Study on the molecular conformational equilibrium of butane for the development of an experimental model system of the hydrophobic effect

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It is well-known that the hydrophobic effect is a critical factor for structural stability of proteins. The purpose of this project is to develop an experimental model for the hydrophobic effect. As a model, we investigated conformational equilibrium of simple chain molecules by using Raman spectroscopy. The most simple and ideal model is the trans gauche equilibrium of butane. We first target this system in aqueous solution. However, it was difficult to observe the equilibrium by Raman spectroscopy due to the low solubility of butane into water. During this project term, we succeed to observe qualitatively the equilibrium, indicating that the relative population of the gauche conformer increased compared with the organic solvent systems. The further experiments for the quantitative analysis are in progress. As another targets we investigated trans-gauche/gauche-gauche equilibrium of diethyl disulfide, which is a model for disulfide bonds for proteins. We elucidated the effect of temperature and pressure on the equilibrium, and discuss the phenomena in comparison with the conformational change of disulfide of a protein when it unfolds. The other research results were obtained for a simple peptide system. We target a simple alanine-rich peptide as a model molecule, which is suitable to observe the helix-coil equilibrium in water. Effect of pressure on the equilibrium showed that the relative population of helix form increases with increasing pressure. This behavior is apparently inconsistent with pressure-induced unfolding of protein. Further projects for elucidating the inconsistency are in progress.