12613
J. Phys. Chem. 1994, 98, 12613-12620
Reaction of Hydroxysulfuranyl Radical with Molecular Oxygen: Electron Transfer vs Addition Christian Schdneich' Department of Pharmaceutical Chemistry, Universiv of Kansas, Lawrence, Kansas 66045 Krzysztof Bobrowski',? Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: June 28, 1994; In Final Form: September 13, 1994@
-
The reaction of the hydroxyl radical with 2-(methy1thio)methyl acetate (2-MTMA) leads to the formation of a hydroxysulfuranyl radical ()So-OH) which exists in a protonatiorddeprotonation equilibrium with pK, = 10.85. Deprotonation most probably occurs via the equilibrium )So-OH H+ )S*-O- but may be followed by an intramolecular proton transfer from the C ~ c a r b o nof 2-MTMA. The hydroxysulfuranyl radical reacts with molecular oxygen with ks = (1.1 f 0.2) x lo8 M-' s-l whereas the deprotonated species reacts with k24 = (5.7 f 1.0) x lo8 M-' s-l. Mechanistically, the reaction of the nondeprotonated species with oxygen involves the intermediary formation of an oxygen adduct as concluded from timeresolved conductivity experiments and attempts to scavenge reducing intermediates with tetranitromethane. The sulfur-oxygen adduct is proposed to be a peroxyl radical bound to a tetravalent sulfur, as in the general structure Rz(OH)SOO', which may exist in equilibrium with a pentavalent sulfur-"side on" oxygen complex
+
n
R2(0H)'S-O-O. Intermediates of the latter type may play an important role in the oxidation of sulfides to sulfoxides, initiated by reactive oxygen species in the presence of molecular oxygen.
Introduction
or with halide X- (reaction 3).12 In the presence of hydroxide
Thioethers serve key functions in chemical and biological systems, for example in organic polymers, or, as the amino acid methionine, in many proteins.' They are highly susceptible to oxidation by reactive oxygen species2 which are formed at significant levels under conditions of oxidative stressS3Important products include sulfoxides: sulfones>*5S-dealkylation products,6 and, from methionine via intermediary formation of methional, e t h ~ l e n e . ~ Large effort has been placed on the characterization of reactive intermediates which are involved in thioether oxidation and their corresponding reaction mechanisms. Such investigations are often compromised by the short lifetime of these intermediates and require fast techniques with sufficient time resolution. Alternatively, reactive intermediates may be produced at low temperatures and trapped in glassy matrices for investigation by conventional techniques. For example, a zwitterionic peroxy sulfoxide, )S(+)-O-O(-), formed via reaction of singlet oxygen with a sulfide, was characterized by FI'IR at 13 K in an oxygen matrix,8 and assigned with the help of theoretical calculationsg of the expected IR spectra. However, such experiments cannot yield definitive conclusion whether such an identified intermediate might indeed be formed at higher temperatures. The oxidation of aliphatic thioethers by strong one-electron oxidants (Ox) such as triplet benzophenone derivatives,lOJ X2'(X = C1, Br, I),l* CC1300',13 T12+,14and the hydroxyl radical (H0')15316has been found to yield sulfur-centeredradical cations, )So+ (reaction 1). The latter associate either with a nonoxidized sulfide to yield the dimeric form [)S:. S(]+ (1) (reaction 2),15,16
'
On leave of absence from the Institute of Biochemistry and Biophysics, Polish Academy of Sciences, 02-532 Warsaw, Poland. Present address: Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland. Abstract published in Advance ACS Abstracts, October 15, 1994. @
0022-365419412098-12613$04.50/0
- ox*-+
ox + )S )S*+
+ S(
+
(1)
)SO+
:.
[)S S(]+ 1
(2)
)S'++X-=)s:.X
(3)
ion and molecular oxygen, the dimeric sulfur radical cations 1 react to s u l f o ~ i d e s . ~ This ~ J ~ process has been proposed to involve the addition of hydroxide to 1 yielding intermediates 2 andor 3 (forward reactions 4 and 5 ) with subsequent reaction of 2 and 3 with molecular oxygen forming sulfoxide and superoxide anion (reactions 6 and 7).lo3l7
1
I I I I
+ HO-
HO-S-Sa
(4)
2 1
2
+ HO- ==
- + + - +
+ 02 3
H+
02
(5)
+ >S=O + SC
(6)
+ >S=O
(7)
3
0 2 . -
H+
I + SC I
HOS.
