Hymidity-induced phase transition of nucleotide hydrates

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Humidity-induced first order phase transition of nucleotides, which are repeating units of nucleic acid, was examined by X-ray analysis and Raman spectroscopy. Main results are as follows :1.In crystals, nucleotides make a molecular layr. Between molecular layrs, there is a region which consists of crystal water molecules and cations.Number of crystal water molecules is a function of humidity. Increase or decrease of crystal water molecules accompanies change of inter layr distances and switching of hydrogen-bond networks and coordination scheme around sodium ions. Amartensite-like transformation was observed in the casa of Na_2CMP.2.Diffused scattering along rows of the reciprocal lattice normal to the molecular plane was observed in the case of Na_2CMP.9H_2O.Static or dynamic fluctuation of number of crystal water molecules should cause layr disorder.3.Intermolecular interactions within a molecular layr in a low hydrated state was found to be essential for reversible transition.4.Rman spectra of a phonon region were measured. S-mode, whose character is common to that of the collective mode of nucleic acid, was conserved through the stepwise transition. The irreducible representation of S-mode was determined based on the polarized Raman spectra.5.Calorimetric measureements revealed that the transition is classified into two groups. For the first group, the observed enthalpy change is in the same order to the latent heat of water. For the second group, the enthalpy change is about a half. Transformation of molecular layrs of the latter group is fairly small.