Photoreactions of p-Quinones with Dimethyl Sulfide and Dimethyl Sulfoxide in Aqueous Acetonitrile[dagger]
The effects of dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO) on the photoreactions of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives in acetonitrile/water were studied. The observed triplet state of the quinones is quenched and the rate constant is close to the diffusion-controlled limit for reactions of most quinones with DMS and lower with DMSO. Semiquinone radical anions (Q*^sup -^) produced by electron transfer from sulfur to the triplet quinone were detected. For both DMS and DMSO the yield of Q*^sup -^ is similar, being generally low for BQ and NQ, substantial for AQ and largest for chloranil. The specific quencher concentrations and the effects of quinone structure and redox potentials on the time-resolved photochemical properties are discussed.
Abbreviations: AQ, 9,10-anthraquinone; BQ, 1,4-benzoquinone; BrNQ, 2-bromo-1,4-naphthoquinone: Cl^sub 2^BQ, 2,6-dichloro-1,4-benzoquinone; Cl^sub 4^BQ, chloranil; Cl^sub 2^NQ. 2,3-dichloro-1,4-naphthoquinone; CT, charge transfer; DMS, dimethyl sulfide; DMSO, dimethyl sulfoxide; Me^sub 2^AQ, 2.6-dimethyl-9,10-anthraquinone; Me^sub 2^BQ, 2,6-dimethyl-1,4-benzoquinone; Me^sub 4^BQ, duroquinone; MeNQ, 2-methyl-1,4-naphthoquinone; MeONQ, 2-methoxy-1,4-naphthoquinone; NQ, 1,4-naphthoquinone; QH^sub 2^, hydroquinones; QOH, 2-hydroxy-1,4-benzoquinone.
© 2006 American Society for Photobiology 0031-8655/06
The photoprocess of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and their derivatives are the subject of intensive investigations (1-16). Most quinones do not show fluorescence of any significance; the triplet state is rapidly populated and the quantum yield of intersystem crossing (Φ^sub isc^) is substantial (1,2). In a variety of solvents (e.g. an alcohol; DH) photoreduction takes place from the triplet state of quinones (3*Q), whereby a solvent radical (D*) and a semiquinone radical (QH*) are formed via H-atom transfer (3-6). Alternatively, a radical ion pair (DH*^sup +^ Q*^sup -^) is formed via electron transfer from an appropriate donor (e.g. an amine) to 3*Q (7-16). The rate constant for triplet quenching (k^sub q^) by amines is diffusion-controlled but rather low, k^sub q^ ≤ 3 × 10^sup 6^ M^sup -1^ s^sup -1^, for alcohols. The photoreduction in nonaqueous solution ultimately leads to hydroquinones (QH^sub 2^) (1,2). Suitable inert solvents for BQ and other strongly photoreactive quinones are rare. Benzene, which is often used in photochemistry as a nonreactive solvent, plays a crucial role for AQ because the triplet lifetime in benzene is relatively short, τT = 0.18 µs (5).
In aqueous solution the triplet states of BQ, NQ and AQ are also rapidly populated, but the photochemistry follows a different pattern (17-31). The products of BQ are QH^sub 2^ and 2-hydroxy-1,4-benzoquinone (QOH) (17,18). For several quinones photochemical release of OH radicals has been claimed (21-24), but for BQ and Me^sub n^BQ (n = 1-4) this can be excluded (19,20,25,26). The well-known semiquinone radicals (32-34) are the observed transients for quinones at high concentrations, for which self-quenching plays a decisive role. For low BQ and Me^sub n^BQ (n = 2-4) concentrations, free radicals play no role with respect to photohydrates, formed by nucleophilic water addition to the triplet state (25,26). Hydroxy products are also formed for NQ and a photohydrate was shown to be the key intermediate (30,31).
Dimethyl sulfoxide (DMSO) is another solvent with specific reactivity toward excited quinones. For parent BQ in aqueous solution, for which the rate constant for quenching of the triplet state by DMSO is k^sub q^ = 3 × 10^sup 9^ M^sup -1^ s^sup -1^, the mechanism has been ascribed primarily to physical quenching (26). DMSO as scavenger upon photolysis of MeBQ (28) and of 2,6-Cl^sub 2^BQ (29) has been addressed, but the role of DMSO in the photochemistry of quinones remained open. In the photosensitized mechanism of DMSO or aromatic sulfides, formation of an exciplex, but no radicals has been proposed (35,36). DMSO has been applied in γ-radiolysis studies for scavenging of OH radicals (37-40). To obtain information concerning electron transfer from sulfides, dimethyl sulfide (DMS) has frequently been used (41-45). Electron transfer from DMS to the triplet state has been established for the cases of benzophenone and 4-carboxybenzophenone (42). The resulting radical cation reacts with DMS. Radicals of aliphatic and aromatic sulfur compounds are known to form dimers, where the two sulfur centers are three-electron bonded (37-45).
