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Free keywords:
Glass transition; Arrhenius behaviour; Na,K-ATPase; Spin label; EPR
Abstract:
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, <alpha(2)>, occurs in the region of 200 K as found for the mean-square atomic displacement, <r(2)>, at the protein dynamic transition by Mossbauer spectroscopy and neutron scattering. The discontinuity in <alpha(2)> vs. T can be described by the Vogel-Tammann-Fulcher equation, implying a finite glass transition temperature. Above the dynamic transition, <alpha(2)> vs. 1/T can be approximated by the Arrhenius law with activation energies similar to those usually found for <r(2)>, 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.