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

关于湿度诱导的核苷酸一级相变动力学，得到了以下信息：Bragg反射的时间依赖性与重力测量监测的晶体水分子数量的增减速率一致。在水的临界蒸汽压或临界温度下，转变速率变为无限大。这一现象可归因于临界点处一阶相变的能量势垒，与观测到的滞后现象相一致。在大多数情况下，不存在中间状态。然而，在Na_2UMP的情况下，发现了瞬态超结构，其细胞长度是原始细胞的两倍。这是由于钠离子和结晶水分子在相变过程中的重定向所致。Na_2IMP的中子衍射分析。用7H_2O来检测氢键网络中氢的位置。水分子无序，氧原子和氢原子的核分布反映了水分子的三种大振幅的振动运动。为了得到与实验结果相补充的信息，对二水鸟苷进行了分子动力学模拟计算。揭示了层间和层内晶体水分子迁移的差异，并发生了水分子的易位。这与结晶水分子的解吸过程密切相关。声子的起源与核酸拉曼光谱中的s模式声子的起源相同，声子被分配为碱基沿叠加方向的平移振动。

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.