In this work, based on the newly gained understanding of the photochemical properties of quinones in aqueous solution (25,26), flash photolysis measurements were carried out with the aim of elucidating the functions of the two sulfur compounds DMS and DMSO (abbreviated as R^sub 2^S) in quinone photoreduction. Quenching of the triplet state of suitable p-quinones by these additives was studied by time-resolved UV-visible spectroscopy at 308 nm. DMSO is widely used in biology to increase the solubility of chromophores in aqueous solution and as a radical scavenger. The literature regarding the reactivity of excited compounds with DMSO in aqueous solutions is puzzling, because DMSO was often thought of as an inert solvent. The specific quinone-DMSO interactions are of importance from the point of view of fundamental mechanistic photochemistry and practically for a better understanding of the photochemistry of protein cofactors in the presence of DMSO.
MATERIALS AND METHODS
Chloranil (Fluka, Ulm, Germany), DMSO (Fisher Scientific, Loughborough, UK). DMS and the other compounds (Aldrich, Taufkirchen, Germany) were used as received. The quinones were the same as those previously described (14-16.27); Me^sub 2^BQ is 2,6-dimethyl-BQ, Cl^sub 2^NQ is 2,3-dimethyl-NQ and Me^sub 2^AQ is 2,3-dimethyl-AQ. Water was from a milliQ (Millipore) system and acetonitrile (Merck, Darmstadt, Germany; Uvasol quality). The absorption spectra were monitored on a UV/visible spectrophotometer (HP 8453). The H^sup +^ or OH^sup -^ concentrations were low, typically 10^sup 8^ M^sup -1^ s^sup -1^ and larger R^sub 2^S concentrations were used only in a few cases for which k^sub q^
The triplet absorption spectra of Me^sub n^BQ (n = 2-4) and Cl^sub n^BQ (n = 3,4) in acetonitrile:water (1:1) are essentially comparable, insofar as they have two maxima around 290-300 nm and at λ^sub T^ = 440-490 nm. The maxima for BQ and MeBQ are both 280 and 430 nm, but the triplet lifetime (τ^sub T^) is much shorter (e.g. 0.1-1 µs with respect to 5-10 µs for Me^sub n^BQ and Cl^sub n^BQ, n = 3,4). A much faster decay is also characteristic for NQ and 2-methyl-1,4-naphthoquinone (MeNQ) (30,31). Photoreduction generally leads to Q*^sup -^ . The wavelengths of maximum absorption of the semiquinone radical anion (λ^sub rad^) in the presence of appropriate amounts of 2-propanol are compiled in Table 1.
Examples of the transient absorption spectra of quinones in the presence of DMS or DMSO are shown in Figs. 1-5. The spectra of Me^sub n^BQ, n = 2,4 in acetonitrile:water (1:1) in the presence of DMS initially reveal the triplet state and Q*^sup -^ as weak, longer-lived species (Fig. 1a,b). The triplet decay kinetics in the presence of DMS or DMSO are also shown for NQ and two substituted NQ (Fig. 3) and Me^sub n^AQ, n = 1,2 (Fig. 5). The semiquinone radical can be kinetically separated from the triplet state, but spectroscopically overlaps in several cases (e.g. MenBQ, NQ and parent AQ). The data for AQ reveal Q*^sup -^ with two maxima at 380 and 500 nm rather than QH with one maximum at λ^sub rad^ = 380 nm (Fig. 4). The yield of Q*^sup -^ is largest for chloranil (Cl^sub 4^BQ) in the presence of DMS (Fig. 2a) and slightly lower with DMSO (Fig. 2b). Triplet quenching by DMS and DMSO also occurs in nonaqueous acetonitrile solution, but virtually no semiquinone radicals could be detected.
