Photochromic coenzyme Q derivatives: Switching redox potentials with light

Article abstract of DOI:10.1039/c7sc00781g

Coenzyme Q is an important redox cofactor involved in a variety of cellular processes, and is thus found in several cell compartments. We report a photochromic derivative of coenzyme Q that combines the molecular structures of the redox active cofactor and a photochromic dye. Light irradiation triggers an electronic rearrangement reversibly changing the redox potential. We used this effect to control the intermolecular redox reaction of the photochromic coenzyme Q derivative with dihydropyridine in solution by light irradiation. On mitochondria, the altered redox properties showed an effect on the respiratory chain. The experiments demonstrate that the redox reactions can be initiated inside the system of interest through irradiation with light and the accompanied photoisomerization.

Full text of DOI:10.1039/c7sc00781g

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Photochromic Coenzyme Q Derivatives: Switching
Redox Potentials with Light
Nadja A. Simeth,a Andrea C. Kneuttinger,b Reinhard Sternerb,* and Burkhard Königa,*
a University of Regensburg, Faculty of Chemistry and Pharmacy, Institute of Organic Chemistry,
Universitätsstraße 31, 93053 Regensburg, Germany.
b
University of Regensburg, Faculty of Biology and Preclinical Medicine, Institute of Biophysics
and Physical Biochemistry, Universitätsstraße 31, 93053 Regensburg, Germany.
* Corresponding authors. Phone: +49 941 943ꢀ4575; Email address: burkhard.koenig@ur.de
(B. König); reinhard.sterner@ur.de (R. Sterner).
Keywords
Photoswitch, Photochromic compound, Photochromism, Photopharmacology, Redox potential,
Coenzyme Q
Abstract
Coenzyme Q is an important redox cofactor involved in a variety of cellular processes, and thus
found in several cell compartments. We report a photochromic derivative of coenzyme Q that
combines the molecular structures of the redox active cofactor and a photochromic dye. Light
irradiation triggers an electronic rearrangement reversibly changing the redox potential. We used
this effect to control the intermolecular redox reaction of the photochromic coenzyme Q
derivative with dihydropyridine in solution by light irradiation. On mitochondria, the altered
redox properties showed an effect on the respiratory chain. The experiments demonstrate that
the redox reactions can be initiated inside the system of interest through irradiation with light and
the accompanied photoisomerization.

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1. Introduction
The concept of employing small, lightꢀresponsive molecules to gain a spatiotemporal control
over biological processes is known for a long time.1 However, in recent years, the field, now
often termed as photopharmacology, expanded rapidly.2ꢀ5 Photochromic inhibitors based on azo dyes
or bridged diarylethenes (DAEs) were used to modulate enzymatic activities.6ꢀ11 Moreover,
switchable antibiotics12ꢀ14 and molecules, which cause lightꢀinduced apoptosis of single cells in
tissue15 illustrate the potential of the approach. Using light as an external stimulant adds an
extrinsic parameter determining the selectivity of the compound.3 Furthermore, light is nonꢀ
invasive, easy to apply, and offers an inimitable spatiotemporal control over the process of
interest.1 Both azobenzenes and DAEs interconvert upon light irradiation between two structural
isomers, which differ strongly in their physical properties, and thus also in their activity towards
biomolecules.1, 2 DAEs generally show good photoconversion, fatigue resistance, and thermal
stability. They are thus frequently used as photochromic tools in biological contexts.1, 16
While several analogues of DNA bases,17, 18 fatty acids, and phospholipids19ꢀ23 have been reported,
the number of photoswitchable derivatives of enzymatic cofactors such as ATP, NAD(P)H,
FAD, CoQ, is small. Recently, Wilson et al. presented a photochromic analogue of pyridoxalꢀ5’ꢀ
phosphate,24 and furthermore ATP has been derivatized into a lightꢀcontrollable mimetic.25
However, redox cofactors, a likewise widely spread family of coenzymes, have not been
considered so far.26 They are involved in many cellular processes, and therefore found in various
cellular compartments. They facilitate e.g. transhydrogenation reactions catalyzed by
oxidoreductases by either accepting or providing electrons.27 In general, they act as an activated
electron carrier for the oxidation of metabolites in aerobic organisms. An important auxiliary
molecule involved in this process is Coenzyme Q (CoQ), also called ubiquinone. The core part of
the molecule, the benzoquinone ring, facilitates the molecule to serve as a twoꢀelectron carrier –
interconverting between a ubiquinone and an ubiquinol form. Due to its large, nonpolar, polyꢀ
isoprenyl chain, the coenzyme is able to diffuse within lipophilic membranes.28 Besides its
30
function as an auxiliary in metabolism,26 it also acts as an antioxidant,29,
regulates the
physiochemical properties of membranes,31 and modulates the amount of βꢀintegrins on the
surface of blood monocytes.32
As most of the cofactors, redox cofactors are not only present in one single cellular
compartment, but widely distributed. Thus, targeting them with conventional strategies of
medicinal chemistry would cause an uncontrollable impact on all cells of the organism and it
would be difficult to regulate the effect in the desired fashion. However, using a

