MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2005; 43: S237–S247 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1669 Extended Car–Parrinello molecular dynamics and electronic g-tensors study of benzosemiquinone radical anion † James R. Asher, 1 Nikos L. Doltsinis 2 and Martin Kaupp 1* 1 Institut f ¨ ur Anorganische Chemie, Universit ¨ at W ¨ urzburg, Am Hubland, D-97074 W ¨ urzburg, Germany 2 Lehrstuhl f ¨ ur Theoretische Chemie, Ruhr-Universit ¨ at Bochum, D-44780 Bochum, Germany Received 20 April 2005; Revised 6 June 2005; Accepted 6 June 2005 Car-Parrinello molecular dynamics simulations of benzoquinone and benzosemiquinone radical anion in both aqueous solution and the gas phase have been carried out at ambient conditions. Hydrogen bonding is considerably more extensive to the anionic than to the neutral aqueous system. In addition to the conventional hydrogen bonding to the carbonyl oxygen atoms, T-stacked hydrogen bonding to the p-system is statistically and energetically significant for the semiquinone anion but not for the neutral quinone. EPR g-tensors have been calculated at DFT level for snapshots taken at regular intervals from the gas-phase and solution semiquinone anion trajectories. Different criteria for extraction of semiquinone/water clusters from the solution trajectory give insight into the effects of different interactions on the g-tensor, as does correlation of the g-tensor with statistically significant hydrogen-bond configurations identified along the trajectories. Comparison of gas-phase and solution results indicates opposite directions of direct electronic and indirect structural influences of hydrogen bonding on g-tensors. Short-time oscillations in g x along the trajectory are due mainly to C – O bond vibrations. Copyright  2005 John Wiley & Sons, Ltd. KEYWORDS: aqueous benzosemiquinone and benzoquinone; bioradicals; Car-Parrinello molecular dynamics simulations; density functional theory; electronic g-tensor; EPR spectroscopy; hydrogen bonding INTRODUCTION Quinones, along with their singly reduced semiquinone and doubly reduced quinol forms, are molecules of considerable biological importance due to their presence in all organisms as electron transfer agents, particularly in respiration and photosynthesis. 1 Electron paramagnetic resonance (EPR) spectroscopy has proven itself a valuable tool for examining the anionic semiquinone radicals. 2 Especially, the recent development of high-field EPR methods has made possible elucidation of the anisotropy of the electronic g-tensor of semiquinones and other bioradicals. 3 The g-tensor is sensitive to intermolecular interactions such as hydrogen bonding, and is thus useful for probing the molecular environment of a bioradical, whether in solution, a protein binding site or a phospholipid membrane. An increasingly important complement to experimental measurements of the g-tensor and other EPR parameters † Presented as part of a special issue on High-field EPR in Biology, Chemistry and Physics. L Correspondence to: Martin Kaupp, Institut f ¨ ur Anorganische Chemie, Universit¨ at W ¨ urzburg, Am Hubland, D-97074 W ¨ urzburg, Germany. E-mail: kaupp@mail.uni-wuerzburg.de Contract/grant sponsor: Deutsche Forschungsgemeinschaft; Contract/grant number: Priority Program SPP1051. Contract/grant sponsor: Leibniz Rechenzentrum Munchen; Contract/grant number: HLRB project h0731. is quantum-chemical calculation of the same. Qualitative insight into factors affecting the g-tensor of a molecule and quantitative checks on the fit between proposed structures and experimental EPR data can both be obtained by construction of appropriate model systems and performance of property calculations thereon. A recent example of this is the identification, based on comparison of computed and measured EPR data, of ‘T-stacked’ hydrogen bonds from a tryptophan group to the -system of the semiquinone carbon ring at the binding site of a reconstituted photosystem I (PS-I) variant 4 – an important interaction which has been suggested to modify the redox potential of the Q/Q ž couple so as to make the neutral quinone a better electron acceptor. Such quantitative applications have become possible because of the recent progress in density functional the- ory (DFT) approaches for calculating g-tensors. 5 Other approaches, based on Hartree–Fock 6 and semiempirical molecular orbital (MO) methods, 7 have also been applied to semiquinone radical anion g-tensors. However, the com- bination of accuracy and computational efficiency makes the DFT-based route currently the preferred choice, 4,5,8–13 even though standard (gradient-corrected) density function- als overestimate the largest (g x ⊳ component of the g-tensor in semiquinones by about 10% (as the overestimate is system- atic, it may be easily corrected by scaling). 4,8–10 The standard modus operandi for quantum-chemical stud- ies of EPR parameters like g-tensors or hyperfine couplings Copyright  2005 John Wiley & Sons, Ltd.