The use of a nitroxide probe in DMSO to capture free radicals in particulate pollution

Article abstract of DOI:10.1002/ejoc.201200903

A profluorescent nitroxide was used to evaluate the oxidative potential of pollution derived from a compression ignition engine fuelled with biodiesel. The reaction products responsible for the observed fluorescence increase when a DMSO solution of nitroxide was exposed to biodiesel exhaust were determined by using HPLC/MS. The main fluorescent species was identified as a methanesulfonamide adduct arising from the reaction of the nitroxide with DMSO-derived sulfoxyl radicals. The oxidative potential of particulate pollution derived from a biodiesel engine exhaust stream was evaluated by using a profluorescent nitroxide. A methanesulfonamide adduct arising from the reaction of the nitroxide with DMSO-derived sulfoxyl radicals was identified as the main fluorescent product.

Full text of DOI:10.1002/ejoc.201200903

Job/Unit: O20903
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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201200903
The Use of a Nitroxide Probe in DMSO to Capture Free Radicals in
Particulate Pollution
Svetlana Stevanovic,[a,b] Branka Miljevic,[b] Geoff K. Eaglesham,[c] Steven E. Bottle,[a]
Zoran D. Ristovski,[b] and Kathryn E. Fairfull-Smith*[a]
Keywords: Atmospheric chemistry / Radicals / Fluorescence / Nitroxides / Environmental chemistry
A profluorescent nitroxide was used to evaluate the oxidative
potential of pollution derived from a compression ignition en-
gine fuelled with biodiesel. The reaction products responsi-
ble for the observed fluorescence increase when a DMSO
solution of nitroxide was exposed to biodiesel exhaust were
determined by using HPLC/MS. The main fluorescent spe-
cies was identified as a methanesulfonamide adduct arising
from the reaction of the nitroxide with DMSO-derived sul-
foxyl radicals.
method are that it facilitates the direct measurement of par-
ticulate-derived free radicals and related ROS during sam-
pling (without relying on post-exposure derivatization) and
allows relatively unstable redox products (such as hydroxyl-
amine 2) to be detected. We have used profluorescent ni-
troxide 1 to detect free radicals and related ROS derived
from PM in cigarette smoke,[11] biomass combustion in a
pellet boiler and logwood stove,[12] and a compression ig-
nition engine using ethanol fumigation technology[13] or di-
esel alternative fuels.[14,15] This probe displays only weak
fluorescence due to the presence of the nitroxide radical,
but exhibits a steady increase in fluorescence emission upon
exposure to various sources of PM.
Introduction
Pollution-derived particulate matter (PM) is widely rec-
ognized as impacting significantly on human health,[1,2]
particularly in the areas of respiratory and cardiovascular
disease.[3–7] The underlying toxicological mechanisms by
which particles induce adverse health effects are complex;
however, central to these mechanisms is the ability of in-
haled PM to induce cellular oxidative stress and promote
inflammatory responses within tissues.[8] PM contains and/
or is able to generate free radicals and related reactive oxy-
gen species (ROS), which result in oxidative stress at the
sites of deposition, thereby triggering a cascade of events
associated with inflammation and, at higher concentrations,
cell death.
The direct detection of free radicals and ROS in PM is
limited by their short half-life, low concentration, and high
reactivity. Recently, we pioneered a new approach that uses
an extremely sensitive profluorescent nitroxide,[9] 9-(1,1,3,3-
tetramethyliosindoline-2-oxyl-5-ethynyl)-10-(phenyl-
ethynyl)anthracene (1),[10] to assess the oxidative potential
of all types of particulate pollution. The advantages of this
[a] ARC Centre of Excellence for Free Radical Chemistry and
Biotechnology, Faculty of Science and Engineering,
Queensland University of Technology,
To date, however, the specific chemical species responsi-
ble for the observed fluorescence increase upon exposure of
1 to PM has not been identified. Herein we describe the
development of a high-performance liquid chromatography/
mass spectrometry (HPLC/MS) method and its use to
identify the fluorescent products that result upon exposure
of profluorescent nitroxide 1 to PM derived from a com-
pression ignition engine using biodiesel. We can now report
that the reaction of 1 in DMSO with biodiesel exhaust gives
predominately methanesulfonamide 3, rather than the ex-
pected methoxyamine 4 or hydroxylamine 2.[16] Further-
2 George St, Brisbane, QLD 4001, Australia
Fax: +61-7-31381804
E-mail: k.fairfull-smith@qut.edu.au
Homepage: http://www.freeradical.org.au/
[b] International Laboratory for Air Quality and Health, Faculty
of Science and Engineering, Queensland University of
Technology
Queensland, Australia
Homepage: http://www.ilaqh.qut.edu.au/
[c] The National Research Centre for Environmental Toxicology
(Entox), University of Queensland,
39 Kessels Rd, Coopers Plains, Queensland 4108, Australia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200903.
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K. E. Fairfull-Smith et al.
SHORT COMMUNICATION
more, we have discovered a hitherto unrecognized fragmen-
tation of DMSO in the presence of biodiesel exhaust.
