Reversible crystal transition of guanosine between the dihydrate and anhydrous states coupled with adsorption-desorption process.

Relative humidity induces the reversible crystal transition of guanosine between the dihydrate and the anhydrous state. The characteristics of the transition was investigated by means of X-ray powder diffraction analysis and high-resolution solid-state 13C NMR spectroscopy. Adsorption-desorption hysteresis was observed. Guanosine dihydrate (the H-state) which is crystallized from an aqueous solution rapidly loses crystal water below 10% relative humidity (rh), and is anhydrous at 0% rh (the A-state). The crystals gradually recover the H-state at approximate 20% rh. In the adsorption process between 10-20% rh, there exists one intermediate state, M, with 1.2-1.3 moles water per mole guanosine. The lattice of the M-state was determined to be orthorhombic with the cell parameters of a = 16.248(1), b = 11.603(1), and c = 13.643(2) A. The base-stacking structure is retained throughout the transition. On the other hand, conformational changes of the riboses and break of the hydrogen-bonding network between the bases would be induced in the A-state in conformity with lack of crystal water.