The rate constants for triplet quenching were obtained from the linear dependence of 1/τ^sub T^ as a function of the donor concentration; examples are shown in Fig. 6. The values with DMS in acetonitrile range from k^sub q^ = 2 × 10^sup 8^ M^sup -1^s^sup -1^ for Me4BQ to (1-3) × 10^sup 10^ M^sup -1^ s^sup -1^ for Me^sub 2^BQ and most NQ or AQ (Table 2). The rate constants are the same in the presence of 50% water. Triplet quenching by DMSO (0.1-10 mM) also takes place and the values in acetonitrile range from k^sub q^ = 4 × 10^sup -1^ s^sup -1^ for Me^sub 3^BQ to (3-6) × 10^sup 9^ M^sup -1^ s^sup -1^ for Me^sub 2^BQ, NQ or AQ. No marked quenching effect was found for 2methoxy-1,4-naphthoquinone (MeONQ) in DMSO or DMSO: water (1:1). In acetonitrile:water (1:1) most k^sub q^ values are similar or slightly lower (Table 2). A quencher concentration of 0.1 mM is sufficient for a typical case. As a measure of the yield of Q*^sup -^, the absorbance of the radical in the presence of DMS or DMSO with respect to that in the presence of 2-propanol (Φ^sub rad^) was taken. This ratio is low for Me^sub n^BQ, n = 2-4, but much larger for Cl^sub 3^BQ (Table 1) and Cl^sub 4^BQ (Fig. 2) in the presence of both DMSO and DMS. Observation was made just after the triplet had decayed, where no possible secondary reduction step occurs (14-16). The Φ^sub rad^ values are also substantial for AQ as long as water is present and generally slightly larger in the presence of DMS than of DMSO (Table 1). In dry acetonitrile triplet quenching remains efficient but Φ^sub rad^ is smaller than 0.1.
Photoreactions of quinones in aqueous solution
The reaction mechanism in aqueous solution is illustrated in Scheme 1. Excitation of quinones in acetonitrile, or a mixture with water, by 308-nm laser pulses produces the lowest triplet state with a high Φ^sub isc^ (1,2,8-16) and a specific absorption maximum, λ^sub T^ (Table 1).
Decay of the triplet state occurs via Reaction 2, but other processes compete. Quenching of the 3Q state by oxygen, Reaction 3; the quinone itself, Reaction 4; water, Reaction 5 and an appropriate donor, Reactions 6/6′, play a role (20,25-27). The rate constant for oxygen quenching of the 3* Q state of Me^sub 3^BQ and AQ in acetonitrile is K^sub 3^ = 2 × 10^sup 9^ and 1 × 10^sup 9^ M^sup -1^ s^sup -1^, respectively.
Unless the solvent is inert. H-utom abstraction reaction 6′ from the solvent leads to the semiquinone radical of BQ, NQ and AQ. This is present as anion in the applied pH range of 5-8 due to equilibrium 7 and pk^sub a^ = 4.2-5 (32-34). Radical termination and protonation reaction 8 eventually yields QH^sub 2^. The triplet quenching by additives competes with the pathways into QH^sub 2^ and QOH, the latter of which is initiated by nucleophilic water addition, Reaction 5.
In Sequences 5, 9 and H) for BQ/Me^sub n^BQ (n = 1-3). 1,2,4trihydroxybenzenes are labile intermediates prior to the reaction with the corresponding quinone to give the two stable products QH^sub 2^ and QOH (25,26). Note that QOH/QO^sup -^ of parent BQ at pH 5-8 is present as anion due to PK^sub a^ = 4.2 (46).
Photoreactions of quinones with DMS
The photoinduced reactions of quinones with DMS in an acetonitrile/water mixture at low quinone concentrations are illustrated in Scheme 2. Competing reactions (e.g. Reaction 3 with oxygen), do not play a role and others. Reactions 4-6, can be suppressed by an appropriate DMS concentration. The proposed (nonobservable) species in the presence of DMS is a charge transfer (CT) complex. The triplet quenching by DMS is proposed to be clue to electron transfer and separation 11 or due to electron transfer and back electron transfer in the solvent cage, Reaction 12, the latter without a change after a few nanoseconds or longer. The major secondary species is the scmiquinone radical anion. The radical cation of DMS is in equilibrium with the dimer radical cation.
The radical cations of aliphatic and aromatic sulfides are known to form dimers, where the two sulfur centers are three-electron bonded, K^sub 13^ = 2 × 105 M^sup -1^ (43). (Me^sub 2^SSMe^sub 2^)*^sup +^ has a molar absorption coefficient at 470 nm of ε^sub 470^ = 6 × 103 M^sup -1^ cm^sup -1^ (37) and is a minor transient in the benzophenone/DMS system (42). The major reaction of the dimer radical cation in aqueous solution is deprotonation (40).
