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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Photoexcitation to generate triplet state has been proved to be the main photoreaction in homogeneous system for many benzoquinone derivatives, including oxidized coenzyme Q (CoQ) and its analogs. In the present study, microemulsion of CoQ, a heterogeneous system, is employed to mimic the distribution of CoQ in biomembrane. The photochemistry of CoQ10 in microemulsion and cyclohexane is investigated and compared using laser flash photolysis and results show that CoQ10 undergoes photoionization via a monophotonic process to generate radical cation of CoQ10 in microemulsion and photoexcitation to generate excited triplet state in cyclohexane. Meanwhile, photoreactions of duroquinone (DQ) and CoQ0 in microemulsion are also investigated to analyze the influence of molecular structure on the photochemistry of benzoquinone derivatives in microemulsion. Results suggest that photoexcitation, which is followed by excited state-involved hydrogen-abstraction reaction, is the main photoreaction for DQ and CoQ0 in microemulsion. However, photoexcited CoQ0 also leads to the formation of hydrated electrons. The isoprenoid side chain-involved high resonance stabilization is proposed to explain the difference in photoreactions of CoQ0 and CoQ10 in microemulsion. Considering that microemulsion is close to biomembrane system, its photoionization in microemulsion may be helpful to understand the real photochemistry of biological quinones in biomembrane system.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Oxidized coenzyme Q (CoQ) or ubiquinone belongs to a series of compounds that share a common benzoquinone ring structure, but differ in the length of the isoprenoid side chain. It functions not only as an electron carrier in the mitochondrial respiratory chain (1,2) and the photosynthesis of bacteria (3,4) but also in its reduced form (ubiquinol) as an antioxidant to scavenge free radical, prevent cell membranes from peroxidation damage, regenerate α-tocopherol, and minimize the effects of oxidative stress (5–9). The main chemical characteristic of CoQ responsible for its various functions is that it can exist in three alternate redox states: the fully oxidized ubiquinone (CoQ), a partially reduced ubisemiquinone (CoQH) and the fully reduced ubiquinol (CoQH2).

For the sake of understanding their role in biological electron transport system, more studies have been carried out by techniques of laser flash photolysis (LFP) and pulse radiolysis to study the photochemistry of CoQ and its analogs (10–17). Previous studies conducted in homogeneous system where CoQ is distributed homogeneously showed that irradiation of absorption light mainly led to photoexcitation of CoQ. In cells, however, CoQ is mainly distributed in cellular membranes where its hydrophobic isoprenoid chain is situated in the hydrophobic region while the polar head group (quinone ring) tends to be close to the water–lipid phase (18–20). Obviously, such heterogeneous distribution of CoQ in biomembrane can not be mimicked by homogenous system.

Microemulsion is thermodynamically and mechanically stable and optically transparent fluids consisting of essentially monodisperse droplets with diameters less than 100 nm, which can be the oil in water (o/w) or water in oil (w/o) type. As CoQ is actually an amphiphilic molecule, its polar head group is able to reach the water/oil interface of microemulsion, whereas its hydrophobic tail is expected to be in the oil phase (Fig. 1). Therefore, microemulsion (o/w) of CoQ is very close to its distribution in membrane. In our research, the photoreaction of CoQ10 in cyclohexane and microemulsion was studied by LFP. It was found that CoQ10 in microemulsion experienced photoionization to produce solvated electron and the radical cation, whereas in cyclohexane underwent photoexcitation to form excited triplet state after irradiation of 266 nm laser pulse. The photoreaction of duroquinone (DQ) and CoQ0 in microemulsion was also investigated, which was compared with that of CoQ10 to make a proposition to explain the different photoreactions of the three quinones in microemulsion.

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Figure 1.  Mimic distribution of CoQ10 in microemulsion.

