs from the nucleation reaction at the GC base pairs during helix formation. The experimental data have been quantitatively analyzed to yield (at 20 degrees) a nucleation formation rate of approximately 10(3) sec-1 for the GC base pairs (bimolecular rate constant of approximately 6 times 10(6) l. mol-1 sec-1) and a dissociation rate of 6 times 10(2) sec-1 at these same base pairs (unimolecular dissociation to strands). The unimolecular propagation reactions at the terminal and terminal base pairs are associated with reaction rates greater than 10(4) sec-1. These values are consistent with a slow formation of a stable nucleus at the GC base pairs followed by a rapid propagation reaction at the AT base pairs. The line width of the (GC) central base pairs in the presence of phosphate is a direct measure of the lifetime of the total helix and yields an activation energy of 45 kcal for helix to coil conversion measured over a narrow temperature range. The exchange from the coil form with water is catalyzed by 0.1 M phosphate with a rate constant kHPO2-/4 = 0.2 times 10(6) 1. mol-1 sec-1.