For quinones the primary observed species is the semiquinone radical (Figs. 1-5). An oxidation could form the dicationic species, Reaction 14, and finally DMS and DMSO, Reaction 15 (41). Reaction 14 is first-order, but its observation is successfully overlapped by second-order Reaction 8 of decay of the semiquinone radical union, which has a half-life in the 0.03-3 ms range (see Figs. 1-5). A disproportionation of (Me^sub 2^SSMe^sub 2^)*^sup +^ into DMS, DMSO and 2H^sup +^ has been considered to be negligible (41). For Cl^sub 3^BQ, Cl^sub 4^BQ AQ and MeAQ no photoconversion of DMS into DMSO could be observed using λ^sub irr^ = 254 nm and HPLC detection.
In the case of benzophenone in acetonitrile, electron transfer from DMS to the triplet state is inefficient (as for quinones) and the yield of ketyl radical is as low as Φ = 0.02 (42). The pK^sub a^ values of the ketones are much larger than for quinones and the yield of the ketyl radical of benzophenone in acetonitrile:water (1:1) is Φ = 0.14 and therefore larger than that (Φ = 0.04) of the radical anion (42). A CT complex with DMS has been proposed for benzophenone and 4-carboxybenzophenone as sensitizers, for which the rate constant for triplet quenching is k^sub q^ = (1.5-5) × 10^sup 9^ M^sup -1^ s^sup -1^ (42). The redox potential of the 3*ketone/ketone*^sup -^ couple is E^sup 3^ = 0.8 V. The redox potential of DMS*^sup +^/DMS is not known. A potential of 1.5-2 V could be assumed on the basis of k^sub q^ = 2.5 × 10^sup 9^ M^sup -1^ s^sup -1^ and the free energy change (ΔG^sub et^ of benzophenone in acetonitrile, using the Rehm-Weller relationship (47) and neglecting the Coulombic term (
For comparison, the redox potentials of tyrosine and tryptophan, 0.94 and 1.05 V, respectively, are smaller. ΔG^sub et^ values of 0.2-0.8 V have been reported for electron transfer from polymethylbenzenes (redox potential: 1.6-2.4 V) to halogenated BQ in acetonitrile (13).
Photoreactions of quinones with DMSO
The radical cation of DMSO is formed after escape of the CTcomplex and could decay by back electron transfer, Reaction 12′, or it could react with water to give methanesulfinic acid, Reaction 17.
Methanesulfinic acid, which is fully dissociated at pH > 3.5, and the methyl radical are also formed by DMSO-scavenging of OH radicals (39). Moreover, in the presence of BQ the methyl radical has been shown to form MeBQ (38). A search for methanesulfinic acid upon irradiation of parent BQ in the presence of DMSO showed that this is not formed as a product (26). Nevertheless, reactions analogous to sequence 6”, 11 and 12′ also take place for the other quinone/DMSO systems. A comparison of the spectra (Figs. 2, 4, 5), the relative yields (Table 1) and the k^sub q^ values (Table 2) reveals that DMS and DMSO behave similarly. A reaction into a dimer radical cation, analogous to Reaction 13 with DMS, has not been considered for DMSO.
The role of DMSO in the photolysis of BQ in aqueous solution is puzzling. One possibility is OH radical attack and, on the basis of radiolysis studies (37), conversion of DMSO into the methanesulfinic acid and the methyl radical. The hypothesis of OH radical release from excited quinones in aqueous solution (21-24) can be excluded (19,20,26). The other, more likely, possibility is that the triplet state is quenched by DMSO.
For MeBQ the half-concentration, [DMSO]^sub ½^: (i.e. the DMSO concentration for 50% change in formation of a radical photoproduct using a nitroxyl trap) is 9 mM (28). Even without the kinetic data for MeBQ in aqueous solution it can be argued that a value of k^sub q^ = 2 × 10^sup 9^ M^sup -1^ s^sup -1^ is in accordance with the values for BQ and Me^sub 2^BQ (Table 2). A relatively large half-concentration of [DMSO]^sub ½^, = 0.01 M is feasible as the lifetime is rather short: τ^sub T^= 50 ns; note that for MeBQ in water τ^sub T^ is shorter than for BQ and Me^sub 2^BQ (25).
Another DMSO half-concentration of ~0.02 M has been reported for Cl^sub 2^BQ in aqueous solution, where the photoconversion into the corresponding QO^sup -^ is lowered in the presence of DMSO (29). The reason could be self-quenching 4 without reaction 6”. If k^sub q^ is 2 × 10^sup 9^ M^sup -1^ s^sup -1^ also for Cl^sub 2^BQ, based on data for Cl^sub n^BQ, n = 3,4 (Table 2), [DMSO]^sub ½^ = 0.02 M is in agreement with a short lifetime τ^sub T^
Enzymatic DMSO reductase, which is able to oxidize DMS and to reduce DMSO in the reverse direction, is, interestingly, of fundamental importance and the subject of recent investigations (48,49).