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Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Chemical reagents.  Acetonitrile, CoQ0 and DQ were purchased from Sigma. Cyclohexane, 1-butanol and phosphate were all analytic grade reagent and commercially available and used without further purification. Sodium dodecyl sulfate (SDS) and CoQ10 (analytic grade reagent) were purchased from Generay Bio-tech (China) and used without further purification. The pH value of the solution was adjusted by phosphate solution (0.02 m pH = 7.2). The solutions were deaerated with high-purity N2 (≥99.99%), N2O, or O2 (≥99.5%) for different purposes by bubbling for at least 20 min prior to the experiments. Ground-state absorption properties were studied using a UV–Vis spectrometer (VARIAN CARY 50 Probe). All experiments were performed in a 1 cm quartz cuvette at room temperature.

Preparation of microemulsion.  In quaternary microemulsion, along with oil, surfactant and water, a long-chain alcohol is used as a cosurfactant. The microemulsion of this study is oil in water (o/w), which was composed of emulsifier 30% (vol/vol), cyclohexane (oil) 10% (vol/vol), and water 60% (vol/vol). The emulsifier was prepared by mixing SDS (surfactant) 25% (m/m), 1-butanol (cosurfactant) 50% (m/m) and water 25% (m/m) togenther. CoQ10 was dissolved in the oil phase (cyclohexane) of microemulsion. The pH values of microemulsion are buffered by 0.02 m phosphate (pH = 7.2).

LFP experiments.  Laser flash photolysis experiments were carried out using Nd:YAG laser of 266 nm light pulses with a duration of 5 ns and the maximum energy of 30 mJ per pulse used as the pump light source. A xenon lamp was employed as detecting light source. The laser and analyzing light beam passed perpendicularly through a quartz cell with an optical path length of 10 mm. The transmitted light entered a monochromator equipped with an R955 photomultiplier. The output signal from the Agilent 54830B digital oscillograph was transferred to a personal computer for data treatment. The LFP setup has been previously described (21–23).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Photoexcitation of CoQ10 in cyclohexane

The triplet state of CoQ0 and CoQ6 (3CoQ0* and 3CoQ6*) in cyclohexane with a maximum absorption around 440 nm have been reported (15,16). The decay at 440 nm including a short-lived process and a longer-lived process can be both accelerated by the presence of O2. In this study, transient absorption spectra from 266 nm LFP of N2-saturated CoQ10 cyclohexane solution shows one maximum absorption peaks around 430 nm (Fig. 2). The decay at 430 nm, also with two different decay processes, which can also be accelerated by O2, has an analogous kinetic character with the decay of 3CoQ6* at 440 nm (16). Therefore, transient absorption around 430 nm is assigned as triplet state of CoQ10 (3CoQ10*). Moreover, the addition of β-carotene (β-Car) leads to the new growth process at 510 nm, which is indicative of generation of the excited triplet state of β-Car (3β-Car*), further confirming the generation of 3CoQ10* (Inset of Fig. 2; Eqs. [1] and [2]). The reaction rate constant of energy transfer from 3CoQ10* to β-Car was estimated as (1.35 ± 0.26) × 1010 m−1 s−1:

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Figure 2.  Transient absorption spectra obtained from LFP (266 nm) of N2-saturated 0.1 mm CoQ10 in cyclohexane recorded at: (■) 0.1 μs, (•) 0.5 μs and (▲) 2 μs after laser pulse. Inset: kinetic decay profile at 510 nm obtained from LFP (266 nm) of N2-saturated 0.1 mm CoQ10 in cyclohexane containing 0.01 mmβ-Car.

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LFP of CoQ10 in microemulsion

As shown in Fig. 3, the absorption of microemulsion at 266 nm is negligible compared with that of CoQ10, CoQ0 and DQ (Scheme 1). LFP (266 nm) of N2-saturated microemulsion of CoQ10 gives a strong and broad absorption band after 500 nm, and three absorption bands centered at 300, 330 and 370 nm, respectively (Fig. 4A). The broad absorption band with maximum absorption around 720 nm can be quenched by typical scavengers of hydrated electrons (eaq), such as O2, N2O and CH3CN, suggesting that eaq is formed immediately after the laser pulse (Fig. 4B). This indicates that photoionization of CoQ10 in microemulsion occurs after irradiation of 266 nm laser pulse.