02.-
Time-resolved studies with numerous aliphatic thioethers have indicated that at pH 10 both equilibria 4 and 5 are located well on the left-hand side. In fact, in most cases hydroxysulfuranyl radicals 3 of aliphatic thioethers decompose into radical cations with k-5 lo8 M-l s-l,18 and intermediates 2 are characterized by even shorter lifetimes.16 The formation of sulfoxides via the reaction sequence 4-7 involves, therefore, 0 1994 American Chemical Society
Schoneich and Bobrowski
12614 J. Phys. Chem., Vol. 98, No. 48, 1994
only small steady-state concentrations of 2 or 3 which are continuously removed from the equilibria 4 and 5 by reaction with molecular oxygen (reactions 6 and 7). In this sequence the forward reactions 4 and 5 are rate-determining at pH 9 and [02] I6 x M, as concluded from pulse radiolysis studies." Thus, although the reaction products sulfoxide and superoxide have been identified,17 uncertainty remains with respect to the nature and reaction kinetics of the involved intermediates, and in particular with regard to the mechanism of the reactions of the sulfuranyl radicals 2 and 3 with molecular oxygen. A detailed kinetic analysis of reactions 6 and 7 is rather difficult owing to the general short lifetime of the sulfuranyl radicals 2 and 3, respectively. We have recently reported the generation of a relatively longlived hydroxysulfuranyl radical 4 from 2-methylthiomethyl acetate (2-MTMA), stabilized through hydrogen-bond formation with the adjacent ester oxygen.19 In preliminary experiments the rate constant for the reaction of 4 with molecular oxygen (reaction 8) was determined to be kg = 1.1 x lo8 M-' s-'.19
.o
O/H'
I
II
4
In the present paper we have utilized this stabilized hydroxy sulfuranyl radical for characterizing its reaction mechanism with molecular oxygen (reaction 8) with particular emphasis on the possibility of the intermediary formation of an oxygen adduct.
4
+ 0,
-
products
(8)
Experimental Section Materials. 2-Methylthiomethyl acetate (ZMTMA), tetranitromethane (TNM), and perchloric acid (HClO4, redistilled) were obtained from Aldrich (Milwaukee, WI), and 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) was from Sigma (St. Louis, MO). All chemicals were of highest commercially available purity. Prior to use 2-MTMA was distilled, and TNM was repeatedly extracted with water until the aqueous phase became colorless in order to remove nitroform anion impurities. Solutions. All solutions were made up freshly in Millipore water (18 MQ) and their respective pH values were adjusted through addition of NaOH or HC104. They were subsequently purged for at least 20 min per 100 mL of sample with the desired gas 02, N20, or N20/02 mixtures prior to the experiment. Mixtures of N20 and 0 2 were calibrated by employing flowmeters with defined flow rates of the respective gases. Pulse Radiolysis. All pulse radiolysis experiments were performed by applying 5 ns pulses of high-energy electrons from the Notre Dame 7 MeV ARC0 LP-7 linear accelerator (for details see ref 20), or 0.3- 1.O p s pulses of high-energy electrons (1.55 MeV) from the Van-de-Graaf accelerator of the HahnMeitner-Institute Berlin (for details see ref 21). Adsorbed doses were in the order of 2-4 Gy (1 Gy = 1 Jkg) corresponding to an average concentration of radicals of (1.2-2.5) x M for a radiation chemical yield of G = 6.0 (G denotes the number of species formed or converted per 100 eV absorbed energy; G = 1.0 corresponds to 0.1036 m o l per J absorbed energy in aqueous solution; for practical purposes the G unit rather than the SI unit is used throughout this paper). Dosimetry was based on the NzO-saturated thiocyanate dosimeter, pH 4.0, using a radiation chemical yield of 6.0, an extinction coefficient of 7580 M-I cm-' for (SCN)2*-, and a conductivity change of A& =
3
I
E
U I
E m
s
d I
'
300
500
400
600
h, nm Figure 1. Transient absorption spectra obtained after pulse irradiation M of an NzO-saturated aqueous solution containing 5.0 x 2;MTMA (a) 40 ns after the pulse at pH 6.4 and (c) 2 ,us after the pulse at pH 11.25. (b) absorption w time profile at 470 nm obtained after pulse irradiation of an Nz0-saturated aqueous solution, pH 4.0, containing 9.1 x M 2-MTMA. -360 S cm2. The formation and reaction of the reactive intermediates were monitored by time-resolved UV and/or conductivity detection (for a detailed description of the conductivity technique see ref 22). The radiolysis of water results in the formation of various species according to reaction 9 with their respective yields23in terms of the G value given in parentheses.
H,O
-
eaq-(2.7), HO' (2.7), H' (0.6), H+ (2.7)
(9)
In oxygenated solutions both H'and eaq- react with molecular oxygen according to reactions 10-12 (klo = 1.9 x 1O'O M-' s-';,~ kll = 2.1 x 10" M-' pKa,12= 4.825).