Effects of quinone structure
Virtually no free radicals are formed in dry acetonitrile via triplet quenching by DMS/DMSO, but in acetonitrile:water 1:1 the reaction occurs via electron transfer. The yield of this photoreduction is large for Cl^sub 4^BQ and AQ (Table 1). The reductive intermediate could be Q*^sup -^ or QH*. The spectra of most semiquinone radicals and the corresponding anion radicals are very similar, but AQ is unique among the quinones in allowing one to distinguish between QH* and Q*^sup -^ as secondary species (14,20,34). The experimental spectra reveal Q*^sup -^ rather than QH* (Fig. 5b). This differs from the benzophenone/DMS system in acetonitrile:water, in which the ketyl radical (Φ = 0.14) and also the (Me^sub 2^SSMe-)*^sup +^ radical were detected (42).
The redox potentials of the quinones E(Q/Q*^sup -^) in acetonitrile and aqueous solution are compiled in Table 2 (50,51). The most positive potential among the BQ relates to Cl^sub 4^BQ, for which the sensitized oxidation of DMS and DMSO has the largest yield (Fig. 2) and in which E^sup 3^ = 2.15 V in acetonitrile (9). The values in water at pH 7 (SHE as reference) differ from those in acetonitrile (SCE as reference) by ~0.6 V (50). The specific quinone structure and the required redox potential of the triplet state are not simply correlated. One reason is the increase in the lowest triplet level ongoing from BQ (E^sub T^ = 2.0-2.3 V) via NQ (E^sub T^ = 2.5 V) to AQ with E^sub T^ = 2.7 V (10,52,53). The ionization potentials of DMS and DMSO in aqueous solution are 6.1 and 7.46 V, respectively, and that of triethylamine, for comparison, is 5.2 V (54). This is in line with the k^sub q^ values, being one to two orders of magnitude smaller for DMSO than for DMS (Table 2).
Another pathway toward QOH and QH^sub 2^ is initiated by the self-quenching Reaction 4. This step is known to play a role at higher quinone concentrations. The self-quenching rate constants for BQ (26) or NQ (20) in neutral aqueous solution are k^sub 4^ = 5 × 10^sup 9^ and 4 × 10^sup 9^ M^sup -1^ s^sup -1^, respectively. Hypothetically, a radical ion pair is the precursor of radical HOQ*, which is spectroscopically very similar to Q*^sup -^ and yields finally QOH (26). However, self-quenching in acetonitrile:water (1:1) does not play a decisive role for continuous irradiation at 254 nm because, due to the low quinone concentration of typically 0.1-0.3 mM, Reaction 5 competes successfully with Reaction 4, Scheme 1. Even for pulsed excitation, self-quenching can be neglected in the presence of either DMS or DMSO at appropriate concentrations because Reaction 6” competes successfully with Reactions 2-5.
The effects of triplet quenching by DMS and DMSO were studied for a series of quinones. The appropriate parameters, such as rate constants for quenching and the yields of the semiquinone radical, are presented. The time-resolved spectra show physical quenching without free radicals in acetonitrile but a contribution from photoreduction in a 1:1 mixture with water. Specific photochemical properties of BQ, NQ and AQ are discussed. The reported DMSO concentrations for 50% change (i.e. formation of a radical photoproduct for MeBQ  and of QOH/QO^sup -^, one of the corresponding products for Cl^sub 2^BQ ), can be attributed primarily to a rather low triplet lifetime. Quinones in the presence of DMS and DMSO are photochemically reactive, like the well-known benzophenone/DMS system (42). Quinone solutions were sometimes prepared by dissolving them in DMSO (e.g. for a study of the kinetics of the redox interaction with ascorbate ). An extension of these thermal into photochemical analyses should be avoided. The results demonstrate that DMSO is not an inert solvent for most quinones and DMSO may be photoreactive for other systems with high Φ^sub isc^.
Acknowledgements-The author thanks Professor Wolfgang Lubitz for his support. Professor C. von Sonnlag for helpful discussions and Mr. Leslie J. Currell and Mr. Horst Selbach for technical assistance.[dagger] This paper is part of a special issue dedicated to Professor J. C. (Tito) Scaiano on the occasion of his 60th birthday.
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Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany
Received 24 May 2005; accepted 19 July 2005; published online 2 August 2005 DOI: 10.1562/2005-05-25-RA-540
* To whom correspondence should be addressed: Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an derRuhr, Germany. Fax: 49 208 306 3951; e-mail: firstname.lastname@example.org
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