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photopharmacological strategy, we are able to activate the agent specifically at the favored site for
a defined time.
For our CoQ mimetic, we merged a redoxꢀactive benzoquinone moiety with two substituted
thiophene moieties, building a photochromic DAE. Through irradiation with light, the 6πꢀsystem
formed by the three parts undergoes an electrocyclic ringꢀclosing reaction. The ringꢀclosure
changes color, shape, and electronic properties of the molecule. Most applications of
photochromic compounds aim at an altered structure with different physiological effect, but only
few studies utilized the different electronic properties of both photoisomeres.24, 33 However, we
focus on changing the redox potentials of the benzoquinone moiety through electronical changes
upon irradiation. This specific mode of action may allow in principle to use one cofactor mimetic
to target all related enzymes even if the active site differs, as the molecule’s mode of action does
not rely on its spatial interaction, but its redox state. Moreover, benzoquinone motifs have been
considered as a scaffold by Deng et al. and Katsumura et al. and were investigated regarding their
switching behavior.34, 35
Lightꢀ and electronicꢀmodulated switches have already been a subject of physicalꢀelectrochemical
as well as spectroscopic studies,36ꢀ41 but to the best of our knowledge, have never been considered
as a the role of a cofactor in biological context. Thus, we present the first photochromic molecule
that has the potential to serve as a redox cofactor.

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2. Results and Discussion
2.1.
Design
To modulate the redox properties of Coenzyme Q by light irradiation, we designed the
photochromic derivatives 1ao and 1bo as depicted in Figure 1 by merging a redoxꢀactive
benzoquinone with thiophene moieties to build a DAE. This allows the molecule to accept
electrons for redox reactions just in one photoisometic state. Upon irradiation by light of 254 nm,
the redoxꢀswitch undergoes a 6πꢀconrotatory electrocyclic ring closure to the closed photoisomer
(1ao,bo to 1ac,bc, Figure 1). The rearrangement delocalizes one double bond of the redoxꢀactive
moiety in the conjugated backbone of the closed switch. The “closed photoisomer” should be
thus mostly redoxꢀinactive as the quinone system was intercepted. In contrast, the “open form”
should be able to undergo oxidation reactions similar to unaltered CoQ (1ao,bo to 1ao,red,bo,red
,
Figure 1). The methoxy groups of the benzoquinone moiety (1bo) are known to be crucial for
interaction at the binding site of CoQꢀconsuming proteins, and are therefore retained in the
model compound (Figure 1).42
Figure 1: General principle of switchable redox potentials based on photochromic CoQ
mimetics.

