Molecular orbital calculations may be used to predict trends in the standard potentials of conjugated molecules, such as the quinones and flavins that are involved in biological electron transfer reactions (Impact I7.2). It is commonly assumed that decreasing the energy of the LUMO enhances the ability of a molecule to accept an electron into the LUMO, with an attendant increase in the value of the molecule's standard potential. Furthermore, a number of studies indicate that there is a linear correlation between the LUMO energy and the reduction potential of aromatic hydrocarbons (see, for example, J.P. Lowe, Quantum chemistry, Chapter 8, Academic Press (1993)).
(a) The standard potentials at pH = 7 for the one-electron reduction of methyl-substituted lA-benzoquinone (13) to their respective SEM quinone radical anions are
Using molecular modeling software and the computational method of your choice (semi-empirical, ab initio, or density functional theory methods), calculate EWMO' the energy of the LUMO of each substituted lA-benzoquinone, and plot EWMO against E~. Do your calculations support a linear relation between EWMO and E?
(b) The lA-benzoquinone for which Rz = R3 = CH3 and Rs = R6 = OCH3 is a suitable model of ubiquinone, a component of the respiratory electron transport chain (Impact I7.2). Determine EWMO of this quinone and then use your results from part (a) to estimate its standard potential.
(c) The l, 4-benzoquinone for which R2 = R3 = CH3 and R6 = H is a suitable model of plastoquinone, a component of the photosynthetic electron transport chain (Impact I7.2). Determine EWMO of this quinone and then use your results from part (a) to estimate its standard potential. Is plastoquinone expected to be a better or worse oxidizing agent than ubiquinone?
(d) Based on your predictions and on basic concepts of biological electron transport (Impact I7.2 and I23.2), suggest a reason why ubiquinone is used in respiration and plastoquinone is used in photosynthesis.

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0.078 CH H CH, H CH CH CH H CH CH CH CH CH, H -0.067 -0.165 0.260