The molecular structure of polyinosinic acid

Fibers of polyinosinic acid produce an oriented X-ray diffraction pattern with a strong meridional spacing of 3.4 Å, and two layer lines at 9.8 and 5.2 Å. The distribution of scattering intensity is characteristic of a helical molecule. The diffraction data is compatible with a molecule consisting of three helically wound polynucleotide chains which are hydrogen bonded to each other through the purine bases. Diffraction patterns calculated from this model agree with the observed data. In solution, polyinosinic acid changes properties when the pH is raised above 10. This is believed to be due to the breakdown of the multistranded molecule. Raising the ionic strength of the solution lowers its optical density and probably stabilizes the molecule in the helical form.