+
H' 0,
-HO,'
(11)
+
(12)
HO,' * H+ 0;-
In N2O-saturated solutions ([NzOIsat x 2 x lo-, M20) the hydrated electrons are converted into hydroxyl radicals via reaction 13 with k13 = 9.1 x lo9 M-I s-'.,O
+
eaq- N 2 0
- HO' +
HO-
+ N,
(13)
Results Formation and pK. of the Hydroxysulfuranyl Radical 4. The absorption spectrum obtained 40 ns after pulse irradiation of an N2O-saturated aqueous solution, pH 6.4, containing 5 x lop3M 2-MTMA is displayed in Figure 1, curve a. It shows an absorption maximum at = 330 nm, formed with a radiation chemical yield of G~330= 18 000 M-' cm-'. This spectrum has previously been assigned to the hydroxysulfuranyl radical 4 which is formed according to reaction 14 with k14 = (6.7 0.2) x 109 M-1 s-1.19
HO' + 2-MTMA
-.4
(14)
The hydroxysulfuranyl radical 4 subsequently decomposes in a fmt-order process with t l = ~ (7.5 & 1.0) x lop6s, corresponding to an overall kexpti = (9.3 & 1.4) x lo4 s-'. In the absence of oxygen, the latter process consists of three distinct pathw a y ~ : '(i) ~ spontaneous elimination of HO- (forward reaction
J. Phys. Chem., Vol. 98, No. 48, 1994 12615
Reaction of Hydroxysulfuranyl Radical with Molecular Oxygen 15; kls = 6.5 x lo4 s - l ) , (ii) proton catalyzed elimination of H2O (reaction 16; k16 = 2.3 x 1O1O M-' s-l), and (iii) displacement of HO- by a second nonoxidized molecule 2-MTMA (forward reaction 17; k17 = 8.9 x lo6 M-I s-l >. 4
4
2-MTMA()S'+) (5)
+ 2-MTMA * (2-MTMA)J)S
+ HO-
:.SO+ (6)
(15)
+ HO-
(17)
The monomeric thioether radical cations 5 formed in reactions 15 and 16 exist in equilibrium 18 with the three-electron-bonded dimeric species 6, characterized through a distinct absorption with ,A = 470 nm.19 However, in the 2-MTMA system a stable dimeric species 6 can only be observed at pH 5 4 and high thioether concentrations. At pH > 4 species 5 and 6 immediately suffer further deprotonation according to reactions 19 andor 20, analogous to various organic thioether radical cations investigated earlier.I5zz6 H+
t-
+
'CH2-S-CH2402CH3
(7)
6-1
I
I
9
10
11
12
PH Figure 2. First-order rate constants as a function of pH for the
decomposition of the hydroxysulfuranyl radical 4; Nz0-saturated M 2-MTMA. aqueous solution containing 5.0 x
SCHEME 1
(19)
(18)
-I
H+
+
CH3-S-C0H-C02CH3 (8)
(20)
The kinetics of the latter processes are displayed in Figure 1, curve b, recorded after pulse irradiation of an NzO-saturated aqueous solution, pH 4,containing 9 x M 2-MTMA. It is evident that the dimeric radical cation 6 decomposes within 25 p s after the pulse (t1/2 = 3.8 p s ) and even shorter lifetimes are expected at pH > 4 and lower thioether concentration^.^^,^^ Parallel performed time-resolved conductivity studies indicate a negligible concentration of ionic species at 25 p s after the pulse confirming that the sulfur-radical cations 5 and/or 6 preferentially decompose via the deprotonation pathways outlined in reactions 19 and 20 with the resulting protons neutralizing the initially formed hydroxide ions (reactions 15 and 17) or substituting for the initially consumed protons according to reaction 16. Approximating k-15 5 1O1O M-' s-l (Le., the diffusioncontrolled limit), and taking k-17 = (2-4) x lo9 M-' s-l 27,28 the decay of 4 at pH 6.4 and [ZMTMA] = 5 x M is accordingly predicted to kexptl = k15 k16[4.0 x -I- k17[5 x = 1.18 x lo5 s-l, in good agreement with the measured value. With the information from Figure 1, curve b, we have ensured that all conductivity data recorded in subsequent experiments at pH =. 4 are free of any contribution from sulfur radical cation chemistry (reactions 15-20) at t > 20 pus after the pulse. Figure 1, curve c, displays the absorption spectrum recorded 2 ps after pulse irradiation of an NzO-saturated aqueous solution, pH 11.25, containing 5 x M 2-MTMA. It shows an absorption maximum with ,Im, = 406 f 2 nm, formed with G~406= 21 350 M-l cm-', and does not display any residual shoulder around 470 nm. Earlier results have shown that the absorption bands of dimeric thioether radical cations do not shift with pHI5 and, therefore, we have to assign this new 406 nm band to a different species. The latter decomposes with firstorder kinetics with t1/2 = 10 p s into as yet unknown products. Figure 2 displays a plot of the first-order rate constants, kexpu, for the decay of 4 measured as a function of pH between pH 8.8 and pH 12. Around pH 9 the decomposition of 4 occurs with kexptl= 5 x lo4 s-l, corresponding to t112 = 13.8 x s, Le., slower by a factor of 2 as compared to pH 6.4 (see above). This can be rationalized by the increase of the HO- concentra-
+
