Kinetics of thermal unfolding of lysozyme

A reversible unfolding of lysozyme has been studied by means of the temperature‐jump method by measuring the absorbance change of pH indicator p‐nitrophenol (p‐NP). In the presence of 4.5M lithium bromide three relaxation processes were found. The fastest process with a relaxation time τ of about 30 μsec is found to come from the proton‐transfer reaction between p‐NP and His 15 in lysozyme. Only the slowest process (τ ≃ 15 sec) is a first‐order reaction and attributable to the unfolding of lysozyme. These results suggest that the all‐or‐none model can be applied to the unfolding of lysozyme as a fairly good approximation. This shows good agreement with the result of Tanford et al. (J. Mol. Biol., 15, 489 (1966); J. Mol. Biol., 73, 185 (1973)). The enthalpy, entropy, and specific heat change between a native and a denatured form are found to be close to 53 kcal/mol, 168 cal/mole/°K, and 2.1 kcal/mol/°K, respectively. The enthalpy and entropy of activation and the specific heat change for the unfolding and the folding are also obtained from kinetic experiments. In the folding reaction, the enthalpy of activation is negative. In the unfolding reaction, the specific heat change is nearly equal to zero. These results suggest that a number of intramolecular hydrogen bonds are broken, but most residues remain tightly folded in the activated state. Our results in concentrated LiBr solutions are compared with those of Tanford in concentrated GuHCl solutions.