HELIX-SPECIFIC INTERACTIONS INDUCE CONDENSATION OF GUANOSINE 4-STRANDED HELICES IN CONCENTRATED SALT-SOLUTIONS

Deoxyguanosine-5'-monophosphate in water self-associates into stable s tructures, which include liquid-crystalline hexagonal and cholesteric phases. The structural unit is a four-stranded helix, composed of stac ked Hoogsteen-bonded guanosine quartets. By using the osmotic stress m ethod, we recently measured the force between helices in KCI solutions up to 2 M. In addition to the long-range electrostatic force, a short -range hydration repulsive contribution was recognized. The hydration repulsion is exponential, and shows a decay length independent from th e ionic strength of the solution. Here, we report that more concentrat ed KCI solutions cause condensation of the guanosine helix in a hexago nal phase with constant equilibrium separation of similar to 7 Angstro m between helix surfaces. Long-range attraction, which induces the sel f-assembly, and short-range repulsion, which prevents the contact betw een the helices, are implied. By using osmotic stress, the force neede d to push helices closer from the spontaneously assumed position has b een measured. The attractive force was then estimated as a difference between the net force and the repulsive contribution, revealing an exp onential decay length about two times larger than that of the short-ra nge repulsion. The agreement with the helix interaction theory introdu ced recently by Kornyshev and Leikin (Kornyshev, A. A., and S. Leikin, 1997, Theory of interaction between helical molecules. J. Phys. Chem. 107:3656-3674) suggests that the repulsive and attractive forces orig inate from helix-specific interactions.