tion enhancing the efficiency of the back reactions of equilibria 15 and 17 (rate constants for the addition of HO- to dimeric sulfur radical cations are in the order of (2-4) x lo9 M-' s-l 27,28). However, a further increase of pH does not result in an even longer lifetime of species 4 but rather in an accelerated decomposition. Between pH 9.8 and 11.8 the kexpuvs pH plot adopts a sigmoidal shape, characteristic for a pH titration curve, with a half-point at pH = 10.85. The first-order experimental rate constant for the decomposition of 4 increases from kexptl= 5 x lo4 s-l at pH 9 to 3.8 x lo5 s-l at pH 11.8. The observed pKa is slightly higher than a similar pKa reported for the acid-base equilibrium of hydroxy sulfuranyl radicals of dimethyl sulfide (reaction 21), pKa,21 = 10.2,29 and might characterize an analogous process for 2-MTMA. (CH,),'S-OH
+ HO-
--L
(CH,),(S'-O-)
+H20
(21)
The observed spectrum and kinetics can then be rationalized by the mechanism displayed in Scheme 1. The hydroxysulfmanyl radical 4 exists in equilibrium 22 with its deprotonated form 9. The pKa of this equilibrium is represented by the titration curve in Figure 2. As structure 9 is drawn, the ester oxygen and the S - 0 - group should repel each other due to their partial and full negative charge, respectively. This repulsion may be relieved either by a conformational change (reaction 23a) yielding structure 10a as one example of all possible conformers with a larger distance between the respective oxygen atoms or by a second proton transfer from the Czcarbon atom yielding 10b (reaction 23b). The latter exists in resonance with structure 1Oc which might receive some ad-
Schoneich and Bobrowski
12616 J. Phys. Chem., Vol. 98, No. 48, 1994
1-
2 -
2
4
6
8
1 0 1 2
[ O , ] , 10-4M
Figure 3. Plots of first-order rate constants for the decay of (a) 4 at pH 6.4 and (b) 10 at pH 11.3 as a function of oxygen concentration; M 2-MTh4A. aqueous solutions containing 5.0 x
ditional stabilization through possible hydrogen bonding between the two oxygen atoms. The pKa for an acid-base equilibrium of 10a is not expected to be around 10.85 but rather 10.2 as for the "open structure" analog dimethyl sulfide (see above). Therefore, reactions 23a and 23b can be treated as quasi irreversible processes contributing to the decomposition of 4 according to the general rate law -d[4]ldt = kexpd x [4] with kexptlbeing defined by the mathematical relation I andf4 = [H'll ([H+] Ka) andfg = Ka/(Ka -t [H+]). Experimentally we cannot differentiate between the formation of loa, lob, or 1Oc via reactions 23a or 23b, respectively. Therefore, from now on the notation 10 represents collectively all possible species loa, lob, and lOc, and k23 stands for k23a k23b, respectively.
+
Figure 4. Conductivity vs time traces obtained after pulse irradiation of oxygen-saturated aqueous solutions: (a) pH 9.2; (b) pH 9.4, containing 1.3 x M 2-MTMA.
8a and 24a) or via formation of an oxygen adduct (reactions 8b and 24b), for example, the sulfur-based peroxyl radicals 12 or 13. In the following we shall discuss experiments which were undertaken in order to distinguish between the two reactions 8a and 8b for the neutral nondeprotonated hydroxy sulfuranyl radical 4.&
4
+ 0,- Hf + 0;- + CH3-S(=O)-CH2C02CH3 (11) (8a)
4
+
kxptl
= k19/2~{k~,/(k-1,[H0-1 + k19/20)+]
k],[2-MTMAl/(k-],[Ho-l
+ k19/20>}+ f4k23f9
10
(I)
The UV spectrum with Amm = 406 f 2 nm displayed in Figure 1, curve c, is consequently assigned to a deprotonated species of structure loa, lob, or 1Oc. These observations define the basis for subsequent investigations of the reaction of 4 and 10 with molecular oxygen, as described below. As an important prerequisite the reaction of 4 with 0 2 has to be investigated at pH 9.5 whereas the reaction of 10 with 0 2 requires pH > 11.3. Reaction of Species 4 and 10 with Molecular Oxygen. The observed first-order decomposition of the hydroxysulfuranyl radical 4 occurs increasingly faster in the presence of increasing concentrations of molecular oxygen. Figure 3, curve a, displays a linear plot of kexpd vs [O,] at pH = 6.4, with the slope yielding a bimolecular rate constant for reaction 8 of k&3= (1.1 f 0.2) x lo8 M-* s-l whereas the intercept corresponds to the combined processes of unimolecular, proton- and thioethercatalyzed decomposition of 4 discussed in the previous section. Figure 3, curve b, shows that a linear plot of keXpdvs [OZ] is also obtained for the reaction of 10 with 0 2 at pH 11.5, monitored at 410 nm, though with a larger slope corresponding to an overall k24 = (5.7 f 1.0) x lo8 M-' s-l.
