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Article

Thermodynamic and Kinetic Studies on the Binding of Nitric Oxide to a New Enzyme Mimic of Cytochrome P450

Supporting Info

Alicja Franke,^† Natalya Hessenauer-Ilicheva,^† Dominik Meyer,^‡ Grażyna Stochel,^§ Wolf-D. Woggon,*^‡ and Rudi van Eldik*^†

Contribution from the Institute for Inorganic Chemistry, University of Erlangen-Nrnberg, Egerlandstrasse 1, 91058 Erlangen, Germany, Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland, and Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30 060 Krakw, Poland.

J. Am. Chem. Soc., 2006, 128 (41), pp 13611–13624

DOI: 10.1021/ja060650o

Publication Date (Web): September 22, 2006

†

University of Erlangen-Nürnberg.

‡

University of Basel.

Jagiellonian University.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, vaneldik@chemie.uni-erlangen.de

Abstract

A new model for the P450 enzyme carrying a SO₃^- ligand coordinated to iron(III) (complex 2) reversibly binds NO to yield the nitrosyl adduct. The rate constant for NO binding to 2 in toluene is of the same order of magnitude as that found for the nitrosylation of the native, substrate-bound form of P450_cam (E·S-P450_cam). Large and negative activation entropy and activation volume values for the binding of NO to complex 2 support a mechanism that is dominated by bond formation with concomitant iron spin change from S = ⁵/₂ to S = 0, as proposed for the reaction between NO and E·S-P450_cam. In contrast, the dissociation of NO from 2(NO) was found to be several orders of magnitude faster than the corresponding reaction for the E·S-P450_cam/NO system. In a coordinating solvent such as methanol, the alcohol coordinates to iron(III) of 2 at the distal position, generating a six-coordinate, high-spin species 5. The reaction of NO with 5 in methanol was found to be much slower in comparison to the nitrosylation reaction of 2 in toluene. This behavior can be explained in terms of a mechanism in which methanol must be displaced during Fe−NO bond formation. The thermodynamic and kinetic data for NO binding to the new model complexes of P450 (2 and 5) are discussed in reference to earlier results obtained for closely related nitrosylation reactions of cytochrome P450_cam (in the presence and in the absence of the substrate) and a thiolate-ligated iron(III) model complex.