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Figure 3.  The UV–Vis spectra of microemulsion, 0.1 mm CoQ10 in microemulsion, 0.2 mm CoQ0 in microemulsion and 0.3 mm DQ in microemulsion.

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Figure  Scheme 1. .  Molecular structures of CoQ10, CoQ0 and DQ.

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Figure 4.  (A) Transient absorption spectra obtained from LFP (266 nm) of N2-saturated 0.1 mm CoQ10 in neutral microemulsion recorded at: (■) 0.5, (•) 3 and (●) 10 μs after laser pulse. (B) Transient absorption spectra obtained from LFP (266 nm) of (■) N2-saturated, (•) N2O-saturated and (●) O2-saturated 0.1 mm CoQ10 in neutral microemulsion at 0.5 μs after laser pulse.

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Kinetic analysis shows the decay processes at 300 and 370 nm are hardly influenced by N2O and O2. The decay curve at 330 nm includes an unobvious growth process, which can be eliminated by N2O and O2 (Fig. 5A), suggesting that products formed from reaction of eaq make partial contribution to the absorption at 330 nm. It is explainable by studies of Land and Swallow which showed that reaction of eaq with CoQ forms ubisemiquinone, which had strong absorption around 320 nm (13). The absorption bands around 300, 370 and 500 nm in O2 or N2O-saturated system should be assigned to the radical cation of CoQ10 (CoQ10+•; Fig. 4B). The possible reactions are presented as follows (Eqs. [3] and [4]):

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Figure 5.  (A) Kinetic decay profiles recorded at 330 nm obtained from LFP (266 nm) of (■) N2-saturated, (•) N2O-saturated and (●) O2-saturated 0.1 mm CoQ10 in neutral microemulsion. (B) Dependence of OD0 of eaq at 720 nm on laser intensity immediately after LFP (266 nm) of N2-saturated 0.1 mm CoQ10 in neutral microemulsion.

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To determine whether the photoionization of CoQ10 by irradiation of 266 nm laser pulse is a one- or a two-photon process, the maximum absorption intensity (OD0) of eaq at 720 nm, i.e. yield of eaq, was measured as a function of laser intensity (24). It suggests that the yield of eaq varies linearly with the light intensity, indicating a monophotonic character of the photoionization (Fig. 5B).

LFP of DQ in microemulsion

Many studies have been made to study the photochemistry of DQ in homogeneous system and micelle system (25–32). The main photoreaction of DQ is its photoexcitation to form excited triplet state of DQ (3DQ*) and the photoreduction of 3DQ* by antioxidants and some solvents. LFP (266 nm) of N2-saturated DQ in microemulsion leads to a broad absorption band in the wavelength region between 400 and 500 nm, which can be eliminated by triplet state quencher O2 (Fig. 6A). Therefore, the absorption band between 400 and 500 nm is assigned as the absorption of 3DQ* by analogous comparison with the previously confirmed absorption of 3DQ* (30,31). Sensitization of 3β-Car* with the growth process at 510 nm by addition of β-Car further confirms the generation of 3DQ* (Inset of Fig. 6A). The reaction rate constant of energy transfer from 3DQ* to β-Car was estimated as (3.62 ± 0.45) × 1010 m−1 s−1.

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Figure 6.  (A) Transient absorption spectra obtained from LFP (266 nm) of 0.3 mm DQ in neutral microemulsion N2-saturated recorded at: (■) 0.1, (•) 1 and (●) 5 μs and O2-saturated recorded at (▼) 1 μs after laser pulse. Inset: kinetic growth profile recorded at 510 nm obtained from LFP (266 nm) of N2-saturated 0.3 mm DQ in neutral microemulsion containing 0.016 mmβ-car. (B) Kinetic decay profiles recorded at 440 (a) and 500 nm (b) obtained from LFP (266 nm) of N2-saturated 0.3 mm DQ in neutral microemulsion, and the growth profile (c) obtained by subtraction of kinetic profile at 500 nm from kinetic profile at 440 nm.