10
+ 0, - products
(24)
The fact that the 406 nm intermediate reacts faster with molecular oxygen than the protonated species 4 can be rationalized by the electrophilic nature of molecular oxygen. It is, however, as yet not possible to state whether the reactions 8 and 24 proceed via an outer-sphere electron transfer (reactions
+ 0, -.CH,-(HO)S(OO')-CH2C0,CH3 (12) (8b) 10 + 0, - 0;- + (11) (244
+ 0, -.CH3-(0-)S(00')-CH2C02CH3
(13)
(24b)
Conductivity Experiments. Figure 4a displays a conductivity vs time profile obtained after pulse irradiation of oxygensaturated deionized water, pH 9.2, with a ca. 0.3 ps electron pulse. The sharp negative peak obtained immediately after the pulse originates from an electric induction by the electron pulse itself whereas the negative signal observable ca. 5 ps after this peak characterizes chemical processes initiated through the pulse. Theoretically the hydrated electron and the hydrogen atoms react with molecular oxygen according to reactions 10 and 11 with the resulting superoxide anion being present exclusively in the deprotonated form at pH 9.2. Thus, both ions, H+ and 0 2 * - , are produced with a combined radiation chemical yield of G = 3.3. At pH 9.2 the protons will immediately neutralize with excess hydroxide ions (reaction 25).
+
H+ HO-
-H,O
(25)
The overall process, therefore, constitutes a substitution of the highly conducting hydroxide ion (& = 180 S cm2 22) by the less conducting superoxide ion (& = 60 S cm2 30), corresponding to a theoretically expected negative conductivity yield of [G(e,-) G(H')][A0(02'-) - &(HO-)] = 3.3 x (60 - 180) = -396 S cm2. Inspection of Figure 4a reveals that the negative conductivity signal slowly decays. Therefore, the exact yield of the chemically induced conductivity signal immediately after the pulse, Le., the time window compromised by the pulse itself, is obtained by extrapolation as G x AA = -360 & 20 S cm2.
+
J. Phys. Chem., Vol. 98, No. 48, 1994 12617
Reaction of Hydroxysulfuranyl Radical with Molecular Oxygen
80
n
1'
reasonably close to the theoretically expected yield within the typical error limit of &lo% for such experiments. It should be noted that a similar signal and yield was obtained upon pulse irradiation of an 02-saturated aqueous solution, pH 5.7, containing 0.25 M methanol, Le., under conditions where the radiation chemically produced hydroxyl radicals were scavenged according to reaction 26 (k26 = 5 x lo9 M-l s-l 24). HO'
ib
"1
4 a
I
Figure 5. Conductivity vs time traces obtained after pulse irradiation of oxygen-saturated aqueous solutions: (a) pH 5.7; (b) pH 5.7, containing 1.3 x M 2-MTMA.
This value is reasonably close to the theoretically expected yield, validating the considerations above. Figure 4b represents the conductivity vs time profile obtained after pulse irradiation of an 02-saturated aqueous solution, pH 9.4, containing 1.3 x M 2-MTMA. It is essentially similar to Figure 4a displaying a negative conductivity yield of G x M = -400 S cm2. This negative conductivity yield can be entirely rationalized by reactions 10-12 and 25, and therefore the reaction of molecular oxygen with the hydroxy sulfuranyl radical 4 does not lead to any additional conductivity change. Thus, a direct formation of superoxide anion according to reaction 8a is very unlikely. Theoretically, such a process would account for an additional negative conductivity of -G(HO')[&(02*-) - &(HO-)] = -324 S cm2 resulting in a combined maximum possible conductivity yield of [G(HO') G(e,,-) G(H')][&(02*-) - Ao(HO-)] = -724 S cm2. A similar picture is obtained upon performing the same experiments at the slightly acidic pH 5.7. Figure 5a displays the conductivity vs time profile obtained after pulse irradiation of 02-saturated water, pH 5.7. Among the species generated according to reaction 9 the Hf/eaq- couple and H' immediately reduce molecular oxygen (reactions 10- 12). Each reduction of 0 2 by H'or the H+/eaq- couple yields a H+/02'- pair which accounts for a theoretical conductivity change of &(H+) A0(02*-) = 315 60 = 375 S cm2 (&(H+) = 315 S cm2 22). For a quantitative treatment we have to calculate (i) the pHdependent ratio of [02'-]/[H02], and (ii) to take into account that the initial formation of H+ with G(H+) = 3.3 in an unbuffered solution of pH 5.7 induces a pH change (e.g., an applied dose of 4.17 Gy corresponding to an initial [H+] = 1.4 x M induces a pH change from 5.7 to 5.5). Thus, at pH 5.5, the ratio of [02*-]/[H02] = 4.9 and G(02'-) = G(eaq-) G(H') - G(H02) = 2.7. Consequently, we expect a theoretical positive conductivity change of 2.7 x 375 = 1012 S cm2. The extrapolation of the conductivity vs time profile in Figure 5a to the absolute yield immediately after the pulse reveals a positive conductivity change of G x AA x 850 S cm2 which is
