Dielectric Response of Micelles Built from Intrinsically Disordered Beta-Casein

For many decades, protein function was associated with a rigid three-dimensional structure. However, in recent years, interest has grown rapidly in proteins that successfully perform their functions despite being intrinsically disordered. Unfortunately, to date, insufficient structural information has been collected on this novel protein family to analyze its structure–function correlations. To obtain the required structural information on intrinsically disordered beta-casein (b-CN), we used broadband dielectric spectroscopy, which has long been used to study structured proteins. Dielectric measurements in the frequency range from 1 Hz to 67 GHz detected three Cole–Cole relaxation processes in a micellar solution of disordered beta-casein. The experimental data obtained were used to estimate the dynamical hierarchy of dipole–dipole correlations involved in the relaxation of each dipole component of the system. The dielectric relaxation of micellized disordered b-CN is rather complex, indicating the cascade nature of protein relaxation, which involves long-range structural correlations of flexible charged protein chains. The relaxation of free and bound water is close to the Debye type, which is entirely consistent with the “wait and switch” model of dielectric relaxation, in which a water dipole waits for the arrival of a structural or ionic defect for reorientation. The relaxation rates of the two water fractions differ dramatically, with bound-water relaxation being two orders of magnitude slower than free-water relaxation. The most reasonable explanation for this difference probably lies in the spatial confinement of bound water resulting from the influence of the charged protein surface. The results obtained demonstrate the research potential of broadband dielectric spectroscopy not only for long-studied rigid, structured proteins with permanent dipole moments but also for the novel family of disordered proteins with fluctuating dipole moments.