One-electron reduction of methanesulfonyl chloride. The fate of MeSO 2CI?- and MeSO2? intermediates in oxygenated solutions and their role in the cis-trans isomerization of mono-unsaturated fatty acids

Article abstract of DOI:10.1021/ja070626q

The one-electron reduction of methanesulfonyl chloride (MeSO₂Cl) leads, in the first instance, to an electron adduct MeSO₂Cl ^?- which lives long enough for direct detection and decays into sulfonyl radicals MeSO₂^? and Cl^-, with k = 1.5 × 10⁶ s^-1. Both, MeSO²Cl ^?- and MeSO₂? showed a similar absorption in the UV with λ_max of 320 nm. In the presence of oxygen, MeSO ₂Cl^?- transfers an electron to O₂ and establishes an equilibrium with superoxide. The rate constant for the forward reaction was measured to 4.1 × 10⁹ M^-1 s ^-1, while for the back reaction only an interval of 1.7 × 10⁵ to 1.7 × 10⁶ M^-1 s^-1 could be estimated, with a somewhat higher degree of confidence for the lower value. This corresponds to an equilibrium constant in the range of 2.4 × 10 ³ to 2.4 × 10⁴. With reference to E°(O ₂/O₂^?-) = -155 mV, the redox potential of the sulfonyl chloride couple, E°(MeSO₂Cl/MeSO₂Cl ^?-), thus results between being equal to -355 and -414 mV (vs NHE). MeSO₂Cl^?- reduces (besides O₂) 4-nitroacetophenone. The underlying electron transfer took place with k = 1.5 × 10⁹ M^-1 s^-1, corroborating an E° for the sulfonyl chloride couple significantly exceeding the above listed lower value. The MeSO₂? radical added to oxygen with a rate constant of 1.1 × 10⁹ M^-1 s^-1. Re-dissociation of O₂ from MeSO₂OO^? occurred only very slowly, if at all, that is, with k ? 10⁵ s^-1. MeSO ₂^? radicals can act as the catalyst for the cis-trans isomerization of several Z- and E-mono-unsaturated fatty acid methyl esters in homogeneous solution. The effectiveness of the isomerization processes has been addressed, and in the presence of oxygen the isomerization is completely suppressed.

Full text of DOI:10.1021/ja070626q

Published on Web 06/20/2007
One-Electron Reduction of Methanesulfonyl Chloride. The
Fate of MeSO2Cl•- and MeSO2 Intermediates in Oxygenated
•
Solutions and Their Role in the Cis-Trans Isomerization of
Mono-unsaturated Fatty Acids
Maurizio Tamba,*,† Katalin Dajka,† Carla Ferreri,† Klaus-Dieter Asmus,‡ and
Chryssostomos Chatgilialoglu*,†
Contribution from the I.S.O.F., Consiglio Nazionale delle Ricerche, Via P. Gobetti 101,
40129 Bologna, Italy, and Faculty of Chemistry, Adam Mickiewicz UniVersity,
Grunwaldzka 6, 60-780 Poznan, Poland
Received January 28, 2007; E-mail: tamba@isof.cnr.it; chrys@isof.cnr.it
Abstract: The one-electron reduction of methanesulfonyl chloride (MeSO2Cl) leads, in the first instance,
to an electron adduct MeSO2Cl•- which lives long enough for direct detection and decays into sulfonyl
radicals MeSO2• and Cl-, with k ) 1.5 × 106 s-1. Both, MeSO2Cl•- and MeSO2• showed a similar absorption
in the UV with λmax of 320 nm. In the presence of oxygen, MeSO2Cl•- transfers an electron to O2 and
establishes an equilibrium with superoxide. The rate constant for the forward reaction was measured to
4.1 × 109 M-1 s-1, while for the back reaction only an interval of 1.7 × 105 to 1.7 × 106 M-1 s-1 could be
estimated, with a somewhat higher degree of confidence for the lower value. This corresponds to an
equilibrium constant in the range of 2.4 × 103 to 2.4 × 104. With reference to E° (O2/O2•-) ) -155 mV, the
redox potential of the sulfonyl chloride couple, E°(MeSO2Cl/MeSO2Cl•-), thus results between being equal
to -355 and -414 mV (vs NHE). MeSO2Cl•- reduces (besides O2) 4-nitroacetophenone. The underlying
electron transfer took place with k ) 1.5 × 109 M-1 s-1, corroborating an E° for the sulfonyl chloride couple
significantly exceeding the above listed lower value. The MeSO2• radical added to oxygen with a rate constant
of 1.1 × 109 M-1 s-1. Re-dissociation of O2 from MeSO2OO• occurred only very slowly, if at all, that is, with
•
k , 105 s-1. MeSO2 radicals can act as the catalyst for the cis-trans isomerization of several Z- and
E-mono-unsaturated fatty acid methyl esters in homogeneous solution. The effectiveness of the isomerization
processes has been addressed, and in the presence of oxygen the isomerization is completely suppressed.
systems ([O2] ) 2.4 × 10-4 M) both, RS• and thiylperoxyl
Introduction
radical RSOO• equilibrate at about equal concentrations, while
Sulfur-centered and sulfur-containing organic free radicals
have received increasing attention in biological and biochemical
mechanisms. The species of main interest in this context are
thiyl radicals, RS•. In many studies this radical has been
characterized as a very versatile species engaged in a variety
of redox, addition, and abstraction reactions. RS• have been
identified as, for example, moderately good oxidants with E°
typically in the 1.0-1.4 V range. Their oxidative power is thus
sufficient to oxidize, for example, Vit C, various phenothiazine
drugs, or NADH.1-4 Thiyl radicals are also readily engaged in
addition processes that are usually reversible. The most relevant
examples from the biological point of view are formulated in
eqs 1-3. The equilibrium constant of the oxygen addition (eq
1/-1) is about 3000, meaning that in air saturated aqueous
under typical physiological conditions ([O2] e 10-5 M) the
equilibrium remains overwhelmingly at the thiyl radical side.5
Equilibrium 2/-2 has an important impact on the redox capacity
of a thiyl/thiol containing system. While RS• is an oxidant, the
conjugated form with a thiolate (i.e., RSSR•-) is a reductant.6
The latter readily transfers an electron to, for example, molecular
oxygen yielding superoxide. Finally, the occurrence of the
reaction 3/-3 has a significant biological consequence for the
integrity of membranes, because it converts cis-unsaturated fatty
acid residues to corresponding trans isomers. Many important
studies have exclusively been devoted to this topic recently.7
(5) Tamba, M.; Simone, G.; Quintiliani, M. Int. J. Radiat. Biol. 1986, 50, 595.