To identify this new product, sonication of 1 was per-
formed on a preparatory scale (20 mg). Methanesulfon-
amide 3 and methanesulfinamide 7, which both gave reten-
tions at 5.5 min following analysis by RP HPLC, were iso-
lated in yields of 28 and 21%, respectively. The structures
of these products were confirmed by NMR spectroscopy
(see Supporting Information) and mass spectrometry
(Table 1, Entry 5). Further evidence for the structure of 3
was provided by the observed strong S=O asymmetric and
symmetric stretching frequencies at 1316 and 1142 cm–1 in
the infrared (IR) spectrum. One plausible mechanism for
the formation of methanesulfonamide 3, secondary amine
6, and methanesulfinamide 7 upon sonication of 1 in
DMSO is shown in Scheme 1. Sonication is known to cause
Results and Discussion
To validate the use of HPLC/MS as a method for the
detection of particulate-derived adducts of 1, we first ob-
tained reverse-phase (RP) HPLC retention times and high-
resolution mass spectrometry (HRMS) data for nitroxide 1,
methoxyamine 4, and acetoxyamine 5 (Table 1, Entries 1–
3). We also formed hydroxylamine 2 (a potential fluorescent
reaction product) by treating 1 with hydrazine hydrate in
DMSO. The formation of 2 was confirmed by HPLC (tR
=
6.3 min) and HRMS (m/z = 492.2313 [M + H]+; Table 1,
Entry 4). Methoxyamine 4 (tR = 10.6 min) was also pro-
duced upon reaction of 1 with hydrazine hydrate in DMSO
(see Supporting Information).
Our previous sampling of PM from various combustion
sources was undertaken by bubbling generated aerosol into
an impinger[17] containing a solution of 1 in DMSO to
avoid solvent loss and subsequent variations in fluorescence
measurements. However, the majority of PM sampling in
this field is undertaken by collecting particles on a filter
(for practicality and efficiency reasons) and often involves
subsequent extraction by sonication into a solvent.[18] Thus,
we next investigated the effect of sonication on a solution
of 1 in DMSO by using our HPLC/MS method.
Interestingly, we observed two peaks in the RP HPLC
chromatogram after sonication of a 10-mm solution of 1 in
DMSO for 60 min (Table 1, Entry 5). The first component
at 4.2 min was assigned by electrospray ionization mass
spectrometry (ESI-MS) to secondary amine 6, but the sec-
ond component at 5.5 min did not give an identifiable ion
by ESI-MS. Its retention time was similar to that of ni-
troxide 1 (at 5.2 min); however, it was substantially more
fluorescent in the HPLC trace with fluorescence detection
Scheme 1. Proposed mechanism for the formation of 3, 6, and 7
(λex = 430 nm, λem = 485 nm; see Supporting Information). from the sonication of 1 in DMSO.
Table 1. Identification of adducts of nitroxide 1 by using HPLC/MS.
Entry
Sample
HPLC retention
(min)[a]
ESI-MS
obtained
Assignment
1
2
3
4
5
1
5.2
10.6
6.2
6.3
4.2
5.5
5.5
4.2
5.5
14.7
5.4
6.4
5.2
491.2252
506.2476
534.2430
492.2313
1 (m/z = 491.2249 [M + H]+)
4 (m/z = 506.2484 [M + H]+)
5 (m/z = 534.2433 [M + H]+)
2 (m/z = 492.2327 [M + H]+)
6 (m/z = 476.2378 [M + H]+)
3 (m/z = 553.2076 [M]+)
7 (m/z = 475.2300 [M + H+ – S(O)CH3])
6[h]
4
5
1 + NH2NH2·H2O
1 + sonication[b]
476.2375
553.2074[f]
475.2302[g]
6
1 + H2O2
–
–
–
–
3[h] and 7[h]
7
8
1 + AAPH[c]
1 + AAPH[d]
10
4[h]
–
3[h] and 7[h]
9
1 + PM (biodiesel)[e]
554.2[i]
3 (m/z = 554.2 [M + H]+)
[a] At 430 nm using a C18 column in 60% THF/40% water at 1 mL/min. [b] Sonication of a 10-mm solution of 1 in DMSO at 460 Hz
for 60 min. [c] Thermolysis of 2,2Ј-azobis(2-methylpropionamidine) dihydrochloride under an Ar atmosphere. [d] Thermolysis of 2,2Ј-
azobis(2-methylpropionamidine) dihydrochloride in air. [e] 100% soy diesel at half load with 4-μm solution of 1. [f] EI-MS on sample
isolated from preparatory scale reaction. [g] ESI-MS on sample isolated from preparatory scale reaction. [h] Assignment based on HPLC
retention time of authentic sample. [i] Obtained by using a photospray photoionization source.
