Energetics Of The Interaction Between Water, Membranes A

这项工作的目的是确定溶液中分子和离子之间形成非共价键所需的能量。了解这种非共价键的能量需求，特别是那些涉及水和具有重要生物学意义的分子的能量需求，是理解分子相互作用及其组成部分的构象变化的基础。本研究采用平衡离子-分子反应化学方法确定了δ tah (std)298、δ tas (std)298和δ tag (std)298的热力学量。在0- 100 mtorr的离子源中，水分压在0-136℃的温度范围内测量平衡常数，计算出水合热力学值。平衡离子强度测量了至少4种水合状态，即0到3个与核心离子相关的水分子，并包括至少60种水分压和温度的组合，涵盖了实验变量的范围。已经得到了三种不同类型分子的结果。首先，对n-烷基铵、CnH(2n+1)NH3+以及二甲基铵和三甲基铵离子周围1-3个水的平衡聚类的详尽研究表明，在离子带电部分周围形成的水簇的行为模式与所附疏水基的性质的影响是一致的。从这些测量中得到的熵值的详细分析与检测与内部氢键形成相关的熵变化是一致的。其次，通过对简单烷基二醇：1,2-(OH)2-丙烷、1,3-(OH)2-丙烷、1,3-(OH)2-丁烷和1,4-(OH)2-丁烷的水化热力学的研究，进一步探讨了水在电荷位点周围组织和内部氢键形成的概念。将水逐步加成成质子化的1,2-(OH)2-丙烷，其放热性随加成次数的增加而递减。虽然我们无法直接测量第一次水的添加量，但我们能够根据水的分压和温度估计出该过程放热的上限。我们得出结论，第一个水化步骤在能量上是非常有利的。1,2-(OH)2-丙烷在加入前两个水分子时，能量学呈下降趋势，但在加入第三个水分子时，能量学没有保持下降趋势。在配合物中加入第三种水被确定为在能量上比加入第二种水更有利，并且发现该过程的熵大大减少。这两个观察结果使我们得出结论：1,3-(OH)2-丙烷三水合物是一个能量和熵有利的状态。另一方面，这两种丁烷二醇的反应方式与1,2-丙烷二醇一致。第三，是最近对几种氨基酸水合作用的研究结果。水与中性氨基酸甘氨酸、缬氨酸和亮氨酸的平衡聚类结果表明，前两个水分子与烷基铵离子的平衡聚类行为非常相似。然而，第三种水的加入伴随着焓和熵的降低，这表明分子内氢键的形成，就像在1,3-丙烷二醇中看到的那样。另一方面，迄今为止对基本氨基酸，精氨酸和赖氨酸的研究结果却大不相同。也就是说，虽然人们可能认为向这些分子中加入第一个水的能量是非常有利的，但我们已经确定，它在能量上不如向中性氨基酸中加入第一个水有利。这表明内部氢键的存在在任何水团形成之前就已经存在了。

The purpose of this work is the determination of the energies required for the formation of non-covalent bonds between molecules and ions in solution. Knowledge of the energetic requirements of such non-covalent bonds, particularly those involving water and biologically significant molecules, is fundamental to understanding molecular interactions and the changes in conformations that are integral to them. This research involves determining the thermodynamic quantities deltaH(std)298, deltaS(std)298, and deltaG(std)298 using the approach of equilibrium ion-molecule reaction chemistry. Hydration thermodynamics values were calculated from equilibrium constants measured over a temperature range of 0-136 degrees C at water partial pressures in the ion source ranging between zero and 100 mtorr. Equilibrium ion intensity measurements were made for at least 4 hydration states, i.e., zero through 3 water molecules associated with a core ion, and include at least 60 combinations of water partial pressures and temperatures covering the ranges of experimental variables. Results have been obtained for three different classes of molecules. First, an exhaustive study of the equilibrium clustering of 1-3 waters around n-alkylammonium, CnH(2n+1)NH3+, and the di- and tri-methyl ammonium ions, has shown a pattern of behavior consistent with water clusters forming around the charged portion of the ion and being influenced by the nature of the attached hydrophobic groups. Detailed analysis of the entropy values derived from these measurements is consistent with detecting the entropy change associated with the formation of an internal hydrogen bond. Second, the concepts of water organizing around a charge site and the formation of internal hydrogen bonds detected by substantial entropy decreases have been explored further with the investigation of the hydration energetics of simple alkyl-diols: 1,2-(OH)2-propane , 1,3-(OH)2-propane, 1,3-(OH)2- butane and 1,4-(OH)2-butane. The stepwise addition of water to protonated 1,2-(OH)2-propane shows a trend of diminishing exothermicity for each addition. While we were unable to obtain a direct measurement for the addition of the first water, we were able to estimate an upper limit on the exothermicity for this process based on the water partial pressure and temperature. We conclude that this first hydration step is energetically very favorable. The decreasing trend in energetics seen for 1,2-(OH)2-propane was observed for the addition of the first two water molecules to 1,3-(OH)2-propane, but was not maintained for the addition of the third water molecule. The addition of the third water to the complex was determined to be energetically more favorable than the addition of the second, and in addition was found to have a substantial decrease in the entropy for the process. These two observations lead to our concluding that the 1,3-(OH)2-propane trihydrate is an energetically and entropically favorable state. On the other hand, both of the butane diols behave in a manner consistent with the 1,2-propane diol. Third, are very recent results from studies of the hydration of several amino acids. Results from the equilibrium clustering of water with the neutral amino acids, glycine, valine and leucine, show behavior that is quite similar to that observed with the alkylammonium ions for the first two water molecules. The addition of the third water however is associated with a decrease in both enthalpy and entropy, suggesting the formation of intramolecular hydrogen bonds as seen in the 1,3-propane diol. in On the other hand, the results obtained to date in studies of the basic amino acids, arginine and lysine, are quite different. That is, while one might expect the energetics of adding the first water to these molecules to be highly favorable, we have determined that it is less energetically favorable than the addition of the first water to the neutral amino acids. This suggests the presence of internal hydrogen bonding being present before any water clusters are formed.