Determination of Conformational Entropy of Fully and
Partially Folded Conformations of Holo- and Apomyoglobin
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Abstract
Holo-
and apomyoglobin can be stabilized in native folded, partially
folded molten globules (MGs) and denatured states depending on the
solvent composition. Although the protein has been studied as a model
system in the field of protein folding, little is known about the
internal dynamics of the different structural conformations on the
picosecond time scale. In a comparative experimental study we investigated
the correlation between protein folding and dynamics on the picosecond
time scale using incoherent quasielastic neutron scattering (QENS).
The measured mean square displacements (MSDs) of conformational motions
depend significantly on the secondary structure content of the protein,
whereas the correlation times of the observed internal dynamics were
found to be similar irrespective of the degree of folding. The conformational
entropy difference Δ<i>S</i><sub><i>conf</i></sub> between the folded conformations and the acid denatured state
could be determined from the measured MSDs and was compared to the
entropy difference Δ<i>S</i> obtained from thermodynamic
parameters reported in the literature. The observed difference between
Δ<i>S</i> and Δ<i>S</i><sub><i>conf</i></sub> was attributed to the entropy difference Δ<i>S</i><sub><i>hydr</i></sub> of dynamically disordered
water molecules of the hydration shell. The entropy content of the
hydration water is significantly larger in the native folded proteins
than in the partially folded MGs. We demonstrate the potential of
incoherent neutron scattering for the investigation of the role of
conformational dynamics in protein folding