Low-frequency dielectric properties of lysozyme as a function of hydration and pH of lyophilisation

Abstract

Dielectric measurements have been made on lysozyme-compressed powders as a function of hydration and of the pH at which the samples were lyophilised. A dielectric dispersion previously described in the literature and known as the alpha-dispersion is found to be strongly influenced by the lyophilisation pH, and reaches a maximum magnitude at around pH 11. A dielectric loss of the form of the alpha-dispersion is observed in polylysine hydrobromide and the sodium salt of polyglutamic acid, but not for polyglycine. These results can be understood in terms of proton transfer between the ionisable side-groups, as well as by the controlling influence of counter-ions in moderating the energetics of the proton transfer processes. Another, weaker and hitherto unreported, dielectric loss process has been found for lysozyme, polyglycine, polylysine and poly(glutamic acid). This is referred to as the alpha 2-dispersion and its characteristics can be understood in terms of vibrational motions of the polypeptide backbones and the plasticising action of bound water.