Hydrogen-Bonding Effect on ¹³C and Proton Hyperfine Couplings of [4-¹³C]-Labeled Ubisemiquinones in Vitro

Abstract

EPR spectra of ubiquinone-0 (UQ 0 ) and UQ 10 anion radicals selectively labeled with 13 C at the 4-CdO position are reported. The environmental effect on the 4-13 C splitting constant was studied in mixed solvents in which the molar fraction of the protic component was changed over a wide range. The quinones were electrochemically reduced, with cyclic voltammetry and concomitant optical spectroscopy being performed as controls. The value and the sign of the 4-13 C hyperfine splitting constant strongly depend on the H-bonding properties (proticity) of the solvent, due to the formation of mono-and disolvates through hydrogen bonding of the carbonyl oxygens in protic solvents. The formation of disolvates is less favorable by a factor of 80 than formation of the monosolvated anion. The results are discussed in light of recent EPR, FTIR, and NMR experiments on the primary 13 C-labeled acceptor quinone Q A in bacterial photosynthetic reaction centers. We propose that in the neutral state both carbonyls of Q A are not or only weakly hydrogen bonded to the protein and that formation of anionic Q A •-results in a much stronger H-bond for the 4-carbonyl only. The large shift of the 4-CdO IR signal reported for UQ in ViVo is in our view mostly due to a change in bond order induced by binding of UQ to the protein, perhaps through a change in sp 2 hybridization at the 4-C position.

Influence of hydrogen bonds on the electronicg-tensor and13C-hyperfine tensors of13C-labeled ubiquinones — EPR and ENDOR study

“…The isotropic 13 Chfcs, including signs obtained from the EPR spectra, are shown in Table 2. They agree well with the values published for (4-13 C) ubisemiquinone-10 [29]. ENDOR experiments were performed in liquid solutions yielding accurate values for the proton hfcs given in Table 2.…”

“…6 1/3Tr(A) is the isotropic part obtained from the full tensor. a l , o is the isotropic hfc measured in liquid solution with generally good agreement concerning magnitudes and signs of as" measured independently in [9,10,29] for UQo' and UQ I O' in alcohols and DME. of the molecules could not be detected in the accessible temperature range (150-330 K) limited by solvent properties.…”

“…Determination of 13 C it-Spin Densities. pr-Spin Densities from the Experiments In a previous EPR study of UQio in liquid solution 13 C it-spin densities were estimated from the isotropic hfcs [29] using the procedure of Fraenkel and Karplus [30]. However, since the isotropic 13 C-hfc is determined by it -6 spin polarization from neighboring carbon and oxygen atoms, and different contributions with opposite signs cancel each other, the 13 C it -spin densities obtained in this way depend strongly on the spin densities of the neighboring atoms (ring carbons and oxygens), which are not exactly known.…”

“…', (Table 3) corrected by the contributions of neighboring carbons and oxygen. About 40% larger values are calculated in[29] from the isotropic hfc using the Karplus/Fraenkel approach (see text). C5 and C6 are calculated from the isotropic hfc ofTable 2using the McConnell and HellerMcConnell equation with QH = -27 G and B2 = 50 G (Eqs.…”

Recent advances in 1,4-benzoquinone chemistry

“…231 This principal is verified by the ab initio studies of neutral and anionic 1,4-benzoquinone-water clusters by Manojkumar et al 232 They observed that when two water molecules are complexing with 1,4-benzoquinone, a conformer exhibiting a H-bond between two water molecules (W 2 Q) is energetically more favoured than the conformer WQW in which there is no direct interaction between the water molecules. The geometry of the structures were optimized at MP2 and B3LYP level using the 6-311++G** basis set.…”