Evidence of alpha fluctuations in myoglobin's denaturation in the high temperature region: Average relaxation time from an Adam-Gibbs perspective.

In this work, we derive an analytical expression for the relaxation time tau as a function of temperature T for myoglobin protein (Mb, PDB:1MBN) in the high temperature limit (T>T(g)=200K). The method is based on a modified version of the Adam-Gibbs theory (AG theory) for the glass transition in supercooled liquids and an implementation of differential geometry techniques. This modified version of the AG theory takes into account that the entropic component in protein's denaturation has two major sources: a configurational contribution DeltaS(c) due to the unfolding of the highly ordered native state N and a hydration contribution DeltaS(hyd) arising from the exposure of non-polar residues to direct contact with solvent polar molecules. Our results show that the configurational contribution DeltaS(c) is temperature-independent and one order of magnitude smaller than its hydration counterpart DeltaS(hyd) in the temperature range considered. The profile obtained for log tau(T) from T=200 K to T=300 K exhibits a non-Arrhenius behavior characteristic of alpha relaxation mechanisms in hydrated proteins and glassy systems. This result is in agreement with recent dielectric spectroscopy data obtained for hydrated myoglobin, where at least two fast relaxation processes in the high temperature limit have been observed. The connection between the relaxation process calculated here and the experimental results is outlined.