Energetics Of The Interaction Between Water, Membranes A

Child Health And Human Development

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Abstract

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 ion source water partial pressures 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, at each of at least 60 combinations of water partial pressures and temperatures covering the ranges of experimental variables. Outside of the 25-60C temperature range, these measurements become very challenging experimentally and require several hours of equilibration at each new temperature point. The initial goal of the project is determining the enthalpy of solvation of a series of alkylammonium ions, CnH(2n+1)NH3+. While these ions are clearly a model system, they offer the possibility of providing insights into the understanding of the relationship between hydrocarbon chain length and solvation as well as the hydration of lipids and membranes. Previous workers have determined thermodynamic parameters for about 25% of the hydrations we have studied and results from our measurements are in close agreement with those published data. In the past year we have completed our measurements in this system. In addition, we have begun extending this work to more biologically significant systems. Observations by several investigators have shown differing effects of 1,2-(OH)2-propane and 1,3-(OH)2-propane on membrane fusion and collagen self-assembly. It has been hypothesized that these differential effects might be attributed to differences in the organization of water around these two molecules. Our equilibrium ion molecule studies of these two simple diols points to substantial differences in their hydration thermodynamics. 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 a 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 energetic and entropic favorable state. The existence of such a favorable state of hydration is consistent with the 1,3-diol incorporating into and disrupting otherwise stable biomolecular structures.