Thermal unfolding of ribonuclease, lysozyme, chymotrypsinogen, and β-lactoglobulin was studied in the absence or presence of poly (ethylene glycols). The unfolding curves were fitted to a two-state model by a nonlinear least-squares program to obtain values of AH, AS, and the melting temperature Tm. A decrease in thermal transition temperature was observed in the presence of poly(ethylene glycol) for all of the protein systems studied. The magnitude of such a decrease depends on the particular protein and the molecular size of poly(ethylene glycol) employed. A linear relation can be established between the magnitude of the decrease in transition temperature and the average hydrophobicity of these proteins; namely, the largest observable decrease is associated with the protein of the highest hydrophobicity. Further analysis of the thermal unfolding data reveals that polyethylene glycols) significantly effect the relation between AH0 of unfolding and temperature for all the proteins studied. For 0-lactoglobulin, a plot of AH versus Tmindicates a change in slope from a negative to a positive value, thus implying a change in ACpin thermal unfolding caused by the presence of poly(ethylene glycols). Results from solvent-protein interaction studies indicate that at high temperature poly(ethylene glycol) 1000 preferentially interacts with the denatured state of protein but is excluded from the native state at low temperature. These observations are consistent with the fact that poly(ethylene glycols) are hydrophobic in nature and will interact favorably with the hydrophobic side chains exposed upon unfolding; thus, it leads to a lowering of thermal transition temperature.
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