The influence of ionic strength and organic solvents on acid transitions of proteins

• 1.

  1. The influence of ionic strength, dioxane, and ethylene glycol on the acid transition of ribonuclease has been studied by difference spectrophotometry. In this transition two of the three buried tyrosyl residues of the molecule can be normalized if the temperature is high enough, but an increase in the ionic strength will repress the normalization of one of them. Dioxane makes the normalization of the second one easier, and ethylene glycol has very little effect.

• 2.

  2. Dioxane also raises the pK of the transition. Ribonuclease, serum albumin, and acetic acid all show the same dependence of Δ pK on dioxane concentration, suggesting that the increase is caused by a change in the bulk dielectric constant of the medium. Chymotrypsinogen is much more strongly affected, and lysozyme is less strongly affected.

• 3.

  3. The denaturation of ribonuclease at pH 2.3 by dioxane shows some very unusual features. The difference spectra are anomalous. The denaturation is very strongly affected by the ionic strength, and in an opposite direction to what is usually observed: denaturation occurs at lower dioxane concentrations as the ionic strength is raised. A possible explanation for these unusual features is based on Weber's observation that dioxane first denatures ribonuclease, and at higher concentrations promotes the formation of helical regions in the molecule.