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As the decay of 3DQ* in microemulsion (Fig. 6A), there appears a transient absorption spectra centered at 440 nm analogous very well to the typical absorption of DQ•− confirmed in previous studies (32). As 3DQ* is a strong oxidant, it is anticipated, in microemulsion, that electron transfer between 3DQ* and 1-butanol happens to generate DQ•− whose absorption is not clearly observed because of overlap of the absorption of 3DQ*. To clearly observe the growth process of DQ•−, the buildup trace at 440 nm is obtained by subtraction of decay curve at 440 nm from decay curve at 500 nm (Fig. 6B). The result shows that the formation of DQ•− is synchronic with the decay of 3DQ*. In conclusion, photoexcitation of DQ in microemulsion occurs after 266 nm laser pulse with the generation of 3DQ*, which is reduced into DQ•− by 1-butanol (Eqs. [5–7]):

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LFP of CoQ0 in microemulsion

Photoreaction of CoQ0 in microemulsion after 266 nm laser pulse leads to a broad absorption band of eaq, which can be quickly scavenged by eaq trap, and another broad absorption band from 320 to 470 nm, which includes various decay processes (Fig. 7A). The growth process at 410 nm can be reduced by N2O and eliminated by O2 (Fig. 7B), which suggests that reactions of eaq and excited state of CoQ0 (CoQ0*) are associated with the growth at 410 nm. By analogous comparison with the absorption of neutral radical of CoQ0 (CoQH0) around 420 nm (17), the growth process at 410 nm is assigned as the generation of CoQH0 by reduction of CoQ0 by eaq and hydrogen abstraction of CoQ0* from 1-butonal. The photoreaction of CoQ0 in microemulsion is very complex so that other decay processes can not be exactly assigned. However, in the presence of O2, all absorption bands disappears quickly (Fig. 7A), which suggests that CoQ0* might be the precursor of photoionization and other photoreactions. Like the result of 266 nm laser photolysis of CoQ0 in isopropanol, no sensitization of 3β-Car* was observed after addition of β-Car in microemulsion (15).

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Figure 7.  (A) Transient absorption spectra obtained from LFP (266 nm) of 0.2 mm CoQ0 in N2-saturated neutral microemulsion recorded at (•) 0.1 and (■) 1 μs, and O2-saturated neutral microemulsion recorded at 1 μs (●) after 266 nm laser pulse. (B) Kinetic decay profiles recorded at 410 nm obtained from LFP (266 nm) of N2-saturated, N2O-saturated and O2-saturated 0.2 mm CoQ0 in neutral microemulsion.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Possible mechanism for photoionization of CoQ0 and CoQ10

The employed three benzoquinone derivatives (DQ, CoQ0 and CoQ10) have different photochemistry in microemulsion after LFP (266 nm). Although photoexcitation of CoQ0 was deduced to initially occur prior to other reactions, no sensitization of 3β-Car* could be detected. It was reported that upon excitation at 266 nm photoexcitation of CoQ0 initially led to excited singlet state and then a singlet-triplet intersystem crossing process can occur with a very low yield (33). Both the triplet state and the singlet state of CoQ0 have a rather short time. Under the conditions of 266 nm LFP of CoQ0 in cyclohexane, the triplet state formed has a lifetime of 0.65 μs, whereas excited singlet state 7.2 ps, and in polar solvent the lifetime of such excited state becomes even shorter (15,33). Previous studies supposed that the protons on the methyl groups attached to the quinone ring of DQ are appreciably acidic in the ground state (29). Therefore, a possibility of proton loss in 3DQ* was proposed to explain the deactivation of 3DQ* by water in polar solvent. A similar situation might be expected for CoQ0, a p-benzoquinone analog in microemulsion (o/w), which is a highly polar environment. Under the irradiation of 266 nm laser pulses, CoQ0 may be photoexcited to form excited state, which might undergo deprotonation to generate an intermediate anion of the o-quinone methide (A in Scheme 2). Two methoxy groups in CoQ0 are electron donating groups and may make A prone to lose an electron to generate eaq. Meanwhile, another way of deactivating CoQ0*via hydrogen abstraction from 1-butanol also exists (Scheme 2). Therefore, deactivation of CoQ0* may lead to the short lifetime and no sensitization of 3β-Car* in polar microemulsion system.

