Librational fluctuations in protein glasses

Librational motions in the region of the protein “glass” (or dynamic) transition are analysed for spin-labelled haemoglobin, serum albumin and -lactoglobulin by EPR spectroscopy. A discontinuity in the temperature dependence of the mean-square librational amplitude, <2>, occurs in the region of 200 K as found for the mean-square atomic displacement, <r2>, at the protein dynamic transition by Mössbauer spectroscopy and neutron scattering. The discontinuity in <2> vs. T can be described by the Vogel-Tamman-Fulcher equation, implying a finite glass transition temperature. Above the dynamic transition, <2> vs. 1/T can be approximated by the Arrhenius law with activation energies similar to those usually found for <r2>, and relaxation processes in glass-forming media and the hydration shells of proteins. Similar results are found for librational fluctuations of membranous Na,K-ATPase spin-labelled either on superficial SH groups or on those essential to activity.