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 beta-lactoglobulin by EPR spectroscopy. A discontinuity in the temperature dependence of the mean-square librational amplitude, , occurs in the region of 200 K as found for the mean-square atomic displacement, , at the protein dynamic transition by Mossbauer spectroscopy and neutron scattering. The discontinuity in vs. T can be described by the Vogel-Tammann-Fulcher equation, implying a finite glass transition temperature. Above the dynamic transition, vs. 1/T can be approximated by the Arrhenius law with activation energies similar to those usually found for , 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. (C) 2013 Elsevier B.V. All rights reserved.