Zhang, X.; Zhang, N.; Schuchmann, H.-P.; von Sonntag, C. J. Phys. Chem.
1994, 98, 6541.
(6) Wardman, P. In Glutathione Conjugation-Mechanisms and Biological
Significance; Sies, H., Ketterer, B., Eds; Academic Press: London, 1988;
pp 44-72. Wardman, P. In S-Centered Radicals; Alfassi, Z. B., Ed.;
Wiley: Chichester, U.K. 1999; pp 289-309.
(7) Chatgilialoglu, C.; Ferreri, C. Acc. Chem. Res. 2005, 36, 441. Ferreri, C.;
Chatgilialoglu, C. ChemBioChem. 2005, 6, 1722. Chatgilialoglu, C.; Ferreri,
C. Cis-Trans Isomerization in Biochemistry; Dugave, C., Ed.; Wiley-
VCH: Wenheim, Germany, 2006, 95. Chatgilialoglu, C.; Ferreri, C.;
Lykakis, I. N.; Wardman, P. Bioorg. Med. Chem. 2006, 14, 6144.
† Consiglio Nazionale delle Ricerche.
‡ Adam Mickiewicz University.
(1) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637.
(2) Sulfur-Centered ReactiVe Intermediates in Chemistry and Biology; Chat-
gilialoglu, C., Asmus, K.-D., Eds.; Plenum Press: New York, 1990.
(3) Asmus, K.-D.; M. Bonifacˇic´, M. In S-Centered Radicals; Alfassi, Z. B.,
Ed.; Wiley: Chichester, U.K. 1999; pp 141-191. Asmus, K.-D. In
Radiation Chemistry: Present Status and Future Prospects; Jonah, C. D.,
Rao, B. M. S., Eds.; Elsevier: Amsterdam, The Netherlands, 2001, 341.
(4) Tamba, M.; O’Neill, P. J. Chem. Soc., Perkin Trans. 1991, 2, 1681.
9
8716
J. AM. CHEM. SOC. 2007, 129, 8716-8723
10.1021/ja070626q CCC: $37.00 © 2007 American Chemical Society

One-Electron Reduction of Methanesulfonyl Chloride
RS• + O2 h RSOO•
A R T I C L E S
(1/-1)
(2/-2)
RS• + RS- h RSSR•-
RS• + -CHdCH- h -C•H-CH(SR)- (3/-3)
The reversible oxygen addition to thiyl radicals (eq 1/-1)
becomes significant also because of the known reactions: (i) a
rearrangement of the thiylperoxyl radical RSOO• to the sulfonyl
radical RSO2• (eq 4)8 and (ii) a further oxygen addition leading
to the sulfonylperoxyl radical RSO2OO• (eq 5).9 All these sulfur-
oxygen radicals must, therefore, be considered as potential
participants in thiol/thiyl based systems exposed to oxygen. For
instance, they are likely to act as oxidants. The question is also
to what extent they may undergo addition and abstraction
reactions like the RS• radicals.
•
RSOO• f RSO2
(4)
(5)
Figure 1. Absorption spectra obtained from the pulse radiolysis of Ar-
purged aqueous solutions containing 5 mM MeSO2Cl and 0.5 M CH3OH:
(b) 0.1 µs after the pulse; (9) 1.0 µs after the pulse. Inset shows the
absorption-time trace at 320 nm. Optical path ) 5.0 cm; dose per pulse )
39 Gy.
RSO2 + O2 f RSO2OO•
•
ESR and optical absorption spectra of sulfonyl radicals
indicated that they are σ-type species with similar spin density
of the unpaired electron on sulfur and oxygen atoms at the SO2
moiety.10-12 Theoretical calculation supported these findings and
showed that the spin distribution for the unpaired electron in
chloride (eq 6)17 or by oxidation of sulfinic acid (eq 7).9 The
present study focuses especially on the former reaction in an
aqueous environment in the absence or in the presence of oxygen
•
MeSO2 is 42% on sulfur and 44% on the oxygen atoms. For
•
and on the ability of MeSO2 radical to act as oxidant and as
PhSO2 the corresponding values are 43% and 39%.12 Moreover,
•
catalyst in the cis-trans isomerization of unsaturated fatty acids.
the ∆H298(RSO2-Cl) were measured for R ) Me or Ph by
photoacoustic calorimetry and found to be equal (295 kJ mol-1),
eaq- + MeSO2Cl f MeSO2 + Cl-
(6)
(7)
•
•
indicating that the radical chemistry of RSO2 should be
independent of the R group.13
•
HO• + MeSO2- f MeSO2 + HO-
The main synthetic applications of RSO2• radicals are based
on their ability to add to carbon-carbon double bonds.10,14 There
is also a clear evidence that the addition of a sulfonyl radical to
an olefin is a reversible reaction.15 Although the absolute rate
Results and Discussion
Reduction of Sulfonyl Chloride in the Absence of Oxygen.
Until now the generation of the sulfonyl radical, as formulated
in eq 6, has been considered a one-step dissociative electron
•
constants for the addition of RSO2 radicals to double bonds
are absent from the literature, rate constants for the â-elimination
of a p-CH3C6H4SO2• radical from a variety of carbon-centered
radicals have been measured.16
•
attachment reaction. As shown below, the formation of MeSO2
involves, however, an intermediate sulfonyl chloride radical
anion. The bimolecular rate constant k6 ) (3.3 ( 0.3) × 1010
M-1 s-1, evaluated for the reduction of MeSO2Cl from the decay
of the eaq- absorption (e.g., at its maximum at 720 nm at three
different sulfonyl chloride concentrations), thus refers only to
the initial electron attachment. This step is clearly controlled
only by the diffusion of the reactants.
•
In radiation chemical studies, MeSO2 radicals have been
generated by dissociative electron capture using sulfonyl
(8) Sevilla, M. D.; Yan, M.; Becker, D. Biochem. Biophys. Res. Commun. 1988,
155, 405. Sevilla, M. D.; Becker, D.; Yan, M. Int. J. Radiat. Biol. 1990,
57, 65. Razskazovskii, Y.; Colason, A.-O.; Sevilla, M. D. J. Phys. Chem.
1995, 99, 7993.
