Pressure Effects on G-Quadruplex Structures Stabilized by Two Different Counter-Ions

It is well known that the formation of G-quadruplex by guanosine derivatives strongly depends on counter-ions. To investigate quadruplex stability in the presence of different counter-ions and in the absence of a covalent axial backbone, high-pressure X-ray diffraction experiments have been performed on lyotropic phases of guanosine 5’-monophosphate (GMP) in the form of sodium and ammonium salts. As a result, concentration-pressure phase diagrams were obtained in a pressure range from 1 bar to about 2 kbar, and the structural properties of the different phases derived. Interestingly, cholesteric (Ch) and hexagonal (H) columnar phases were found in both GMP salts, but during compression a reverse behavior was detected: in GMP sodium salt, pressure induces a H-Ch phase transition, while in GMP ammonium salt the induced transition is Ch-H. Different counter-ion stabilization, which control quadruplex length and the consequence of lateral interactions, probably explain such observation