Pressure effects on water-swollen elastin. A model for hydrophobic interactions in proteins

The effect of pressure on the swelling of elastin in pure water was investigated. Because elastin is a very non-polar protein, and because the swelling of elastin can be directly related to changes in the strength of hydrophobic interactions, we have used elastin as a model to study the effect of pressure on hydrophobic interactions in proteins. The elastin swelling model is particularly useful because it is based on a macromolecular system very similar to a globular protein and not on dilute aqueous solutions of small non-polar compounds. Increased pressure causes elastin to increase its swollen volume, and the observed swelling changes were analyzed in terms of the Flory-Rehner theory (Flory, P.J. and Rehner, Jr., J (1943) J. Chem. Phys. 11, 521--526) for the swelling of kinetically free, random polymer networks. Our calculations provide a measure of the volume change for the process of transferring 1 mol of an average non-polar amino acid side chain from a region where the side chains are surrounded by other non-polar groups and have no contact with water (i.e. a hydrophobic region) into contact with water. The results indicate that there is a small, negative volume change associated with this process, and quantitative estimates indicate that the volume change is of the order of --6 ml/mol side chain. The results support the hypothesis that the free energy required to transfer a non-polar side chain from a hydrophobic region into water becomes less positive (i.e. the hydrophobic interaction becomes weaker) as hydrostatic pressure is increased.