+
+
+
+
+
+ CH,OH - H,O -I-'CH20H
(26)
A conductivity vs time profile obtained after pulse irradiation of an 02-saturated aqueous solution, pH 5.7, containing 1.3 x M 2-MTMA is displayed in Figure 5b. It shows a positive conductivity yield of G x AA = 1000 S cm2, i.e., close to that obtained in the absence of 2-MTMA. It appears, therefore, that this conductivity change is solely caused by reactions 10-12. This yield is much below the theoretically expected combined yield of G x AA = 1840 if the hydroxyl radicals would yield additional H+/02'- pairs via the reaction sequence 14 and 8a (the latter value is based on an additional radiation chemical yield, at pH 5.5, of G(02'-) = 2.24 and G(H02) = 0.45 from G(HO') = 2.7. In conclusion, conductivity experiments at both pH values 5.7 and 9.4 do not indicate the formation of superoxide anion as an immediate product of the reaction of molecular oxygen with the hydroxysulfuranyl radical 4. These observations can be rationalized by invoking the formation of a sulfur-oxygen adduct, and are corroborated by further experiments employing tetranitromethane as a scavenger for superoxide, as described below. Scavenging of Reducing Species by Tetranitromethane. Tetranitromethane (TNM) is easily reduced by superoxide anion (reaction 27; k27 = 1.9 x lo9 M-' s-l 2 5 ) and by some a-(alky1thio)alkyl radicals from aliphatic thioethers (reactions 28 and 29; kZ8= 2.8 x lo9 M-l s-l and kZ9= 9 x lo4 s-l for 'CHZ-S-CH~~~).
+ C(N02), - 0, + NO2 + C(N02),- (27) 'CH2-S-CH3 + C(N02), - CH3SCH20W(0)C(N02)3 0;-
CH3SCH20N'(0)C(N02),
-
(28) +CH2-S-CH3 4NO2
+ C(N02),-
(29)
The product nitroform anion is characterized by a strong absorption with ,Imm= 350 nm and €350 = 15 000 M-' cm-1.32 Figure 6a displays the absorption vs time profile obtained at 350 nm after pulse irradiation of an NzO-saturated aqueous solution, pH 7.2, containing 5 x lo-, M 2-MTMA and 7.5 x M TNM. The species present immediately after the pulse with GE350 = 22 350 M-' cm-' represents the combined yields of some nitroform anion, formed via reduction of TNM by H', and the hydroxysulfuranyl radical 4. The latter decomposes with first-order kinetics with t l = ~ 6.9 f 1.0 ps. Thus, the presence of TNM does not influence the rate of decomposition of 4. Most importantly, however, this reaction is not accompanied by any significant additional formation of the 350 nm band of the nitroform anion indicating that 4 does not reduce TNM to the nitroform anion. Similar results were obtained at pH 6.4 and 9.2 (data not shown). This observation also indicates that any potential deprotonation product from the radical cations 5 and/or 6 does not reduce TNM. Species 7 is expected to exhibit similar reactivity towards TNM as the 'CH2-S-CH3 radical. Taking k28 = 2.8 x lo9
Schoneich and Bobrowski
12618 J. Phys. Chem., Vol. 98, No. 48, 1994
0 25
53
time,
b
b
e.
.&...-- ...(....*"-..--.,..-~'..#.
f : :
,-..*-,. 53
100
time,
Figure 6. Absorption vs time traces obtained after pulse irradiation of (a) an NzO-saturated aqueous solution, pH 7.2, containing 5.0 x M TNM, and (b) an 02-saturated M 2-MTMA and 7.5 x M 2-MTMA and 4.0 x M TNM. solution containing 5.0 x Note: the y axes are displayed in terms of E ; the radiation chemical yields GEgiven in the text are obtained by multiplication of E by G =
Figure 7. Absorption vs time traces obtained after pulse irradiation of aqueous solutions containing 5 x M 2-MTMA: (a) N20saturated, pH 7.9, in the presence of 2.8 x M ABTS; (b) 0 2 saturated, pH 8.1, in the presence of 2.2 x M ABTS. Note: the y-axes are displayed in terms of E ; the radiation chemical yields GE given in the text are obtained by multiplication of E by G = 6.0.
6.0.
