UNUSUAL LYOTROPIC POLYMORPHISM OF DEOXYGUANOSINE-5'-MONOPHOSPHATE - X-RAY-DIFFRACTION ANALYSIS OF THE CORRELATION BETWEEN SELF-ASSEMBLING AND PHASE-BEHAVIOR

The concentration-temperature dependent phase diagram of the deoxyguanosine-5'-monophosphate [d(pG)]-water system was determined by x-ray diffraction measurements. The columnar lyotropic polymorphism has been confirmed. However, at medium concentrations and high temperatures, we found an unusual and large region of coexistence of isotropic and hexagonal phases (maybe in equilibrium with the cholesteric phase). Increasing the ionic strength of the solution by adding KCl, resulted in a stabilization of the high-temperature hexagonal phase and a direct isotropic to hexagonal (with increasing concentration) and a cholesteric to hexagonal (with increasing temperature) phase transition were observed. The phase diagrams completely agree with the one predicted for hard rod-shaped particles. In order to understand the phase behavior, the cholesteric and hexagonal unit cell parameters were analyzed at different temperatures as a function of the volume concentration of d(pG) on the basis of recent statistical-mechanical calculations. Moreover, by analyzing the high-angle [s = (3.4 angstrom)-1] peak, the average number of coherently scattering tetramers was also determined. For d(pG) in pure H2O, we obtained values between 35 and 53. After adding KCl, the functional dependence turned out to be more complex, and we obtained values between 26 and 58.