Solvation and conformational dynamics of dicarboxylic suberic acid

摘要

Solvation structure acid conformational preferences of the dicarboxylic suberic acid, HOOC(CH2)(6)COOH, both neutral and protolyzed in water and neutral in methanol have been studied using molecular-dynamics (MO) computer simulations. According to results from MD simulations in water solution, the backbone hydrocarbon chain shows a very clear tendency to curl up into a helical structure, forming either a tgggt or a tg'g'g't conformation. The carboxylic head groups are strongly hydrated in the water solution, while the hydrophobic hydrocarbon skeleton is surrounded by water molecules in a packed structure. In the helical conformation, the surface area of the nonpolar part of the fatty acid is minimized against water phase. Transitions between the right and the left-handed helices are observed in neutral and mono-anionic forms of suberic acid in water solution. Suberic acid dissolved in methanol does not show any conformational preferences. Along the hydrocarbon chain, g, g', and t conformers are equally populated. The head group torsional angles, however, strongly prefer trans conformation due to dipolar interactions between the carboxyl groups and the solvent hydroxyl groups. In addition to MD simulations, corresponding water and methanol solutions are prepared and (13)e NMR (nuclear magnetic resonance) chemical shifts are measured in both solutions. Using a time-averaged geometry for suberic acid from MD simulations in water, chemical shielding constants are calculated quantum chemically. Agreement between the theoretical and the experimental chemical shifts is good, and gives indirect support to the simulated conformations of suberic acid in the investigated solutions. The simulation results are also consistent with recent Raman investigations of suberic acid in both water and methanol solutions. (C) 1998 American Institute of Physics. [References: 26]