M. Ignasiak et al. / Chemical Physics Letters 502 (2011) 29–36
35
(Supporting information Figure S2). In addition to this peak, the
irradiated samples exhibit a peak at m/z 294, i.e. an addition of
16 to the initial compound (Table 3 and Figure S2). The relative
intensity of this peak, with respect to that of the initial compound
(m/z 278) increases linearly with the dose (Figure 4). This peak is
higher if irradiation took place without catalase than in the pres-
ence of catalase, showing that a noticeable amount of this com-
pound comes from the two-electron oxidation by hydrogen
peroxide. Nevertheless, a substantial amount of (Met–Lys+16) is
noticeable if irradiation took place with catalase.
unambiguously revealed the presence of a band characteristic of
the SO bond elongation in the 1000 cmꢀ1 region. Moreover, the
addition of catalase prior to irradiation, which efficiently removed
H2O2, did not suppress this peak. Its height was less, as expected.
There is no doubt that the species having m/z 294, corresponding
to the addition of oxygen to the protonated dipeptide (Met–Lys),
exhibits a SO bond and thus is identified as the peptide having
methionine sulfoxide.
As for the mechanism of sulfoxide formation in one-electron
Å
oxidation, the first steps are well known. The OH radicals add on
The CID-MS2 mass spectra of the dipeptides [(Met–Lys)H]+ non-
the sulphur atom, which is followed by OHꢀ elimination. The sulf-
uranyl radical cation undergoes stabilization by the formation of a
two centre-three electron bond with any atom having a lone pair
doublet available ([7,8] and references therein). This stabilization
notably increases the lifetime of the radical. We propose that this
longer lifetime allows the radical to disproportionate, which would
be accompanied by hydration of the sulphur. Our findings allow
explaining why one finds methionine sulfoxide in proteins from
the oxidation of methionine residue after oxidative stress.
Å
oxidized and oxidized by OH radicals produced by water gamma
radiolysis have been recorded. The m/z values of the most intense
peaks present in the fragmentation mass spectra of the non-irradi-
ated and irradiated dipeptides with or without catalase are re-
ported in Table 3.
The non-irradiated samples behave quite similarly upon frag-
mentation at both pH values (6 or 9.8).
The IRMPD spectrum of [(Met–Lys)H]+ (Figure 5a) shows some
similarity with that of protonated methionine (Figure 1) and some
features resemble the IR spectrum of protonated L-lysine in the gas
Acknowledgments
phase [33]. Four bands are observed. The band around 1200 cmꢀ1
are reminiscent of that at 1170 cmꢀ1 in [MetH+] (Table 1 and Fig-
ure 1) and might be attributed to the bending mode of the C–O–H
group. Those at 1500–1600 cmꢀ1 should be due to the amine scis-
soring like in [MetH+].
Financial support by the European Commission to the EPITOPES
project (Electron Plus Infrared TO probe and Elucidate Structures,
EC project 15637) founded through the NEST (New and Emerging
Science and Technology) program is gratefully acknowledged. We
thank J.M. Ortega and the CLIO team for technical assistance. The
very efficient assistance of V. Steinmetz during the IRMPD experi-
ments is greatly acknowledged. We are indebted to the COST
CM0603 (Free Radicals in Chemical Biology) for very fruitful dis-
cussions. We thank Prof. K.-D. Asmus for very stimulating discus-
sions about the mechanism, and Prof. B. Marciniak for his
support in this work.
In addition to these bands, [(Met–Lys)OH]+ exhibits a sharp
band centred at 1050 cmꢀ1 (Figure 5b and c) that falls in the range
of the S@O bond stretching mode although it is blue-shifted com-
pared to that of free methionine sulfoxide. The presence of this
sharp band is in agreement with the formation of a sulfoxide func-
tion. This compound results not only from the oxidation by hydro-
gen peroxide, but also to the one-electron oxidation by ÅOH radicals
since it is still present with an unchanged shape if irradiation took
place in the presence of catalase. We conclude that upon oxidation
of the methionine residue in the dipeptide Met–Lys, methionine
does lead to its sulfoxide and that the amount is proportional to
the dose.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
4. Conclusion
References
In this Letter we have unambiguously established the vibra-
tional signature of the SO bond in methionine sulfoxide by IRMPD
spectroscopy that exhibits an additional band compared to methi-
onine in the 1000 cmꢀ1 region. DFT calculations have allowed an
interpretation of the spectrum and have confirmed that this band
was due to the stretching mode of the SO bond.
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form (PrP(Sc)) is the key event in neurodegeneration [34]. The sul-
foxidation of the methionine might be the switch for triggering the
pathogenic conversion [35].
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oxygen. We used
c radiolysis in N2O atmosphere to oxidize the
Å
peptide. OH radicals are the major oxidants and H2O2 is a minor
compound (the yields are 0.55 and 0.07 l
mol Jꢀ1, respectively),
thus the oxidants are the same as in oxidative stress. H2O2 was re-
moved by the addition of catalase. The MS spectra of the Met–Lys
peptide oxidized showed that the main oxidation product was the
peptide to which an oxygen atom was added. IRMPD spectra