(9) Sehested, K.; Holcman, J. J. Radiat. Phys. Chem. 1996, 47, 357.
Razskazovskii, Y.; Sevilla, M. D. J. Phys. Chem. 1996, 100, 4090. Flyunt,
R.; Makogon, O.; Schuchmann, M. N.; Asmus, K.-D.; von Sonntag, C. J.
Chem. Soc., Perkin Trans. 2001, 2, 787.
(10) Chatgilialoglu, C. In The Chemistry of Sulfones and Sulfoxides; Patai, S.,
Rappoport, Z., Stirling, C. J. M., Eds.; Wiley: Chichester, U.K. 1988; pp
1089-1113.
(11) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C. J. Chem. Soc. Perkin
Trans. 1979, 2, 770. Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C.
J. Chem. Soc., Perkin Trans. 1980, 2, 1429.
Tracing the sulfonyl chloride reduction at 320 nm, that is, at
•
the maximum of the MeSO2 radical absorption, revealed the
above indicated, more complex situation. This can be realized
by inspection of the inset in Figure 1, which specifically pertains
to an argon-saturated aqueous solution containing 50 mM
MeSO2Cl and 0.5 M methanol.18 Two distinct processes are
apparent. An initial fast rise occurred practically within the
duration of the pulse and accounted for almost two-thirds of
the final absorption. The second, considerably slower step, made
up for the remaining third.
(12) Chatgilialoglu, C.; Griller, D.; Guerra, M. J. Phys. Chem. 1987, 91, 3747.
Alberti, A.; Chatgilialoglu, C.; Guerra, M. J. Chem. Soc., Perkin Trans.
1986, 2, 1179.
(13) Chatgilialoglu, C.; Griller, D.; Kanabus-Kaminska, J. M.; Lossing, F. P. J.
Chem. Soc., Perkin Trans. 1994, 2, 357. See also: Laarhoven, L. J. J.;
Mulder, P.; Wayner, D. D. M. Acc. Chem. Res. 1999, 32, 342.
(14) Chatgilialoglu, C.; Bertrand, M. P.; Ferreri, C. In S-Centered Radicals;
Alfassi, Z. B., Ed.; Wiley: Chichester, U.K. 1999; pp 312-354. Bertrand,
M. P.; Ferreri, C. In Radical in Organic Synthesis; Renaud, P., Sibi, M.,
Eds.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 2, pp 485-503.
(15) Asscher, M.; Vofsi, D. J. Chem. Soc. 1964, 4962.
(17) Eriksen, T. E.; Lind, L. Radiochem. Radioanal. Lett. 1976, 25, 11.
(18) Radiolysis of neutral water leads to eaq- (0.27), HO• (0.28), and H• (0.062);
the values in parentheses represent the radiation yields in units of µmol
(16) Timokhin, V. I.; Gastaldi, S.; Bertrand, M. P.; Chatgilialoglu, C. J. Org.
Chem. 2003, 68, 3532.
J-1. In the presence of 0.5 M CH3OH, both HO• (k ) 9.7 × 108 M-1 s-1
)
and H• (k ) 2.6 × 106 M-1 s-1) are scavenged efficiently.19
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8717

A R T I C L E S
Tamba et al.
The same picture was observed when methanol is replaced
studies of radiolysis of a solid sample of piperidine-1-sulfonyl
chloride.22 Its radical anion is believed to possess the σ*-type
rather than the π-type quasi-trigonal bipyramidal structure, on
the basis of the spectral parameters, that is, the incoming electron
is accommodated in an antibonding orbital of the sulfur-chlorine
bond, yielding a 2σ/1σ* three-electron bonded species (2). The
fact that both radicals show similar optical absorptions in the
UV (with λmax at around 320 nm) with relatively low extinction
coefficient, suggests that in the radical anion 2 the valence
transitions also involves transfer of electrons from the lone pair
orbitals of the oxygen atoms to the SOMO σ*.
•
by tert-butanol. Neither CH2OH (as abstracting or reducing
•
species) nor CH2C(CH3)2OH (as abstracting species) contrib-
uted to the observed effects on this early time scale reported in
the figure.20 The absorption spectrum of the slowly formed
species, recorded at times g1 µs, is identical to that of the
MeSO2• radical. Qualitatively the same spectrum was obtained
for the transient formed in the much faster initial process. This
was recorded at 100 ns, that is, practically immediately after
the pulse. It also exhibited a maximum at around 320 nm but
differed from that of the sulfonyl radical by a generally lower
absorptivity. This difference was slightly more pronounced at
the high-energy side than at the low-energy side, indicating an
•
apparent slight red-shift relative to MeSO2 radical. For com-
parison, both spectra are shown in Figure 1.
Kinetically, the slow step can be fitted by an exponential.
The first-order rate constant corresponded to k ) (1.5 ( 0.5)
× 106 s-1. The same value was obtained at any other, for
example, a ten times lower (5 mM), sulfonyl chloride concen-
tration, identifying this step as a true first-order process. This,
together with the slight difference in absorption characteristics,
suggests that we are dealing with two different, although
spectroscopically similar species, namely, a transient im-
Reduction of Sulfonyl Chloride in the Presence of Oxygen.
In the presence of oxygen a two-step absorption increase was
also observed in the UV, which qualitatively resembles that
displayed in the inset of Figure 1. With respect to yields and
kinetics certain differences were noted. Based on current
knowledge, at least two features can be anticipated, namely, (i)
competition between MeSO2Cl and O2 for the hydrated electron
-
mediately formed in the eaq + MeSO2Cl reaction, which
subsequently decays into the sulfonyl radical. The intermediate
is considered to be the electron adduct, that is, the sulfonyl
chloride radical anion. Mechanistically, both species are linked
through the reaction sequence formulated in eqs 8 and 9
•
and (ii) reaction of the MeSO2 radical with O2. Concerning
MeSO2Cl + eaq- f MeSO2Cl•-
(8)
(9)
the first aspect, the rate constant for the sulfonyl chloride
reduction (eq 8) is almost 50% higher than that for the oxygen
reduction (eq 10, k10 ) 2.1 × 1010 M-1 s-1).19 In 5 mM MeSO2-
Cl solution, for example, which contains also 1 × 10-4, 2.4 ×
10-4 (air sat.) or 5 × 10-4 M oxygen, about 1.2, 3, and 6% of
the hydrated electrons will be scavenged by O2, respectively.
