The folded protein as a viscoelastic solid

We apply a nanorheology technique to explore the mechanical properties of a globular protein in the frequency range 10Hz-10 kHz and find that the folded state of the protein behaves like a viscoelastic solid. For increasing amplitude of the forcing, we observe three different regimes: linear elasticity, then a regime of viscoelastic but reversible deformations, and finally an irreversible regime. The second regime, which has the signature of a viscoelastic solid, gives access to the internal dissipation coefficient of the folded state, for which we find gamma approximate to 4 x 10(-5) kg/s, corresponding to an internal viscosity eta similar to 10(4) Pa.s for frequencies below similar to 10 Hz. We propose that the large discrepancy between this value, which agrees with previous AFM indentation experiments, and the value of the internal viscosity extracted from refolding experiments is a consequence of the viscoelastic nature of the protein's mechanics. Thus the present method yields detailed measurements of the mechanics of the folded state. Copyright (C) EPLA, 2011