M-' scl and k29 = 9 x lo4 s-l 31 any formation of nitroform anions through 7 would occur with t1/2 = 7.7 ps, Le., well within the time frame of 40 ps covered in Figure 5a. On the other hand, species 8 is stabilized by the combined effects of an electron-donating (CH3-S-) and an electron-withdrawing substituent (-CO*CH3). The resonance stabilization energy of the analogous Et-S-C'H-CO2Et has been calculated to RSE = 18 kcaVmol as compared to 512 kcaVmol for 'CH2-S-R.33 Thus, we expect species 8 to exhibit much lower reactivity toward TNM, if at all. The absence of any nitroform anion formation appears to indicate the exclusive formation of species 8, rationalized by the high RSE. Figure 6b displays the absorption vs time profile at 350 nm obtained after pulse irradiation of an 02-saturated aqueous solution, pH 7.3, containing 5 x lop3M 2-MTMA and 4.0 x M TMN. The absorption present immediately after the pulse with G6350 = 11 340 M-' cm-' is attributed to a small amount of nitroform anion formed through reduction of TNM by eq- and H',and species 4 which, under these conditions, is formed with half of the yield as compared to N20-saturated solutions. The subsequent growth of the additional absorption occurs with t1/2 = 11.5 ps and attains a final radiation chemical yield of G6350 = 50 800 M-' cm-'. Division of the latter yield by 6350 = 15 000 M-' cm-' of the nitroform anion reveals that it is formed with G = 3.4. This value corresponds to the combined initial radiation chemical yield of hydrated electrons and H' atoms which reduce molecular oxygen according to reactions 10- 12. Superoxide anion subsequently reduces TNM according to reaction 27, corroborated by the measured tl12 = 11.5 ps of the growth of the signal at 350 nm which, under pseudo-fiist-order conditions, corresponds to k = 1.5 x lo9 M-' s-' (Le., is close to the reported k27 = 1.9 x lo9 M-' s-'). On
the other hand, we note that any reaction of 4 with molecular oxygen does not immediately produce additional yields of superoxide anion which should have been detected through higher yields of nitroform anion. These observations, together with the conductivity results, strongly favor the formation of the oxygen adduct 12 according to reaction 8b. The Potential Reaction of the Oxygen Adduct with ABTS. ABTS is an electron donor which reacts via electron transfer with many organic and inorganic radicals.34 It has been often used to compare the reactivities of various substituted peroxyl radicals (reaction 30). Rate constants €or ABTS oxidation range from ca. k30 < lo5 M-' s-l for the methylperoxyl radical (CH300') to k30 = 1.9 x lo9 M-' s-' for the trichloromethylperoxyl radical (CC1300').34
ROO'
+ ABTS - ROO- + ABTS'+
(30)
The formation of the ABTS'+ radical cation can be conveniently monitored at 415 nm (€415 = 36 000 M-' cm-' 34). Figure 7a displays the absorption vs. time profile at 415 nm obtained after pulse irradiation of an NzO-saturated aqueous solution, pH 7.9, containing 5 x M 2-MTMA and 2.8 x lop4 M ABTS. The growth of the signal is characterized by a fast (tl/z = 7.2 & 1.0 ps) and a slow process (t1/2 < 50 ps). The fast process corresponds well to the lifetime of species 4. The latter decomposes into sulfur-centered radical cations (reactions 1517) which are strong oxidants35subsequently oxidizing ABTS to its radical cation. Division of the radiation chemical yield of ABTS'+, G6415 = 105 000 M-' cm-', by 6415 = 36 000 M-' cm-I yields G = 2.9. This indicates that ABTS, at the employed concentration, is able to scavenge ca. 50% of the sulfur-centered radical cations with the residual species entering other pathways such as deprotonation (reactions 19 and 20). The slow growth
Reaction of Hydroxysulfuranyl Radical with Molecular Oxygen
SCHEME 2
H,C 12
iI
J. Phys. Chem., Vol. 98, No. 48, 1994 12619
stannyl peroxyl radicals R3SnOO' 36 which, in tum, was used to rationalize their pronounced stability toward further decomposition via dimerization. We have attempted to characterize the oxygen adduct derived from 4 by time-resolved ESR spectroscopy (pulse radiolysis-ESR), However, low sensitivity owing to the necessary low solute concentrations and line broadening did not permit any definitive conclusion on the structure of the oxygen adduct. An interesting analogy with regard to the formation of oxygen-centeredradicals at tetravalent sulfur is provided by the reaction of hydroxyl radicals with aliphatic sulfoxides. The addition of HO' to the sulfoxide sulfur yields a short-lived oxyl radical 15 containing a tetravalent sulfur atom.37 0 . H3C-S
14
of the 415 nm band with t112 < 50 p s may well be ascribed to a reaction of species 8 with ABTS. Figure 7b displays the absorption vs time profile at 415 nm obtained after pulse irradiation of an 02-saturated aqueous solution, pH 8.1, containing 5 x M 2-MTMA and 2.2 x M ABTS. The ABTS'+ radical cation is, again, formed in a biphasic growth. The fast growth accounts for the majority of ABTS'+ formation, with G6415 = 26 600 M-' cm-' and occurs with t112 = 3.6 p s , Le., approximately the half-life for the decomposition of 4 in an 02-saturated system. It represents the fraction of sulfur radical cations which are (i) formed via reactions 15-17 in competition to the addition of molecular oxygen to 4 (reaction 8), and (ii) are scavenged by ABTS. Taking G(HO') = 2.7, k(HO' ABTS) = 1.2 x 10" M-' s - ' , ~k8~ = 1.1 x lo8 M-' s-', and considering that ABTS should scavenge ca. 50% of the thioether radical cations, competition kinetics predict that ABTS'+ should theoretically be formed with G = 0.7, in good agreement with the experimentally obtained value of G(ABTS'+) = 0.74 for the fast growth. Both the half-life and yield of the slow formation of ABTS'+ in the 02-saturated system are comparable to the slow growth in the absence of 0 2 and correspond, at best, to a yield of G = 0.2. The absence of any additional formation of ABTS'+ with t1/2 < 2 x s in the 02-saturated system suggests that any oxygen adduct 12 should be a relatively weak oxidant, which reacts with ABTS with a rate constant smaller than 1.6 x lo7 M-' s-l.