Accordingly, the yields of the sulfonyl radical and its presumed
radical anion precursor should be lowered by similar percent-
ages. In the presence of the above listed oxygen concentrations
MeSO2Cl•- f MeSO2 + Cl-
•
The reaction of hydrated electrons with the 5 mM MeSO2Cl
occurs with t1/2 ) 4.2 ns and is completed within a few
nanoseconds. For such fast reactions the yield of scavengable
electrons is expected to increase noticeably with the scavenger
concentration in the upper mM range, because of an increased
probability to intercept the radiation chemical geminate recom-
bination process. In accordance with corresponding findings in
other systems with efficient electron scavengers,21 the measured
yield of MeSO2Cl•- (initial absorption) increased by about 10-
15% when the MeSO2Cl concentration was increased from 5
to 50 mM.
•
the yields of MeSO2 (as measured at 320 nm) dropped,
however, by 20 (24% according to averaged curve), 43%, and
56%, respectively (see Figure 2).
Raising the MeSO2Cl concentration to 50 mM (all other
conditions unchanged) left even only a tenth of the above
calculated yields for the direct contribution by reaction 8, that
•
The optical absorption spectra of MeSO2 and other substi-
•
is, 0.6% at most. Yet, the MeSO2 radical yield was still reduced
tuted sulfonyl radicals have been the subject of a detailed
study.12 The maximum around 325 nm (ꢀ ≈ 800 M-1 cm-1) in
water was assigned by multiple scattering XR calculations to
valence transitions involving transfer of electrons from the lone
pair orbitals of the oxygen atoms to the SOMO (1), which is in
accord with a red shift observed in going from water to
hydrocarbon solvents, owing to stabilization of the ground state
by hydrogen bonds. Regarding the transient radical anion of
sulfonyl chlorides, evidence for its existence came from the ESR
by the same percentages observed with the oxygen-free system
as in the 5 mM MeSO2Cl system. This clearly hints at a reaction
of MeSO2Cl•- (i.e., the precursor of MeSO2 ) with O2 (eq 11)
•
as a process that competes with eq 9.
•-
O2 + eaq- f O2
(10)
(11)
MeSO2Cl•- + O2 f MeSO2Cl + O2
•-
(19) Buxton, G. V.; Greenstock, C. L.; Helman, P. W.; Ross, A. B. J. Phys.
Chem. Ref. Data 1988, 17, 513.
Kinetically this competition has been evaluated by measuring
the decrease in sulfonyl radical yield (which equals the yield
•
(20) A slow reaction of HOCH2 radical with MeSO2Cl is observed at longer
time-scale. Detailed studies of a variety of R-hydroxyalkyl radicals with
few sulfonyl chlorides are under investigation and will be published
elsewhere. (Bobrowski, K.; Chatgilialoglu, C.; Tamba, M., et al. Unpub-
lished results.)
(22) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C.; Symons, M. C. R. J.
Chem. Res. 1980, 185, 2610.
(21) Asmus, K.-D.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 348.
9
8718 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

One-Electron Reduction of Methanesulfonyl Chloride
A R T I C L E S
•
Figure 4. Plot of kobs for decay of MeSO2 absorption at 320 nm as a
function of O2 concentration.
Figure 2. Optical density at 320 nm recorded 1 µs after the electron pulse
from aqueous solutions containing 5 mM MeSO2Cl, 0.5 M CH3OH, and
various O2 concentrations. Optical path ) 5.0 cm; dose per pulse ) 39 Gy.
Points represent the mean of at least three independent measurements. In
the errors bars, the values of the low and high arguments are absolute values.
reaction of MeSO2Cl•- with O2. At 5 × 10-4 M O2, reaction 9
occurred with t1/2 ≈ 0.5 µs, that is, at 100 ns (the time of signal
recording) ca. 13% of MeSO2Cl•- has already reacted. This,
together with the 6% contribution of reaction 10, plausibly
makes up for the observed overall 20% lower signal.
The fact that in oxygenated systems the absolute yields of
MeSO2Cl•- and MeSO2• became about 15-20% higher, when
the sulfonyl chloride concentration is increased from 5 to 50
mM, can be again reasonably explained by an increased
efficiency of the eaq- and H• scavenging (increased interference
with the geminate recombination effect, see above).
Reaction of the Sulfonyl Radical with Oxygen. Our present
results unambiguously confirm a reaction between the sulfonyl
radical and oxygen (eq 13). This expresses itself in an
•
increasingly faster decay of the MeSO2 radical absorption with
increasing oxygen concentration. Figure 4 shows the plot of
the observed first-order rate constants at various [O2]. From the
slope a rate constant of k13 ) (1.1 ( 0.4) × 109 M-1 s-1 is
derived for the oxygen addition. Our present measurement is
in excellent agreement with earlier values around 1 × 109 M-1
s-1 and confirms that the underlying reaction is practically a
diffusion-controlled process.9
Figure 3. Evaluation of rate constant k11 according to eq 12.
of reaction 11) at the various oxygen concentrations. A plot of
the appropriate relationship (eq 12), namely
MeSO2 + O2 h MeSO2OO•
(13/-13)
•
•
•
•
[MeSO2 ]0/([MeSO2 ]0 - [MeSO2 ]) ) 1 + (k9/k11) ×
1/[O2] (12)
Figure 4 shows an intercept of 6.3 × 104 s-1. A smaller part of
this reflects the decay of MeSO2• in the absence of O2 (ca. 1 ×
104 s-1 contribution based on the first half-life of the bimolecular
termination). In addition, the intercept would also include the
rate constant for a possible back reaction (eq -13). The
assumption of such an oxygen addition equilibrium, in principle,
is most reasonable in view of a corresponding equilibrium
involving thiyl radicals (eq 1). For any quantitative assessment
it must, in any case, be recognized that even small changes of
the slope (remaining well within the above-mentioned error
limits) strongly affect the absolute value of the intercept. The
low rate constant k-13 e 5 × 104 s-1 derived in this way is,23
however, fully corroborated by the experimental results con-
•
•
is shown in Figure 3, with [MeSO2 ]0 and [MeSO2 ] referring
to the measured absorptions at 320 nm in the absence and
presence of oxygen, respectively. As can be appreciated upon
inspection of the data, a good straight line can be drawn through
the origin (infinite oxygen concentration) and the three experi-
mental oxygen concentrations. From the slope of the straight
line in Figure 3 and with the above evaluated k9 ) 1.5 × 106
s-1, a bimolecular rate constant for the electron transfer of k11
) 4.1 × 109 M-1 s-1 is derived.