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2.2.
Synthesis of the Photochromic Coenzyme Q Derivatives
The key step of the synthesis towards compounds 1ao and 1bo was a Stille crossꢀcoupling reaction
of 5a or 5b onto quinone 8a or 8b, respectively (Scheme 1). The thiophene precursors (5a and
5b) were easily synthesized from 2,5ꢀdimethyl thiophene (2) or 2ꢀmethyl thiophene (3),
43
respectively, partly following known protocols.34, After bromination and arylation through
SuzukiꢀMiyaura crossꢀcoupling the preꢀfunctionalized thiophenes 5a and 5b were treated with
secBuLi and subsequently SnClBu3 yielding organo stannates 6a and 6b.35 The quinone 8b was
synthesized through oxidation of 7 using Fenton’s reagent in acetic acid.44 Benzoquinone (8a) as
well as 2,3ꢀdimethoxy benzoquinone (8b) were then halogenated utilizing bromine under
oxidative conditions.45 The final step was performed employing Pd(PPh3)4 and elevated
temperatures for 16h to give 1ao and 1bo in acceptable yields (10% or 29%, respectively).34
Scheme 1: Synthesis of photochromic Coenzyme Q mimetics 1ao and 1bo: a: NBS, AcOH, 2h,
66%. b: Br2, AcOH, 18h, r.t., 69%. c: PhB(OH)2, Pd(PPh3)2Cl2, AsPh3, Na2CO3 (2M, aq), THF,
sec
16h, 80 °C, 80%. d BuLi, SnBu3Cl, dry THF, ꢀ78 °C to r.t., quantitative. e: K3[Fe(CN)6], H2O2,
AcOH, r.t., 28%. f: Br2, H2SO4, Et2O, 1h; then Ag2O, Et2O, 1h, 30% for 9a; 55% for 9b. g:
Pd(PPh3)4, CuI, dry toluene, 85 °C (1ao) or 115 °C (1bo), 16h, 10% (1ao) or 29% (1bo).
The open form of the photochromic quinones (1ao and 1bo) were converted into the
corresponding closed isomers (1ac and 1bc) through irradiation with UV light (Scheme 2).
Compound 1ao did not tolerate the longꢀterm irradiation and decomposed. However, closed
isomer 1bc was isolated from reversed phase column chromatography (13% yield).

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Scheme 2: Synthesis of closed isomers 1ac and 1bc.
2.3.
Spectroscopic, electrochemical and spectroelelectrochemical studies
The photoisomerization of compound 1ao and 1bo was investigated in toluene, dimethylsulfoxide
(DMSO), dichloromethane (DCM), dioxane and MeCN. Unfortunately, compound 1ao did not
tolerate irradiation with UV light (312 nm) and resulted in decomposition into uncharacterized
products (for spectra see Supporting Information). However, compound 1bo switches in polar
and nonꢀpolar solvents (Figure 2 and Supporting Information). Upon irradiation with UV light
(312 nm or 254 nm, respectively), the color of the solution changed from bright red to blue. The
photoisomerization was monitored by UV/Vis spectroscopy; associated spectra in DMSO are
depicted exemplary in Figure 2 (further spectra are shown in the Supporting Information). New
absorption maxima at 410 nm and 594 nm show the formation the photoisomer; the isosbestic
point at 323 nm indicates a distinct twoꢀcompound isomerization. Photostationary states (PSSs)
were determined by HPLC measurements (Supporting Information). The PSS for the closing
reaction was determined to be 52% in DMSO, whereas the opening reaction was achieved
quantitatively through irradiation with orange light (590 nm, 30ꢁM, 2 min). An intramolecular
electron transfer (TICT) as reported for other DAE derivatives bearing an electronꢀacceptor core
moiety rationalizes the generally rather low PSS for the closing reaction.46, 47 Thus, we focused on
the opening reaction for further experiments, which generates a redox active compound from a
redox silent molecule upon irradiation.

