Double Helix Formation of Oligoresorcinols in Water: Thermodynamic and Kinetic Aspects

We previously reported that the oligoresorcinols formed double-stranded helices in neutral water through interstrand aromatic interactions. In the present study, we synthesized a new series of oligomers from the 2mer to the 15mer to explore the thermodynamics, kinetics, and mechanism of the double helix formation of the oligoresorcinols in water. The double helix formation was dependent on the chain length of the oligomers and significantly affected by solvent, pH, salt, and temperature. The free energy change (-Delta G) for the double helix formation linearly increased with the chain length from the 4mer to the 11 mer (Delta Delta G = -0.94 kcal mol(-1) unit(-1)), whereas it did not change for the oligomers longer than the 11 met. The van't Hoff analysis of the 9mer revealed that the double helix formation was an enthalpically driven process (Delta H = -27 +/- 1.5 kcal mol(-1) and AS = -70 +/- 5 Cal mol(-1) K-1), which was consistent with the upfield shifts in the H-1 NMR spectra and the hypochromicity of the absorption spectra as a result of the interstrand aromatic interactions in water. Furthermore, the kinetic analysis of the chain exchange reaction between the double helices of the optically active and optically inactive 11 mers revealed a small Delta S double dagger, suggesting that the chain exchange proceeds not via the dissociation-association pathway, but via the direct exchange pathway.