The yield of the initial (MeSO2Cl)•- absorption at 320 nm
(recorded at 100 ns) was shown to decrease slightly with
increasing [O2] and, at higher oxygen concentration, remained
almost constant. An up to 6% decrease over the experimental
[O2] range is due to the above-mentioned scavenging of hydrated
electrons by O2 (eq 10). The fact that the actual experimental
readings are still somewhat lower (up to about 20% below the
initial level) is plausibly explained by an increasingly faster
(23) An equilibrium may also be reflected in the incomplete decay of the
absorption at 320 nm, which seems to remain at a finite level of about
15% of initial absorption (see Figure 5e). If all of this was due to the
•
equilibrium concentration of MeSO2 , K13 and k-13 could be evaluated more
explicitly. However, because of other possible long-lived absorptions
(sulfonyl peroxyl, final products), the values thus derived should again only
be regarded as limiting values, that is, K13 > 2.4 × 104 M-1 and k-13
<
4.6 × 104 s-1
.
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8719

A R T I C L E S
Tamba et al.
In an air-saturated solution of an otherwise identical system
a significantly enhanced absorption signal was observed at 260
nm (Figure 5b). In particular, it is noted that the signal stays
practically stable over a time range of 10 µs, while at 320 nm
(Figure 5d) the signal (representing the forward reaction of
equilibrium 10) decayed significantly within this period. There-
fore, the long-term absorption at 260 nm cannot be attributed
anymore to the MeSO2• radical but is, accordingly, assigned to
the long-lived O2•-. The possibility that the absorbing species
may be the MeSO2OO• is discarded, because the rate of the
•
oxygen addition to MeSO2 (t1/2 ) 2.6 µs) is considerably slower
than even the slowest step of the formation of the absorption at
260 nm (t1/2 ≈ 0.5 µs).
•-
A further piece of information concerns a reaction of O2
with sulfonyl chloride. Occurrence of this process is concluded
from an acceleration of the decay of the 260 nm absorption
with increasing sulfonyl chloride concentration. The underlying
reaction is considered to be an electron transfer by which a
sulfonyl chloride radical anion is regenerated. The O2•-/MeSO2-
Cl•- system thus constitutes an equilibrium, and eq 11 is,
therefore, re-formulated as a reversible process.
MeSO2Cl•- + O2 h MeSO2Cl + O2
(11/-11)
•-
Further support for the equilibrium is provided by the slight
•-
drop in O2 yield upon increasing the sulfonyl chloride
concentration from 5 to 50 mM (in air-saturated solutions), as
it can be seen from the long-term absorptions at 260 nm in
Figure 5b,5c, respectively. This observation reflects the higher
efficiency of the back reaction (eq -11) at the higher sulfonyl
Figure 5. (a-c) Absorption-time traces at 260 nm obtained from pulsed
aqueous radiolysis of 0.5 M CH3OH, containing also (a) 5 mM MeSO2Cl,
no O2 (Ar-saturated); (b) 5 mM MeSO2Cl, air-saturated; and (c) 50 mM
MeSO2Cl, air-saturated. (d-e) Absorption-time traces at 320 nm obtained
from pulsed aqueous of 0.5 M CH3OH, containing also (d) 5 mM MeSO2-
Cl, air-saturated; (e) 50 mM MeSO2Cl, air-saturated. Optical path cell )
5.0 cm; dose per pulse ) 39 Gy.
•
chloride concentration. The fact that simultaneously the MeSO2
yield increases (Figure 5e compared with Figure 5d) can, in
part, also be interpreted along this line. For a quantitative
assessment of k-11 several factors need to be taken into account.
The observed decay rate at 260 nm thus reflects not only the
inherent equilibrium kinetics and the various possible radical-
radical termination reactions but also possible unknown con-
tributions by other species (radicals and molecular products) to
the absorption at this wavelength, as well as complications
introduced by the chosen composition of the investigated
systems. A desirable parameter is, for example, a sufficiently
high O2•- concentration. This can be achieved by several means.
The highest direct formation of this species via reaction 10 (and
corresponding reaction of O2 with H• atoms) requires high O2
concentration. This, in turn, accelerates the forward reaction in
equilibrium 11/-11. As a consequence, any electron transfer from
O2•- to MeSO2Cl will increasingly, and eventually much faster,
be reversed than the irreversible removal of MeSO2Cl•- through
chloride ion cleavage (eq 9). The apparent yield of O2•- decay
thus becomes increasingly smaller. A higher yield of O2•- can
alternatively (although indirectly) be generated by increasing
the sulfonyl chloride concentration, namely, through the se-
quence of reactions 8 and 11. Addition of formate (instead of
methanol) can potentially double the achievable O2•- yield via
conversion of •OH and H• into superoxide.18,24 Reactions
contributing in such systems are formulated in eqs 14-16 and
11.
cerning isomerization of double bonds in systems containing
sulfonyl radicals and oxygen (see later).
Involvement of O2•-. Direct observation of O2•-, formed in
reaction 11, is, in principle, possible through its optical
absorption in the UV (λmax ) 245 nm, ꢀ ≈ 2350 M-1 cm-1).24
Because of experimental limitations (e.g., self-absorption of
solutes, other absorbing radicals, insufficient intensity of the
analyzing light) meaningful measurements are often hampered
at this low wavelength. This applied also to our sulfonyl chloride
system, restricting us to the region g260 nm. The molar
•-
absorptivity of O2 at this wavelength is, however, still high
enough (ꢀ ≈ 1700 M-1 cm-1) for an identification of the
superoxide anion.24 Figure 5 displays three absorption-time
traces recorded at this wavelength in pulse irradiated solutions
containing 0.5 M CH3OH and (a) 5 mM MeSO2Cl, no O2
(Argon-saturated), (b) 5 mM MeSO2Cl, air-saturated, and (c)
50 mM MeSO2Cl, air-saturated. Additionally shown are the
traces of the latter two solutions at the wavelength of 320 nm
(Figure 5d,e).
In the oxygen-free system (Figure 5a) the trace resembled
the two-step features of the recording at 320 nm (Figure 1) and
is, therefore, mainly attributed to the contributions of the MeSO2•
radical and its MeSO2Cl•- precursor at this wavelength.
(24) Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. J. Phys. Chem.
Ref. Data 1987, 14, 1041. Bielski, B. H. J. Photochem. Photobiol. 1978,
28, 645.
