Reduction Potentials of P450 Compounds I and II: Insight into the Thermodynamics of C–H Bond Activation

Abstract

We present a mixed experimental/theoretical determination of the bond strengths and redox potentials that define the ground state thermodynamics for C-H bond activation in cytochrome P450 catalysis. Using redox titrations with [Ir(IV)Cl 6 ] 2− we have determined the compound II/ ferric (or Fe(IV)OH/Fe(III)OH 2) couple and its associated D(O-H) Ferric bond strength in CYP158. Knowledge of this potential as well as the compound II/ferric (or Fe(IV)O/Fe(III)OH) reduction potential in horseradish peroxidase and the two-electron compound I/ferric (or Fe(IV)O(Por •)/ Fe(III)OH 2 (Por)) reduction potential in aromatic peroxidase has allowed us to gauge the accuracy of theoretically determined bond strengths. Using the restricted open shell (ROS) method as proposed by Wright and coworkers, we have obtained O-H bond strengths and associated redox potentials for charge-neutral H-atom-reductions of these iron(IV)-hydroxo and −oxo porphyrin species that are within 1 kcal/mol of experimentally determined values, suggesting that the ROS method may provide accurate values for the P450-II O-H bond strength and P450-I reduction potential. The efforts detailed here indicate the ground state thermodynamics of C-H bond activation in P450 are best described as follows: E 0' Comp-I = 1.22 V (at pH 7, vs. NHE) with D(O-H) Comp-II = 95 kcal/mol, and E 0' Comp-II = 0.99 V (at pH 7, vs. NHE) with D(O-H) Ferric = 90 kcal/ mol.

Computational Approaches for Redox Potentials of Iron(IV)‐oxido Complexes

“…The thermodynamics of PCET (or CPET, concerted proton electron transfer) have been studied extensively, and there are common protocols based on a Bordwell‐Polanyi analysis that allow to relate the redox potential E ° with the p K a value and the bond dissociation free energy [Equation ()]: [ 16 ] where the bond dissociation free energy BDFE (O–H) is in kcal · mol –1 , the proton coupled redox potential E ° in V vs. Fc/Fc + , and C G,sol relates to the standard reduction potential of H + and has been determined for a number of solvents; the values used for acetonitrile and water at 298 K vs. Fc/Fc + (the latter vs. SHE) in kcal · mol –1 are 54.9 and 57.6, respectively, and the latter was used here – the experiments for obtaining the PCET thermodynamic data were performed in “wet” acetonitrile, and the differences in computed p K a data for the two solvents are minor. [ 16 ] The BDFE derives from Equation (): [ 58 ] …”

“…This was followed by a frequency analysis at the UB3LYP/def2‐TZVP level. The last step involved scaling of the zero‐point energies and enthalpy corrections by 0.9806 and 0.9989 [ 58 ] – admittedly, the scaling is dependent on the system and in the present case this might be the source for a minor systematic error.…”

Deciphering the origin of million-fold reactivity observed for the open core diiron [HO–FeIII–O–FeIVO]2+species towards C–H bond activation: role of spin-states, spin-coupling, and spin-cooperation

“…Further, we have also estimated the BDE OH /D CH values 5,72,[164][165][166][167][168][169][170][171] for the three different pathways we described and our calculated BDE O1H1 /D C1H1 values for the I-2 Int hs-hs , II-2 Int hs-hs , and III-2 Int hs-hs are 382.0/388.6, 311.0/390.7 and 284.6/389.0 kJ mol À1 (see the Thermochemistry section for equations eqn (1)-(3) in the ESI † for details). These values suggest that the terminal Fe IV ]O species is a much better oxidant than either terminal Fe III -OH or bridged m-oxo for hydroxylation as well as desaturation reactions.…”

Strongly Coupled Redox-Linked Conformational Switching at the Active Site of the Non-Heme Iron-Dependent Dioxygenase, TauD

“…The E½ of the ferrous isomer of TauD reported here is over 0.5 V higher than the Fe 2+/3+ transition in CYP450. 45 If this isomer conformation is transiently retained during the catalytic cycle, the F3/F4 transition in TauD may also have a much higher E½ than that of Cmp I/II of CYP450 while the pKa of F3 is lower than that of Cmp II. In this case, the ferrous isomer conformation would promote deprotonation of F3 that favors the alkoxide pathway in TauD over the hydroxyl radical rebinding pathway, as found in CYP450.…”