+
Discussion According to the observations discussed above we can characterize the reaction of a hydroxysulfuranyl radical with molecular oxygen as proceeding via the intermediary formation of a novel oxygen adduct. These findings confirm an earlier hypothesis on the mechanism of the hydroxyl radical induced sulfoxide formation from aliphatic sulfides where molecular oxygen was found to be e~sential.'~On the other hand, scavenger experiments indicated that it was likely not an outersphere electron transfer but an addition-elimination mechanism which led to the formation of sulfoxide and superoxide.17 The formation of sulfoxide and superoxide from hydroxysulfuranyl radical-oxygen adducts, thus, occurs on a much longer time scale with t1/2 > 200 p s . The structure of the oxygen adduct may be best characterized as a sulfur-based peroxyl radical 12, although there is the likelihood that such a peroxyl radical may exist in equilibrium 31 with a pentavalent "side-on" oxygen adduct 14 (see Scheme 2). Low-temperature ESR experiments, for example, have indicated the occurrence of such an equilibrium for trialkyl-
I I
-CH3
HO 15
We shall also note that various sulfur-based oxyl and peroxyl radicals have been characterized for the reaction of thiyl radicals, RS', with molecular oxygen, e.g., RSO', RSOO', R-502, and RS0200',38 and some perthiyl based peroxyl radicals such as RSSOO' and RSS0200' have been h y p o t h e ~ i z e d .Thus, ~ ~ the formation of peroxyl radicals at hypervalent sulfur centers does not appear to be an unusual chemical reaction. It has been argued that sulfur-based peroxyl radicals are relatively weak oxidant^.“^,^^ From that point of view we do not expect structures 12 or 14 to react rapidly with ABTS, as confirmed experimentally. Intermediates with tetravalent sulfur structurally related to species 12 have been postulated for several reactions. For example, the reaction of hydrogen peroxide with dialkoxysulfuranes was proposed to involve the intermediate R'2S(OR)OOH42and a similar species appears to be formed upon addition of singlet oxygen to thioesters in alcoholic solvents." On the basis of ESR and optical studies, various bonding models for sulfuranyl radicals have been advanced describing them as u, n, or (r* radicals, re~pectively."~We cannot distinguish between a potential o or z configuration of 4 from our experiments. However, the observation of a relatively stable oxygen adduct appears to exclude the localization of the single electron in a o*-antibonding orbital. The latter structure of 4 would have been expected to undergo an outer-sphere electron transfer (reaction 8a) rather than to add molecular oxygen (reaction 8b). Hydroxysulfuranyl-oxygen adducts may play an important role as intermediates in the biological process of protein oxidation during oxidative stress. Deactivation of several enzymes, for example, may be correlated with the oxidation of methionine to the corresponding sulfoxide.44 Hydroxysulfuranyl radicals formed at methionine might be stabilized through hydrogen bonding to carbonyl oxygens located within the peptide linkage. Evidence for the latter has already been found for the model peptide G l y - M e t - G l ~ . ~ ~
Acknowledgment. The work described herein was supported by the Association for International Cancer Research (AICR) (ChS.) and by the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) (K.B.). We gratefully acknowledge the kind hospitality of Prof. K.-D. Asmus during a scientific stay of Ch.S. at the Hahn-Meitner-Institute of Berlin, F.R.G., and also express ourthanks to Drs. G. Merenyi and J. Lind for some fruitful discussions. This paper is Document No. NDRL3725 from the Notre Dame Radiation Laboratory.
12620 J. Phys. Chem., Vol. 98, No. 48, 1994
References and Notes (1) (a) Organic Chemistry of Sulfur; Oae, S . , Ed.; Plenum Press: New York, 1977. (b) Huxtable, R. J. Biochemistry of Sulfur; Plenum Press: New York, 1986. (2) Scislowski, P. W. D.; Davis, E. J. FEES Left. 1987,224, 177181. (3) Halliwell, B.; Gutteridge, J. M. C. Methods Enzymol. 1990,186, 1-85. (4) Liang, J.-J.; Gu, C.-L.; Kacher, M. L.; Foote, C. S. J . Am. Chem. SOC. 1983,105,4717-4721. 15) Watanabe. Y.: Numata. T.: Ivanaei. T.: Oae, S. Bull. Chem. SOC.