•OH/H• + HCO2- f CO2•- + H2O/H2
(14)
9
8720 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

One-Electron Reduction of Methanesulfonyl Chloride
CO2•- + O2 f O2•- + CO2
A R T I C L E S
constant for the PNAP•- formation was measured to be 7.5 ×
104 s-1. This is 20 times lower than the radical anion decay
into sulfonyl radicals via eq 9 (k9 ) 1.5 × 106 s-1). Accordingly
only about 5% of MeSO2Cl•- can react with PNAP in this
system. This was fully corroborated by the absorptions measured
after completion of reactions 9 and 18. On the basis of the
known molar absorption coefficients of the reaction products
(15)
(16)
CO2•- + MeSO2Cl f MeSO2Cl•- + CO2
Considering all these aspects, we chose to conduct experiments
with a system containing 1.25 × 10-3 M oxygen, 0.5 M HCO2
,
-
and various concentrations of MeSO2Cl. The observed first-
order decay (kobs) at 260 nm did not increase, however, linearly
with sulfonyl chloride concentrations, but the plot of kobs versus
[MeSO2Cl] showed a continuous curvature. Thus, it was not
possible to extract an unambiguous value for k-11 from these
data. The bimolecular rate constant evaluated from the lowest
MeSO2Cl concentration (0.1 mM), under the assumption that
eq -11 is the only process controlling the decay at 260 nm (1.7
•
•-
MeSO2 (ꢀ ≈ 1000 M-1 cm-1 at 320 nm) and PNAP (ꢀ ≈
18200 M-1 cm-1 at 350 nm),25 the PNAP reduction occurred
with an efficiency of about 6%. The above first-order rate for
the formation of PNAP•- gives a bimolecular rate constant k18
) 1.5((0.2) × 109 M-1 s-1. This efficient electron-transfer
suggests that K11 must be high enough to yield a redox potential
for the MeSO2Cl/MeSO2Cl•- couple which is reasonably more
negative than that of (E° ) -353 mV)1 for the PNAP/PNAP•-
couple, that is, K11 > 2240 in agreement with our above
× 106 M-1 s-1), must be corrected at least for the mutual O2
•-
termination. Additionally, eq 17 may have to be considered
•-
conclusion from the O2 experiment.26
owing to the reductive power of the CO2 (E0 ) -1.1 V)1
•-
Cis-Trans Isomerization of Mono-unsaturated Fatty Acid
Methyl Esters by Sulfonyl Radical. The γ-radiolysis of
ethanol/water (2/1, v/v) in N2 flushed solutions led to the
transient species shown in eqs 19 and 20, where R• represents
which should even reduce the superoxide to hydrogen peroxide
(E(O2•-, 2H+/H2O2) ≈ 0.9 V at pH 7).1
2H
O2•- + CO2
+8 H2O2 + CO2
(17)
•-
•
•
the produced alcohol radicals (i.e., CH2CH2OH, CH3 CHOH,
CH3CH2O•).27 Hydrogen abstraction from ethanol by HO• radical
and H• atoms increased the production of alkyl radical species
(eq 21). Solvated electrons as well as possibly all R• radicals
reacted with methanesulfonyl chloride to give the corresponding
sulfonyl radical (eqs 22 and 23).28
With a CO2•- concentration of about 10-5 M in our experiment
this electron-transfer reaction would become effective if k17
approaches or exceeds a value as low as 107 M-1 s-1. Reaction
•-
17 and the O2 termination should have less, if any, of effect
at the higher sulfonyl chloride concentrations. As mentioned
above, these systems are affected by an almost complete and
immediate back electron transfer (eq 11). The formally calcu-
lated k-11 values vary between 1.7 × 106 and 1.7 × 105 M-1
s-1 for the 0.1-5 mM MeSO2Cl range. Accordingly, the
equilibrium constant K11 would range from 2.4 × 103 to 2.4 ×
104. Because the k-11 and K11 values derived from the high
concentration system lack the complication by reaction 17, they
can probably be viewed with a comparatively higher degree of
confidence. Irrespective of these uncertainties there seems to
be no ambiguity though about the reversibility of eq 11.
Knowing the equilibrium constant allowed us to calculate the
difference in redox potentials between the MeSO2Cl/MeSO2-
H2O
9
γ8 eaq-, HO•, H•
γ8 esol-, R•
(19)
CH3CH2OH
9
(20)
(21)
(22)
(23)
HO•/H• + CH3CH2OH f H2O/H2 + R•
eaq-/esol- + MeSO2Cl f MeSO2 + Cl-
•
R• + MeSO2Cl f MeSO2 + RCl
•
A solution of 2.5 mM methyl palmitoleate (9c-C16:1) and 1.25
mM MeSO2Cl in ethanol/water (2/1, v/v) was deoxygenated
by N2 stream and then irradiated using a 60Co-Gammacell (dose
rate ) 10 Gy min-1) at 22 °C. The reaction was monitored by
GC, and the dose profile of 9c-C16:1 disappearance (0) and
methyl palmitelaidate (9t-C16:1) formation (9) is shown in Figure
6. The two geometrical isomers reached an equilibrium after
300 Gy, the cis/trans ratio being 18/82. Figure 6 (2, 4) shows
also that the same equilibrium is reached in a practically equal
time if one starts from the trans isomer 9t-C16:1.
•-
Cl•- and O2/O2 couples:
∆E° ) E°(O2/O2•-) - E°(MeSO2Cl/MeSO2Cl•-) )
RT ln K/nF ≈ 59.1 log K
Our experimental limits for K11 translate into a ∆E° ) 200-
259 mV range. With E°(O2/O2•-) ) -155 mV1 the redox
potential range for the MeSO2Cl/MeSO2Cl•- couple thus
calculates to
E°(MeSO2Cl/MeSO2Cl•-) ) E°(O2/O2•-) - ∆E° )
-355 to -414 mV (vs NHE)
(25) Whillans, D. H. Radiat. Phys. Chem. 1977, 10, 335.
(26) In the interpretation of the above results, a possible reaction of PNAP with
•CH2OH radicals should be considered. Having a rate constant as low as
8.6 × 106 M-1 s-1 (see: Jagannadham, V.; Steenken, S. J. Am. Chem.
Soc. 1988, 110, 2188) this reaction is, however, too slow to account for
the observed PNAP•- formation. Furthermore, this reaction is not an electron
transfer but has been identified as an adduct formation. As in the MeSO2-
Cl/O2 system, the underlying electron transfer may also be reversible in
the MeSO2Cl/PNAP system. Unambiguous proof of this reasonable
assumption was, however, not anticipated in this study.