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Figure 2: Left: Photoisomerization of 1bo in DMSO (30 ꢁM, irradiation with 312 nm). Right:
Spectra of isolated photoisomers 1bo (red line, maxima at 247 nm and 297 nm) and 1bc (blue line,
maxima at 307 nm, 415 nm and 583 nm) in MeCN (isosbestic point at 317 nm). For further
details see Supporting Information.
To determine the redox potential of CoQ derivatives 1bo and 1bc, we performed cyclic
voltammetric (CV) studies. The compounds included in the study are depicted in Chart 1.
Compound 12 was hereby synthesized from commercially available 11 similar to 9b in one step.
All 1,4ꢀbenzoquinone derivatives (9b, 10: Figure 3, C and D; 8b, 9a, 11, 12, Supporting
Information) including the open photochromic compound 1bo (Figure 3, A) show similar
behavior as CoQ10 (Figure 3, D). Specifically, all compounds exhibit two wellꢀdefined, fully
reversible reduction signals. This finding is in agreement with previous investigations on
benzoquinones. The first step corresponds to the formation of a semiquinone radical, while the
second step corresponds to the formation of a dianion (or hydroquinone if protons are
present).48 The potentials of the molecules are predestined by a substitution pattern of the
respective compound and are summarized in Table 1. The substituents of the benzoquinone core
affect the reduction potential values. With an increasing number of electron withdrawing
bromine substituents the reductions occur at less negative potentials. In contrast to all other
investigated paraꢀbenzoquinones, the closed isomer 1bc generates three – not two – cathodic
waves as well as one anodic one (Figure 3, B), which are reproducible (cf. Supporting
Information). Also, the intensities of the respective waves differ, and hence indicate that redox
steps involve a different number of electrons.

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B
A
4.0x10-5
o
c
1b in MeCN
-5
1b in MeCN
1.0x10
2.0x10-5
0.0
-6
5.0x10
0.0
-6
-5.0x10
-2.0x10-5
-4.0x10-5
-6.0x10-5
-5
-1.0x10
-5
-1.5x10
-5
-2.0x10
-5
-2.5x10
-1.5
-1.0
-0.5
0.0
0.5
1.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential/V
Potential/V
D
C
4.0x10-5
9b in MeCN
CoQ10 in MeCN
*
1.0x10-5
2.0x10-5
0.0
5.0x10-6
0.0
-2.0x10-5
-4.0x10-5
-6.0x10-5
-5.0x10-6
-1.0x10-5
*
*Ferrocene
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0 1.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential/V
Potential/V
Figure 3: Cyclic voltammogram of compounds 1bo (A, 30 ꢁM), 1bc (B, 2 ꢁM), 9b (C, 30 ꢁM),
and CoQ10 (D, 2 ꢁM, *ferrocene (0.005M) as internal standard) in MeCN + 0.1M Bu4NBF4.
Table 1: Redox potentials of benzoquinone derivatives 8b, 9a, 9b, 1bo, 1bc, CoQ10(10), 11, and 12
(cf. Chart 1) vs. SCN49 (determined by CV measurements in MeCN + 0.1M Bu4NBF4 using
ferrocene (0.005M) as internal standard).
Compound
Potentials vs. SCN49
ꢀ0.58; ꢀ1.23
1bo
1bc
+0.83; ꢀ0.24; ꢀ0.55; ꢀ1.39
ꢀ0.66; ꢀ1.37
8b
9a
ꢀ0.24; ꢀ0.97
9b
ꢀ0.35; ꢀ1.1
CoQ10 (10)
ꢀ0.77; ꢀ1.39

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11
12
ꢀ0.69; ꢀ1.34
ꢀ0.54; ꢀ1,27
Next, we investigated the redox states by spectroelectrochemistry, using several different 1,4ꢀ
benzoquinone derivatives for comparison ( (Chart 1).
Chart 1: 1,4ꢀBenzoquinones included in the electrochemical studies.
Exemplary, changes of the UV/Vis spectra of compounds 1bo, 1bc, and 9b during one cathodic
potential sweep are depicted in Figure 4 (Difference spectra of 1bo, 1bc, and 9b as well as UV/Vis
spectra of 9a, 10, 11 and 12 can be found in in Supporting Information). All paraꢀbenzoquinones
– besides the closed photoswitch 1bc – show similar absorption bands with the changing
potential; without applying an external potential, the benzoquinones show an absorption band at
around 280–300 nm depending on their substitution pattern. During the first reduction step
corresponding to the formation of the semiquinone intermediate,48 this band decreases to give
rise to two new bands– one at around 310 nm and a broader one at around 410 nm exhibiting a
wellꢀdefined double maximum, which is also present in the case of 1bo and CoQ10, but not so
wellꢀdefined.50 For CoQ10, this might be due to solubility issues in MeCN. The second cathodic
wave is the formation of the dianion,48 generating a novel band at around 370 nm, whereas the
two bandes of the semiquinone radical decrease again. All spectra show clear isosbestic points
indicating a distinct twoꢀcompound process. The reduction process is fully reversible and can be
repeated over several cycles.

