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

关于湿度引起的核苷酸一级相变的动力学，得到了以下信息。布拉格反射的时间依赖性与重量测量监测到的结晶水分子数量的增加或减少的速度一致。当水或温度达到临界蒸汽压时，转变速率变为无穷大。这一现象可归因于临界点一级相变的能垒，与观测到的磁滞现象相一致。在大多数情况下，不存在中间状态。而在Na_2UMP的情况下，发现了暂态超结构，其胞长是原来的两倍。它们归因于钠离子和结晶水分子在相变过程中的重新取向。Na_2IMP的中子衍射分析用7H2O研究了氢键网络中的氢位置。存在无序的水分子，氧原子和氢原子的核分布反映了水分子的三种较大幅度的振动运动。对鸟苷二水合物进行了分子动力学模拟计算，得到了与实验结果互补的信息。揭示了层间和层内结晶水分子迁移率的差异，并发生了水分子的移位。这可能与结晶水分子的脱附过程密切相关。声子的起源与在核酸拉曼光谱中观察到的S模的声子起源相同，它被认为是碱基沿堆积方向的平移振动。

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.