The MeSO2Cl•- radical anion is, accordingly, a significantly
better reductant than O2•-, well in accord with our experimental
finding. To further confirm this conclusion, we tested it with
the MeSO2Cl•--induced reduction of 4-nitroacetophenone (PNAP):
(27) Asmus, K.-D.; Mo¨ckel, H.; Henglein, A. J. Phys. Chem. 1973, 77, 1218.
(28) Primary and secondary alkyl radicals react with sulfonyl chlorides with
rate constants in the range of 3 × 105 to 1.3 × 106 M-1 s-1 at 25 °C in
organic solvents. See: Chatgilialoglu, C. J. Org. Chem. 1986, 51, 2871.
Horowitz, A. Int. J. Chem. Kinet. 1976, 8, 709. The rate constants of the
same reaction in water could be larger, because of the polar contributions
that stabilize the transition state.
MeSO2Cl•- + PNAP f MeSO2Cl + PNAP•- (18)
For a solution containing, for example, 5 × 10-5 M PNAP
(besides 5 mM MeSO2Cl and 0.5 M CH3OH) a first-order rate
9
J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8721

A R T I C L E S
Tamba et al.
Assuming that G(MeSO2 ) is 0.65 µmol J-1 29
,
we calculated a
•
catalytic cycle for the isomerization of ca. 120 at the initial
phase. Scheme 1 shows the mechanism that we conceive for
this process of cis-trans isomerization by the MeSO2• radical.
The percentages of trans/cis isomers after completion are 82/
18 at 22 °C for all compounds, corresponding to an equilibrium
constant K ) 4.6. The reported difference in the stability of the
two isomers in 2-butenes, that is, ∆G°(trans) - ∆G°(cis) )
-3.1 kJ mol-1, corresponds to K ) 3.5, since ∆G ) -RT ln K.
Therefore, in mono-unsaturated acid methyl esters the final
isomeric composition should account for the difference in the
stability of the two isomers and further support the role of
•
MeSO2 radicals as an efficient catalyst in the cis-trans
isomerization.31
We also tested the effect of molecular oxygen in the cis-
trans isomerization. In particular, the isomerization of 9c-C14:1
was repeated under the above-described experimental conditions,
without degassing and leaving the sample to the open air. After
400 Gy of irradiation dose, no isomerization was observed. A
similar experiment was carried out with 6c-C18:1 and the sample
was air-saturated prior to irradiation. After 300 Gy again no
isomerization was observed. These results indicate an inhibitory
effect of oxygen toward isomerization.31 Although absolute rate
Figure 6. γ-Irradiation (dose rate ) 10 Gy min-1) of 2.5 mM methyl
palmitoleate (9c-C16:1) or methyl palmitelaidate (9t-C16:1) with MeSO2-
Cl (1.25 mM) in N2-fluxed ethanol/water (2/1, v/v) at 22 °C. Dose profiles
of disappearance of 9c-C16:1 (0) and formation of 9t-C16:1 (9) starting
from cis isomer or disappearance of 9t-C16:1 (2) and formation of 9c-
C16:1 (4) starting from trans isomer.
•
constants for the addition of RSO2 radicals to double bonds
are absent from the literature, rate constants for the â-elimination
•
of p-CH3C6H4SO2 from a variety of carbon-centered radicals
have been measured.16 Of interest to us is the rate constant of
1.5 × 106 s-1 (at 20 °C) obtained for PhCH2CH(•)CH2SO2Ts.
This value is expected to be one or more orders of magnitude
higher when a second â-substituent is attached to the carbon
bearing the sulfonyl moiety and the efficiency of the alkyl
radical trapping by oxygen should, accordingly, be very low
(cf. Scheme 1).32 Therefore, the observed inhibitory effect of
oxygen could indeed plausibly be explained by the scavenging
of the sulfonyl radicals.33
Conclusion
In the present study unambiguous evidence is provided for a
relatively long-lived electron adduct to sulfonyl chlorides,
RSO2Cl•-. It acts as reductant and establishes an equilibrium
with O2•-. In the absence of a suitable redox partner, it decays
Figure 7. (left side) Formation of trans isomers from γ-irradiation (dose
rate ) 10 Gy min-1) of different mono-unsaturated fatty acid methyl esters
(2.5 mM) with MeSO2Cl (1.25 mM) in N2-fluxed ethanol/water (2/1, v/v)
at 22 °C: (b) 9t-C14:1, (9) 9t-C16:1, ([) 6t-C18:1, (2) 9t-C18:1, (1)
11t-C18:1, (left-pointing triangle) 11t-C20:1. (right side) Each point is the
sum of cis and trans isomers.
•
under elimination of chloride into the RSO2 radical. Oxygen
•
addition to RSO2 constitutes another likely equilibrium. The
radical chemistry of sulfonyl chloride, particularly if occurring
in an oxygenated environment, constitutes a complex system
of several equilibria. Combined with or embedded into a thiol/
oxygen containing system, which itself is characterized by redox
and addition equilibria, as well as rearrangement reactions (e.g.,
We extended this approach to five other cis-mono-unsaturated
fatty acid methyl esters, that is, myristoleate (9c-C14:1),
petroselinate (6c-C18:1), oleate (9c-C18:1), vaccenate (11c-C18:
1), and eicosenoate (11c-C20:1), and the results were very
similar in all the experiments. Figure 7 (left side) shows the
dose profile of the six trans isomer formations to be practically
the same. These results also indicate that the position of
unsaturation and the hydrocarbon chain length have no influence
on the cis-trans isomerization process. Figure 7 (right side)
shows no loss of unsaturated esters since the loss of cis
isomer is exactly balanced by the formation of geometrical trans
isomer.
(29) On the basis of the total G ) 0.61 µmol J-1 in water (eq 19) and the total
G ) 0.67 µmol J-1 in ethanol (eq 20),30 the assumption of G ) 0.65 µmol
J-1 seems reasonable.
(30) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry,
3rd Ed.; Wiley: New York, 1990.
(31) It is worth also recalling that the equilibrated cis/trans mixtures are similar
to those reported for the isomerization of unsaturated fatty acid methyl
esters by thiyl radicals, see: Chatgilialoglu, C.; Ferreri, C.; Mulazzani, Q.
