An intermediate state in phase transition cycles of Na2dGMP hydrate

School of Science, Kitasato University
○Shigefumi Yamamura Tadashi Moriguchi Shigetaka Yoneda Yoko Sugawara

The first-order phase transitions of nucleoside and nucleotide hydrates are induced by humidity and temperature changes [1]. We found bifurcation of the phase transition in the case of 2´-deoxyguanosine 5´-monophosphate (Na2dGMP), i.e. the tetrahydrate transforms to the anhydrous form A or B [2]. One intermediate state (form M) was observed in the reverse transition from the anhydrous form B to the tetrahydrate. Around room temperature, the anhydrous form B changes to the form M, and then the form M gradually changes to the tetrahydrate. Therefore the form M would be a metastable state. We have successfully determined the crystal structure of the form M.
A single crystal of the form M was obtained by rapid cooling of the anhydrous form B to -100 °C, while the anhydrous form B was prepared by heating the tetrahydrate to 80 °C in a glass capillary. X-ray analysis of the form M was carried out, and the form M was found to be the dihydrate. Water molecules of the form M occupy vacant channel sites between the arrays of dGMP molecules in the anhydrous form B. The arrays of dGMP molecules are maintained in the form M. The conformation of dGMP molecule in the form M is similar to that in the anhydrous form B but largely different from that in the tetrahydrate. It is concluded that the large conformational difference of dGMP molecule between the anhydrous form B and the tetrahydrate plays a key role for the emergence of the intermediate state.
[1] Urabe et al. (1995) Phys. Rev. B, 51, 5666. [2] Yamamura et al. (2005) Acta Crystallogr. A61, C326.