Redox potentials of ubiquinone, menaquinone, phylloquinone, and plastoquinone in aqueous solution

Yuki Kato,Keisuke Saito,Shinnosuke Kishi,Hiroshi Ishikita

doi:10.1007/s11120-017-0433-4

Yuki Kato, Keisuke Saito + Show 2 more

Open Access

https://doi.org/10.1007/s11120-017-0433-4

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Journal: Photosynthesis Research	Publication Date: Aug 22, 2017
Citations: 38	License type: open-access

Affiliation: Nagoya University, The University of Tokyo

Abstract

Quinones serve as redox active cofactors in bacterial photosynthetic reaction centers: photosystem I, photosystem II, cytochrome bc1, and cytochrome b6f. In particular, ubiquinone is ubiquitous in animals and most bacteria and plays a key role in several cellular processes, e.g., mitochondrial electron transport. Their experimentally measured redox potential values for one-electron reduction Em(Q/Q·−) were already reported in dimethylformamide (DMF) versus saturated calomel electrode but not in water versus normal hydrogen electrode (NHE). We calculated Em(Q/Q·−) of 1,4-quinones using a quantum chemical approach. The calculated energy differences of reduction of Q to Q·− in DMF and water for 1,4-quinone derivatives correlated highly with the experimentally measured Em(Q/Q·−) in DMF and water, respectively. Em(Q/Q·−) were calculated to be −163 mV for ubiquinone, −260 mV for menaquinone and phylloquinone, and −154 mV for plastoquinone in water versus NHE.

Highlights

Quinones can accept two electrons and two protons via the initial protonation of semiquinone (Q·− to QH·) and the second protonation of hydroquinone (QH− to QH2)
The calculated ΔEQM/polarizable continuum model (PCM) for reduction of deprotonated Q to Q·− for ten 1,4-quinones in DMF (ΔEQM/PCM(DMF)) and water (ΔEQM/PCM(water)) were highly associated with the experimentally measured Em(Q/Q·−) in DMF, ranging from −401 to − 751 mV versus saturated calomel electrode (SCE) (Prince et al 1983), and the experimentally measured Em(Q/Q·−) in water, ranging from −240 to 99 mV versus normal hydrogen electrode (NHE) (Swallow 1982), which were best fitted to the following equations (Figs. 3a, b): Em(Q∕Q⋅−) in DMF versus SCE [mV] = − 32.1 (ΔEQM∕PCM(DMF) + 108.54 kcal∕mol ) (4)
The present study shows that Em(Q/Q·−) is −260 mV for menaquinone in water versus NHE (Table 1); the calculated Em(Q/Q·−) can be confirmed by Eq 6, which can be reproduced by adding 480 mV to Em(Q/Q·−) in DMF versus SCE

Summary

Introduction

Quinones can accept two electrons and two protons via the initial protonation of semiquinone (Q·− to QH·) and the second protonation of hydroquinone (QH− to QH2). Ubiquinone serves as an electron acceptor at the QA and QB binding sites in reaction centers of purple bacteria (PbRC) from Rhodobacter sphaeroides and serves as an electron donor in cytochrome bc. Plastoquinone serves as an electron acceptor at the QA and QB sites in photosystem II (PSII) (Fig. 1) (Robinson and Crofts 1984; Rutherford et al 1984; Okamura et al 2000; Brettel and Leibl 2001; Wraight 2004) and serves as an electron donor in cytochrome b6 f. In PbRC and PSII, both QA and QB are located near the non-heme Fe2+, and the Fe2+ ligands (i.e., His-L190 and His-M217 (or M219) in PbRC and D1-His215 and D2-His214 in PSII) donate an H-bond to the carbonyl O atoms of quinones that are proximal to the Fe complex (Oprox) (Fig. 1a–c). Redox potential values for one-electron reduction, Em(Q/Q·−), for 1,4-quinones, including ubiquinone, menaquinone (phylloquinone), and plastoquinone, were experimentally measured in dimethylformamide (DMF) versus saturated calomel electrode (SCE) by Prince et al. Vol.:(0123456789)

Computational procedures

Results and discussion

Conclusion