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B
A
C
D
E
F
G
Figure 4: Spectroelectrochemical studies of compounds 9b (A: first peak, B: second peak), 1bo
(C: first peak, D: second peak) and 1bc (E: first peak, F: second peak, G: third peak). One
spectraꢀset collerates with one cathodic wave. Black arrows indicate increase/decrease of maxima;
blue arrows assign isosbestic points.
In contrast to the other investigated benzoquinones, the closed photoswitch 1bc showed a slightly
different spectrum in the beginning due to the attached chromophore. The molecule exhibits
maxima at 307 nm, 415 nm, and 583 nm. During the first reduction step, the maximum at
415 nm broadens, while the other maxima remain unchanged. Changes are more drastic as the
second reduction occurs; the longꢀwave band around 580 nm fully decreases, the band at around
410 nm broadens even more and the shortꢀwave band at around 300 nm experiences a
hypsochromic shift. While the last reduction step takes place, the band around 410 nm is also
hypsochromically shifted.

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Although there are similarities to the nonꢀphotochromic benzoquinones and to 1bo, the
spectroelectrochemical studies clearly reveals a different behavior of 1bc. Especially, the wellꢀ
pronounced decrease of the longꢀwave band around 580 nm, which can be assigned to the
conjugated thiophene backbone in the chromophore of the photoswitch,16 indicates a redox
process in this moiety of the molecule. However, this appears rather counterintuitive, as though
impaired the benzoquinone moiety still should be more prompt to reduction. Yet very recently,
Saito et al. report EPR and computational studies on a related molecule and could show the
formation of a radical anion similar to a semiquinone.51 A charge delocalization over the whole
molecule is likely to be allowed through two nearly degenerated LUMO and LUMO+1. Thus, we
suggest that the second reductive wave also has semiquinone character but delocalized over the
whole molecule (loss of longꢀwave band around 580 nm, which is associated with the closed ring
in DAEs). The first and the third reduction potential show less prominent changes in UV/Vis
and are yet to be assigned.
Overall, the cyclovoltammetric and spectroelectrochemical studies revealed a different redox
behavior for both photoisomers of 1b. Spectroscopic investigations show that 1bo and 1bc can be
in fact interconverted into each another through light, but the PSS values of the closing reaction
in all investigated solvents (see Supporting Information) is rather low (maximum 52% in DMSO).
In addition, the time required for the opening and the closing reaction are in the minuteꢀrange.
However, both characteristics are crucial for many redox mediated reactions, especially in
biological systems as e.g. enzymatic reactions take place in the milliꢀsecond range or even faster.52
Therefore, compound 1b can only be considered for a limited set of experiments up to now. The
investigated redox reactions should be slow or independent of diffusion of the cofactor mimetic
during irradiation. The quantitative opening reaction can be used to provide a redox partner
within a system upon irradiation similar to caged compounds. However, the reverse process
would not be quantitative. Considering these properties, we investigated the applicability of the
redox switch as a tool in reactions.