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Figure  Scheme 2. .  The proposed ways of deactivating excited state of CoQ0via hydrogen abstraction and photoionization.

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However, as for CoQ10 in microemulsion, only photoionization is detected and not influenced by O2. The different photoreactions of CoQ10 and CoQ0 in microemulsion may lie in the isoprenoid side chain linked to benzoquinone ring. Considering that deprotonation of the methylene carbon, which binds the quinone ring and the isoprenoid side chain together will lead to an higher resonance stabilized anion intermediate, we propose, as presented in Scheme 3, that excited CoQ10 undergoes rapid protons loss in the oil/water interface, which is followed by intramolecular electronic rearrangement and proton transfer to form anion of bisphenol CoQ10 (C), which loses an electron to form a neutral radical and eaq. As the higher resonance stabilization make C more stable than o-quinone methide of CoQ0 and DQ, deactivation of excited CoQ10via deprotonation might be much more favorable than CoQ0 and DQ so that transient excited state of CoQ10 that can be quenched by O2 may not form. An alternative possibility is that deprotonation and intramolecular rearrangement of CoQ10 may exist in its stable state. The anion of bisphenol CoQ10 (C) could undergo monophotonic photoionization under irradiation of 266 nm laser pulse, as anion form of phenols are prone to be photoionizated by absorption light via monophotonic process (34,35).

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Figure  Scheme 3. .  The proposed deprotonation and intramolecular rearrangement of CoQ10 in microemulsion.

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Photochemistry of CoQ in biological system

The excited state of quinones produced by absorption light can not only directly damage lipids, peptides and DNAs via hydrogen abstraction or electron transfer (Type-I mechanism) but also react with O2 to induce generation of reactive oxygen species (Type-II mechanism), which are highly harmful to biological system (36–44). The photoexcitation of CoQ in homogeneous system suggests CoQ may potentially cause photodamage to biological system via Type-I or -II mechanism. However, the result in our study suggests CoQ with isoprenoid side chain, may undergo photoionization rather than photoexcitation under irradiation of absorption light in biological systems. Therefore, photodamage to biomolecules induced by triplet state of CoQ in biological systems may not exist. Moreover, in view of the proposed deprotonation and intramolecular rearrangement of CoQ10 in microemulsion, CoQ10, if possible, in its phenolic resonance structure, may serve as an antioxidant to reduce oxidative pressure.

As we proposed elsewhere (Scheme 3), the isoprenoid side chain linked to benzoquinone ring plays a critical role in photoionization of CoQ10 in microemulsion. However, more studies should be carried out to see whether biological quinones (including CoQ and vitamin K) with different length of isoprenoid side chain can be photoionized by absorption light in microemulsion or not, which may give us new insight into the photochemistry of hydrophobic quinones in membrane.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

We have investigated the photochemistry of CoQ0, CoQ10 and DQ in microemulsion using LFP. Monophotonic photoionization of CoQ10 was detected after 266 nm LFP of CoQ10 in microemulsion. The reaction products were characterized as neutral radical of CoQ10 with maximum absorption bands centered at 300, 370 and 500 nm. Under the irradiation of 266 nm laser pulse, photoexcitation is the initial photoreactions of DQ and CoQ0 in microemulsion and followed by other reactions, which can be prevented by the presence of O2. The isoprenoid side chain-involved high resonance stabilization is proposed to lead to different photoreactions of CoQ0, DQ and CoQ10 in microemulsion.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References

Acknowledgements— This work was financially supported by the 973 Program (grant no. 2010CB912604 and 2010CB933901), the National Natural Science Foundation of China (grant no, 31140038), Science and Technology Commission of Shanghai Municipality (grant no. 11411951500), and the Genetically Modified Organisms Breeding Major Projects (no. 2009ZX08011-032B).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
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