Time resolved analysis of the first order phase transition of nucleotide hydrates

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Concerning the dynamics of humidity-induced first order phase transition of nucleotides, following information was obtained.1. Time dependence of Bragg reflections coincides with the rate of increase or decrease of the number of crystal water molecules monitored by gravimetric measurements. The transition rate becomes infinite at the critical vapor-pressure of water or temperature. The phenomenon would be attributed to energy barrier of the first-order phase transition at the critical point, and is consistent with the observation of hysteresis. In most cases, there exists no intermediate state. However, in the case of Na_2UMP, transient super-structures were found, the cell lengths of which are twice as long as the original one. They are attributed to reorientation of sodium ions and crystal water molecules during the phase transition.2. Neutron diffraction analysis of Na_2IMP. 7H_2O was carried out to examine the hydrogen positions in the hydrogen-bond networks. There are disordered water molecules, and the nuclear distribution of oxygen and hydrogen atoms was found to reflect three types of librational motions of water molecules with large amplitude.3. Calculations of molecular dynamics simulation of guanosine dihydrate was carried out to obtain the complemental information to the experimental results. The difference in mobilities of interlayer and intralayer crystal water molecules was revealed, and translocation of water molecules occurred. It would be in close relation to the desorption pass of crystal water molecules. The phonon, whose origin would be common to that of the S-mode observed in Raman spectra of nucleic acid, was assigned to the translational vibration of bases along the stacking direction.