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2.4.
Photoactivated Oxidation Reactions
In the mitochondrial respiratory chain, CoQn oxidizes NADH to NAD+ in Complex I (NADHꢀ
Q oxidoreductase).26 Thus, we decided to let our photochromic CoQ mimetic 1bo react with
Hantzsch ester 13, which belongs to the same family of dihydropyridines as NADH, and hence
serves as an excellent test substrate for a photoactivated oxidation reaction.
Figure 5: Photoactivated redox reaction between photochromic CoQ derivative 1bo and an
NADH modelꢀ Hantzsch ester 13. Compound 1bo is generated in situ from closed isomer 1bc via
irradiation.
A solution of each photoisomer (5 mM, 1bo, 1bc) was treated with a solution of Hantzsch ester 13
(12 mM) in DMSO and reacted at ambient temperature in the dark. We took samples in certain
time intervals and analyzed them through analytical HPLC (UV/Vis detection at 317 nm which
equals PSS, HPLC traces see Supporting Information) to follow the consumption and the
generation of the respective redox pairs. The formation of 1bo,red from 1bo and 1bc, respectively,
through reduction with 13 is depicted in Figure 6A (normed areas of HPLC traces). The open
photoisomer 1bo is rapidly reduced by Hantzsch ester 13 (red line, Figure 6A), whereas the closed
one 1bc shows almost no conversion to 1bo,red or any other product (blue, Figure 6B, minor
background reaction results from thermal ringꢀopening). Thus, compound 1bc is unreactive
before irradiation.
Then, we investigated whether the formation of 1bo,red can be triggered through lightꢀactivation
of 1bc. The closed isomer (10 mM, DMSO) was treated with Hantzsch ester 13 (12 mM) and left
stirring in the dark for three hours. Subsequently, the solution was irradiated with a 590 nm
singleꢀspot LED for 10 min (picture, Figure 6) to be quantitatively converted into 1bo. HPLC
analysis confirmed a full conversion into only the open photoisomer and also an immediately
initialized reduction into 1bo,red (Figure 6B, for HPLC traces see Supporting Information). The
initial slope of the reaction seems to be larger than utilizing 1bo. This could be due to a preꢀ

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aggregation of 1bc and the Hantzsch ester. A preliminary computational analysis was carried out
in order to unravel the presence of an interaction between both photoisomers and Hantzsch ester
13. The DFT method at the B3LYP/6ꢀ31G(d) level of theory (cf. Supporting Information).53ꢀ56
We decided to follow a modification of an approach already found in the literature57 to gain
qualitative insight on the presence of a nonꢀcovalent π stacking between the two photoisomers of
1b and 13.
From an energetic point of view, the closed form appeared to be more stable (ca. 7 Kcal molꢀ1), cf.
Supporting Information) due to the interaction with 13. Moreover, the geometry of the optimized
structures clearly shows the Hantzsch ester resides in closer proximity to 1bo than to 1bc (see
Figure 6C and D).
In summary, we can conclude that the intermolecular reaction does relay on the respective redox
potentials of the compounds indicating that we are truly able to change redox potentials of 1b by
photoꢀinduced ringꢀopening of the electronically caged closed photoisomer.
A
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
Time/h
B
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
Time/h
C
D

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Figure 6: A: Formation of 1bo,red via reduction of 1bo (5 mM, DMSO, red line) or 1bc (5 mM
DMSO, blue line) through dihydropyridine derivative, Hantzsch ester 13 monitored through
HPLCꢀUV/Vis. B: Formation of 1bo,red after photoactivation (arrow) of 1bc (10 mM, DMSO)
after three hours of initial reaction time with a 590 nm singleꢀspot LED for 10 min (picture).
Values are typically the means ± SEM of three individual experiments. C: Optimized geometry of
the 1bo–13 adduct. D: Optimized geometry of the 1bc –13 adduct (All the distances are reported
in Ångstroms).

