Nanosecond structural dynamics of intrinsically disordered -casein micelles using neutron spectroscopy

Nakagawa, Hiroshi   ; Appavou, M.-S.*; Wuttke, J.*; Zamponi, M.*; Holderer, O.*; Schrader, T.*; Richter, D.*; Doster, W.*

-casein undergoes a reversible endothermic self-association with increasing temperature, forming micelles of limited size. We characterize the structural flexibility in terms of nano-second molecular motions. We report on two relaxation processes on a nano-second and a sub-nano-second time scale for -casein in solution. Both processes are analyzed by Brownian Oscillator model, by which the spring constant can be defined in the isotropic parabolic potential. The slower process seems a characteristic feature of the unfolded structure, and dividing the relaxation time by the solvent viscosity removes most of the temperature dependence, indicating that indicates, the process involves density fluctuations of the solvent. The faster process has a smaller amplitude and requires hydration water. The flexibility of a -casein monomer is preserved in the micelle.