Dielectric properties of polyelectrolytes. V. Dielectric dispersion of heavy meromyosin

Dielectric constant and dielectric loss of heavy meromyosin (HMM) were measured with varying pH. HMM showed a broader dispersion pattern than that with a single relaxation time especially on the high-frequencey side. The dielectric increment increased sharply with pH, above p^{H 6}, whereas the mean relaxation time and whole dispersion pattern were unchanged in the same region. The values of the increment and the mean relaxation time were much larger than those of usual globular proteins. The dispersion profile, pH dependence, and values of the increment are well explained by Oosawa's counterion fluctuation theory. Other mechanisms are more or less inadequate to our results. In the low pH region below the isoelectric precipitation region, both the increment and the mean relaxation time decreased; this is probably due to partial denaturation and suppression of the dissociation of carboxyl groups. An experiment on a urea-denatured sample supports this assumption. The biological significance of the pH dependence is discussed.