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2.5.
Photoactivated mitochondrial reduction of 2,6ꢀdichlorophenol
indophenol
In order to test the effect of 1bc and 1bo on a natural system, mitochondria were isolated from
the wildtype yeast strain BY4742.58 CoQn serves as electron transmitter in the respiratory chain of
mitochondria from Complex II (succinate dehydrogenase) to Complex III (cytochromeꢀC
oxidoreductase). By using 2,6ꢀdichlorophenol indophenol (DCIP) as electron acceptor instead of
Complex III, CoQ analogues can be tested on isolated mitochondria.59 The reduction of DCIP
with 1bc and 1bo was followed spectrophotometrically (Figure 7), continuously for 30 min or
interrupted by irradiation with 590 nm, in order to switch the closed 1bc into the open, more
active 1bo isomer.
Figure 7: Reduction of 2,6ꢀdichlorophenol indophenol (DCIP) by mitochondria over time
monitored spectrophotometrically at 520 nm. 1bc and 1bo assays were carried out continuously
(upper panels) or interrupted by irradiation at 590 nm as indicated by arrows (lower panels). The
data were fitted for linear activity at 0ꢀ10 min (red) and 20ꢀ30 min (blue). Whereas activity
decreases with time without irradiation of either the closed or the open form as well as after
irradiation of the open form (blue slope < red slope), it increases after irradiation of the closed
form (blue slope > red slope).

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For both isomers of our CoQ mimetic 1b, reduction of DCIP could be observed in the
continuous assay (Figure 7, no irradiation) where a fast initial rate (red) was followed by a lower
rate (blue). Comparing the initial rate of DCIP reduction with 1bc and 1bo, the open isomer is
only approximately 70% more active than the closed isomer. We assume that this seemingly high
activity of the closed isomer is due to residual activity of bound natural CoQn, which we cannot
exclude in isolated mitochondria.
To emphasize the activity difference of 1bc and 1bo, we irradiated both isomers after 10 min
reaction time (Figure 7, 590 nm irradiation). The activity for the open isomer decreased as in the
nonꢀirradiated assay (blue slope < red slope). However, the rate for the closed isomer after
irradiation increased (blue slope > red slope) indicating that the reaction was restarted.
Thus, despite the complex system of mitochondria, in which CoQ is deployed in several
reactions, we were able to see an activation of the DCIP reduction after irradiation of 1bc.
Consequently, the photochromic CoQ mimetic 1b cannot only replace CoQn in a natural system,
but can also activate the same after irradiation.

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3. Conclusion
In conclusion, we have developed a photochromic CoQ derivative (1b) combining a redox active
and a photochromic moiety in one molecule. We could show by cyclic voltammertry that the
redox potential of both photoisomers differs, and thus can be altered through
photoisomerization. In addition, the mode of action of cathodic waves in spectroelectrochemical
studies was examined indicating the nature of the reductive steps. Furthermore, we could show
that the closed form of the switch (1bc) is redox inactive in a reaction with Hantzsch ester, a
member of the dihydropyridine family, and has to be photoactived to its open isomer (1bo) to be
redox active. This proves that the reactivity of 1b relies on the altered redox potentials of the two
photoisomers, and is thus controllable through electronic changes rather than by conformational
and geometry changes as seen in most other studies. Finally, we were able to show that treatment
of isolated mitochondria with both photoisomers led to a difference of approximately 70% of the
velocity of DCIP reduction. We could further show that the closed isomer 1bc can restart the
DCIP reduction upon activation through irradiation. Thus, we present the first photochromic
redox cofactor that operates in a complex, biological context. The photochromic compound
converts upon irradiation from the formally caged closed form of the mimetic into an active
redox probe inside the system. However, reversibility of the redox reaction could not yet be
achieved due to low PSS values and insufficiently fast reaction rates. Optimization of both issues
should be addressed in further research to finally obtain a tool facilitating the control of redoxꢀ
dependent biological functions in cells by light.

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4. Supporting Information
Detailed experimental procedures and analytical data are available in the supporting information.

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5. Acknowledgments
We thank Regina Hoheisel for CV measurements, Antonín Králík and Morten Grøtli for fruitful
discussions, and Stefano L. Crespi for assistance regarding computational studies. We thank Lisaꢀ
Marie Altmann and Lorena Oegl for technical assistance. NAS thanks the Studienstiftung des
Deutschen Volkes for a doctoral scholarship.

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