G.; Ballestri, M.; Landi, L. J. Am. Chem. Soc. 2000, 122, 4593.
Chatgilialoglu, C.; Zambonin, L.; Altieri, A.; Ferreri, C.; Mulazzani, Q.
G.; Landi, L. Free Radical Biol. Med. 2002, 33, 1681.
(32) Chatgilialoglu, C.; Altieri, A.; Fischer, H. J. Am. Chem. Soc. 2002, 124,
12816. Chatgilialoglu, C.; Samadi, A.; Guerra, M.; Fischer, H. ChemPhy-
sChem. 2005, 6, 286.
The product yield (mol/kg) divided by the absorbed dose (1
Gy ) 1 J kg-1) gives the radiation chemical yield or G(isomer-
ization). Plotting the G(isomerization) versus dose and extrapo-
lating the curve to zero dose, a G = 80 µmol J-1 is obtained.
(33) It is worth mentioning that the thiyl radical-catalysed isomerization process
does not suffer from the presence of oxygen,31 thus indicating that the
equilibrium reaction of thiyl radical by oxygen is shifted to the left (see
introduction).
9
8722 J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007

One-Electron Reduction of Methanesulfonyl Chloride
A R T I C L E S
Scheme 1. Reaction Mechanism for the Cis-Trans Isomerization
RSOO• f RSO2 ) based on the RS• radical chemistry, the overall
•
thickness). Temperature started from 160 °C held for 55 min, followed
by an increase of 5 °C/min up to 195 °C, held for 10 min, followed by
a second increase of 10 °C/min up to 250 °C. The carrier gas pressure
was helium at a constant pressure of 29 psi. Trans methyl esters were
identified by comparison with the retention times of commercially
available authentic samples.
chemistry may thus assume a high degree of delicately balanced
complexity. In the present study this was demonstrated, for
example, with the cis-trans isomerization of mono-unsaturated
fatty acid compounds.
Isomerization of Mono-unsaturated Fatty Acids Methyl Esters.
A 2.5 mM solution of fatty acid methyl ester in ethanol/water (2/1,
v/v) was placed in a 4 mL vial equipped with an open-top screw cap
and a Teflon-faced septum and added with 1.25 mM methanesulfonyl
chloride and methyl stearate (1 mM) as the internal standard. The
mixture was flushed with N2 for 15 min prior to γ-irradiation. To follow
the reaction course by GC analysis, 100 µL aliquots were withdrawn
at different times. The aliquots were processed by the addition of an
aqueous saturated solution of NaHCO3 and the evaporation of ethanol
under vacuum. Then, n-hexane was added, and the organic phase,
containing the methyl ester, was washed with water and dried over
anhydrous sodium sulfate. After evaporation of the solvent under
vacuum, the residue containing the fatty acid methyl esters was
examined by GC for the determination of cis/trans isomeric ratio.
Isomerization of Mono-unsaturated Fatty Acids Methyl Esters
in the Presence of Oxygen. The experiment reported above was
repeated using methyl myristoleate as the substrate and avoiding the
degassing procedure. After the above-reported workup of the reaction
samples at different times reaching 400 Gy of total irradiation dose,
GC analysis was carried out in comparison with an authentic com-
mercially available sample of myristelaidic acid methyl ester. No traces
of trans isomer were detected. Another experiment was carried out with
methyl petroselinate under the same condition, except that the sample
was air-saturated prior to irradiation. After the above-reported workup
of the reaction samples at different times reaching 300 Gy of total
irradiation dose, GC analysis was carried out in comparison with an
authentic commercially available sample of petroselaidic acid methyl
ester, showing the absence of trans isomer.
Experimental Section
Materials. Methanesulfonyl chloride (Aldrich) was distilled before
use under nitrogen. Sodium formate and p-nitroacetophenone (PNAP)
from Merck and alcohols (HPLC grade) and mono-unsaturated fatty
acid methyl esters from Sigma-Aldrich Co. were used without further
purification. Solutions were freshly prepared using water purified with
a Millipore (Milli-Q) system.
Pulse Radiolysis. All time-resolved investigations were conducted
by means of the radiation chemistry technique of pulse radiolysis, using
the 12 MeV LINAC (linear accelerator) CNR facility at the ISOF
Institute in Bologna. Details of the optical detection system, methods,
and computer treatment of data are described elsewhere.34 Pulses from
50 to 200 ns were used according to the dose (10-40 Gy) to be
delivered to the samples. The pulse irradiation was performed at room
temperature (22 ( 2°) on samples contained in Spectrosil quartz cells
of 5 cm optical path length. In the presence of oxygen, the investigated
aqueous systems generally contained (5-50) mM MeSO2Cl, 0.5 M
CH3OH, and various O2 concentrations (5 × 10-5 to 1.2 × 10-3 M).
Air- and oxygen-saturated solutions were directly used, other oxygen
concentrations were attained by flushing solutions with varying
percentages of Ar/O2 or air/gas mixtures. Experimental error limits for
individual radiation chemical experiments are typically 10%. Deviating
figures are specifically noted.
Continuous Radiolyses. Continuous radiolyses were performed at
room temperature (22 ( 2 °C) on 1 mL samples using a 60Co-
Gammacell at dose rate 10 Gy min-1. The exact absorbed radiation
dose was determined with the Fricke chemical dosimeter, by taking
G(Fe3+) ) 1.61 µmol J-1 30
.
GC Analyses. GC analyses for the determination of the isomeric
ratio of the unsaturated fatty acids were performed using a Varian CP-
3800 gas chromatograph equipped with a flame ionization detector and
a Rtx-2330 column (90% biscyanopropyl-10% phenylcyanopropyl
polysiloxane capillary column; 60 m, 0.25 mm i.d., 0.20 µm film
Acknowledgment. K.D.A. gratefully acknowledges the sup-
port provided by ISOF-CNR for several scientific visits to
Bologna. We thank Professor Krzysztof Bobrowski for helpful
discussions. Work supported in part by the European Com-
munity’s Marie Curie Research Training Network under contract
HPRN-CT-2002-000184 [SULFRAD].
(34) Emmi, S. S.; D’Angelantonio, M.; Poggi, G.; Beggiato, G.; Camaioni, N.;
Geri, A.; Martelli, A.; Pietropaolo, D.; Zotti, G. Res. Chem. Intermed. 1998,
24, 1.
JA070626Q
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J. AM. CHEM. SOC. VOL. 129, NO. 28, 2007 8723