Temperature dependent experiments allow the determination of enthalpy and volume changes
for each of the measured transients.  For transients associated with protonation of carboxylates and
histidines it is possible to calculate the activation energy.
The results obtained at prepulse pH = 4.5 and pH = 7 by plotting E_lj_i vs the thermoelastic parameter (C_pr/b) are reported in Table 2.

The enthalpic change (the heat release) accompanying each process (obtained from the intercept of the linear interpolation) is small, of the order of a few kcal/mol, in accordance with the expectation that these reactions are essentially entropy driven.
In particular, the heat realeased when a proton is bound by histidines 24 and 119 is compatible with the enthalpy changes measured with steady state techniques.

The structural volume changes determined with this method (from the slope of the linear interpolation) are in agreement with the values determined with the two-temperatures method.
 
The temperature dependence of the rate  constants allows the determination of the activation energy of the process. The activation of carboxyls is unaffected by the state of the protein before the pH jump.  On the other hand, the barrier for protonation (and the following unimolecular step) is rather high, indicating that this event is responsible for the transition from the stable native state to the less stable compact acid intermediate.
This conclusion supports the results of Baldwin and coworkers (Barrick, Hughson and R. Baldwin (1994).  J. Mol. Biol. 237: 588-601) who suggested that the protonation of his24 and his119 should represent a rate limiting step and a key stabilization factor in the transition from N to I in apomyoglobin.  In our understanding the protonation of these two residues is followed by a rupture of the hydrogen bond between them and leads to a substantial solvation of the hidden hydrophobic residues at the interface between helices A, G and H and the remaining helices. The two histidines stabilize the interaction between the two subdomains by means of their hydrogen bond.