Premium
Theory of chemical bonds in metalloenzymes. XIV. Correspondence between magnetic coupling mode and radical coupling mechanism in hydroxylations with methane monooxygenase and related species
Author(s) -
Saito Toru,
Shoji Mitsuo,
Isobe Hiroshi,
Yamanaka Shusuke,
Kitagawa Yasutaka,
Yamada Satoru,
Kawakami Takashi,
Okumura Mitsutaka,
Yamaguchi Kizashi
Publication year - 2010
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.22918
Subject(s) - diradical , chemistry , singlet state , hydroxylation , methane monooxygenase , density functional theory , computational chemistry , photochemistry , spin states , triplet state , radical , singlet fission , molecule , methane , excited state , organic chemistry , atomic physics , physics , inorganic chemistry , enzyme
Abstract Broken‐symmetry (BS) and approximate spin‐projected (AP) BS hybrid density functional theory (DFT) calculations were performed to elucidate possible mechanisms of hydroxylation reactions of methane and alkanes with soluble methane monooxygenase (sMMO) and related metalloenzymes. The BS HDFT (UB3LYP) method was employed to elucidate electronic and spin structures of the key intermediate “Q” and to locate transition structures for hydroxylation reactions in the lowest‐spin (LS) singlet and the highest‐spin (HS) states of sMMO. The spin density populations and chemical indices obtained by the BS B3LYP calculations were found to be consistent with orbital interaction models for hydroxylation with MMO. However these indices in turn indicated significant spin contamination errors in the BS LS solution. The elimination of the errors with the AP procedure indeed reduced the barrier height for the recombination step of alkyl and hydroxyl radicals in the pure LS singlet state, leading to a rebound process. Then present computational results indicated that hydroxylation reactions proceed through the continuous diradical (diradicaloid) mechanism without discreet free radical fragments in the pure LS singlet state. The computational results are, respectively, compatible with local singlet (SD) and local triplet (TD) diradical mechanisms for hydroxylation in the LS and HS states; those were already applied to P450 successfully. Thus magnetic (exchange) coupling modes (LS and HS) in MMO, P450 and related metalloenzymes are directly related to local SD and TD mechanisms for hydroxylation, indicating the correspondence between the magnetic coupling mode and the radical reaction mechanism. These theoretical results enable us to examine recent BS hybrid DFT computational results for hydroxylation reactions with sMMO by several groups. Implications of the present theoretical and computational results are also discussed in relation to several experimental aspects of hydroxylation reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010