Reorganization enthalpy

Intramolecular Ru(II) to Cu(II) ET rates have been measured in two other blue copper proteins, stellacyanin [42, 43] and azurin [9, 13, 28]. Pseudomonas aeruginosa azurin has been ruthenated at His83 [13] (Fig. 5). The intramolecular Ru(II) to Cu(II) ET rate of 1.9 s was found to be independent of temperature [28]. The Cu reorganization enthalpy was estimated to be < 7 kcal/mol [13, 28], a value confirming that blue copper is structured for efficient ET. Again, a blue copper ET rate is low in comparison with heme protein rates over similar distances (at similar driving forces) (Table 1). 

Additional insight into the various energetic contributions to X may be derived from the temperature dependence of k. Within the Marcus framework, it is possible to extract the enthalpy of reorganization of the protein from the observed activation enthalpy. The reorganization enthalpies for cytochrome c and azurin have an upper limit of about 0.3 eV (7 kcal/mol), whereas the value for myoglobin is about 0.9 eV (20 kcal/mol) (77). The experimental reorganization energy for cytochrome c is consistent with a theoretical prediction of this value obtained... 

The first studies of ground-state ET in modified proteins gave ET rates of 0.02-53 s for ET distances of 11.7-12.7 A (see Table VII). The activation enthalpy for cytochrome c is lower than that for myoglobin. Equation 10 (111) was used to evaluate the reorganization enthalpy due to the heme center in each of these proteins. In Eq. 10, AHf2 is the reorganization enthalpy for... 

According to the Marcus theory [9], the electron transfer rate depends upon the reaction enthalpy (AG), the electronic coupling (V) and the reorganization energy (A). By changing the electron donor and the bridge we measured the influence of these parameters on the charge transfer rate. The re-... 

T - mean enthalpy of disruption to ground state products AHcq - valence reorganization energy of CO ... 

Figure 5.5 Thermochemical cycles relating O-H bond enthalpy contributions ( s) with bond dissociation enthalpies (DH°) in phenol and ethanol. ER are reorganization energies (see text).

The value obtained for the reorganization energy of PhO, -29.3 kJ mol together with the PhO-H bond dissociation enthalpy, lead to... 

Let us concentrate on the thermochemical cycle of figure 5.6 that involves the disruption of the complex Cr(CO)3(C6H6). The enthalpy of this reaction, previously calculated as 497.9 10.3 kJ mol-1 from standard enthalpy of formation data, can be related (equation 5.24) to the bond enthalpy contributions EsfCr-CO ) andE s(Cr (V.He) through the reorganization energies ER(C() ) and ER(C(tHf )- Two asterisks indicate that the fragment has the same structure as... 

Figure 5.6 Thermochemical cycles to estimate the Cr-CgHg bond enthalpy contribution (fs) in Cr(CO)3(C6hl6). ER are reorganization energies. One asterisk indicates that the fragment has the same structure as in Cr(CO)6, and two asterisks mean that the fragment has the same structure as in Cr(CO)3(CgH6)-...

We are now left with the evaluation of E s (Cr—CO), the Cr-CO bond enthalpy contribution in Cr(CO)6. The third thermochemical cycle in figure 5.6 shows how this bond enthalpy contribution can be evaluated from the Cr-CO mean bond dissociation enthalpy (107.0 0.8 kJ mol-1 see section 5.2) and the reorganization energy ER(CO ). 

Figure 2.3. The enthalpy dependence (—AE, kcal/mol) as a function of the solvent reorganization energy for the rate of proton transfer when Ea — 1.0 kcal/mol, Eq = 1.0 kcal/mol, AQ — 0.1 A and Gig = 200 cm-1. Rates are normalized to the maximum rate constant for proton transfer. Larger graph Es = 2.0 kcal/mol. Smaller graph Es = 7.0kca/mol.

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