Close packing of side chains of a protein and protein-solvent contact. The overlap of excluded volumes leads to an increase in the total volume available to the translational displacement of solvent molecules. The contact of a solvent molecule with the protein also induces the overlap of excluded volumes generated by the protein and the solvent molecule, which causes a larger translational restriction for the contacting solvent molecule but brings an increase in the total volume available to the translational displacement of the other solvent molecules (see the third paragraph in Sec. IV D ).
(a) Physical picture of thermal denaturation for different proteins in water with no sugar added. Three proteins, “a,” “b,” and “c,” are considered. ΔS w and ΔS conf are the solvent-entropy gain and the conformational-entropy loss upon protein folding, respectively. T m is the denaturation temperature. The subscripts, “a,” “b,” and “c,” denote the values for proteins “a,” “b,” and “c,” respectively. The minimum value of the abscissa is 298 K. (b) Physical picture of thermal denaturation for a protein in different solvents. Three solvents, “α,” “β,” and “γ,” are considered. ΔS is the solvent-entropy gain in each solvent upon protein folding. The subscripts, “α,” “β,” and “γ,” denote the values for solvents “α,” “β,” and “γ,” respectively.
Dependence of the criterion of the thermal stability on sugar species, sucrose (square) and glucose (circle), and sugar concentration.
Denaturation temperatures and measures of the thermal stability ΔS w, mp/(k B N r) and ΔS w, hs/(k B N r) for Yfh1, CyaY, and hfra. The superscript “w” represents that the solvent-entropy gain upon protein folding is calculated for water with no sugar added, and the superscripts “mp” and “hs” represent that the multipolar-model water and the hard-sphere solvent are employed for water, respectively. The number density and molecular diameter are set at those of real water.
Physical origins and signs of the four constituents of the solvation entropy for water with no sugar added.
Change in the solvent-entropy gain upon protein folding caused by sugar addition and its constituents. They are scaled by the Boltzmann constant k B.
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