2
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Nitroxide Probe to Capture Free Radicals in Particulate Pollution
cleavage of DMSO to give sulfoxyl (CH3SO·) and methyl derived from a compression ignition engine fuelled with bio-
(·CH3) radicals.[19] The radical trapping reactions of sulfoxyl diesel. Aerosol generated from a compression ignition en-
(CH3SO·) radicals with nitroxides have been previously es- gine employing 100% soy diesel at half-load was bubbled
tablished[20,21] and proceed through rearrangement of the into an impinger containing a solution of nitroxide 1 in
initial methanesulfinate adduct 8 to give sulfonamide 3. DMSO (without sonication). The fluorescence increased
Aminyl radical 9 can also add to DMSO to form the ob- linearly over 60 min by spectrofluorimetry (no increase in
served methanesulfinamide 7 in an analogous mechanism fluorescence was observed for an untreated solution of 1 in
to the reactivity of hydroxy radicals.[16] Secondary amine 6 DMSO over the same time period). Analysis of the sample
could result following hydrogen abstraction by aminyl radi- by HPLC/MS gave a surprisingly simple chromatogram
cal 9. Secondary amines have been detected in materials with a major component at 5.2 min, which was significantly
where nitroxides are involved in radical scavenging reac- more fluorescent than starting nitroxide 1 (Table 1, Entry 9;
tions,[22,23] but in this context, where low (ambient) op- Figures 1 and 2). Notably, only very small amounts of
erating temperatures and low (μm) concentrations are in- methoxyamine 4 were observed in the HPLC trace. The
volved, these scavenging reactions are unlikely to oper- mass spectrum of the major component at 5.2 min gave a
ate.[24]
The isolation of DMSO adducts of nitroxide 1 demon-
strates for the first time that radicals formed during the son-
ication of DMSO can be captured by using a profluorescent
nitroxide. Moreover, this finding indicates that a cautious
approach should be taken when detecting reactive oxygen
species with other probes (such as 2Ј,7Ј-dichlorofluorescein
diacetate: DCFH-DA) by sonication in DMSO or other sol-
vents known to produce radicals when sonicated.
To further explore the chemistry behind the observed
fluorescence increase arising from exposure of nitroxide 1
to various sources of PM, we examined reactions between
1 and several model compounds that are common constitu-
ents of aerosol pollution and analyzed the results by HPLC/
MS. The reaction of nitroxide 1 with hydrogen peroxide in
DMSO produced two main products following analysis by
HPLC (Table 1, Entry 6). These were assigned as secondary
amine 6 and DMSO adducts 3 and 7 on the basis of their
retention times and substantial fluorescence (as no identifi-
able ions could be obtained by ESI-MS for these com-
Figure 1. HPLC chromatograms from the reaction of nitroxide 1
pounds). Resulting products 3 and 7 are presumably formed
(4 μm in DMSO) with particulate matter derived from a compres-
sion ignition engine employing biodiesel: (a) absorbance at 430 nm,
from the reaction of 1 with sulfoxyl (CH3SO·) radicals
through the same mechanism as that shown in Scheme 1. (b) fluorescence detection λex = 430 nm, λem = 485 nm.
In this case, sulfoxyl (CH3SO·) radicals are derived from
·
methanesulfinic acid, which is obtained along with CH3
when hydroxy (HO·) radicals react with DMSO.[19]
Reaction of 1 with alkyl radicals produced by the ther-
molysis of 2,2Ј-azobis(2-methylpropionamidine) dihydro-
chloride (AAPH) under anaerobic conditions gave a new
fluorescent product (assumed to be adduct 10) with a reten-
tion time of 14.7 min as well as unreacted nitroxide 1
(Table 1, Entry 7). The reaction of peroxyl radicals (pro-
duced by the thermolysis of AAPH under aerobic condi-
tions) with 1 gave two major components after analysis by
HPLC (Table 1, Entry 8). These products were assigned as
hydroxylamine 2 and DMSO adducts 3 and 7 on the basis
of their retention times and strong fluorescence.[25] Thus,
the nitroxide reacted to form fluorescent products when
treated with high concentrations of hydrogen peroxide, per-
oxyl radicals, or carbon-centered radicals.
Following the development of our HPLC/MS method
and insight into the reactivity of probe molecule 1 towards
common aerosol constituents, we investigated the use of
(a) absorbance at 430 nm, (b) fluorescence detection λex = 430 nm,
this approach to detect free radicals and related ROS in PM λem = 485 nm.
Figure 2. HPLC chromatograms of nitroxide 1 (10 mm in DMSO):
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K. E. Fairfull-Smith et al.
SHORT COMMUNICATION
strong signal at m/z = 554.2 when a photospray photoioni- the use of a profluorescent nitroxide to detect radicals gen-
zation source was used, which corresponds to methanesul- erated through DMSO sonication. These results will im-
fonamide 3 ([M+ H]+, Figure 3). An authentic sample of prove our ability to quantify free radicals and related ROS
methanesulfonamide 3 (isolated from the sonication of 1 in derived from diesel/biodiesel exhaust and are an important
DMSO) gave an identical mass spectrum with the photoio- step towards our goal of directly detecting free radicals and
nization source.
related ROS in PM to aid future improvements in air qual-
ity.
Experimental Section
Reaction of Model Aerosol Compounds with Compound 1
Sonication: A solution (10 mL) of 9-(1,1,3,3-tetramethylisoindolin-
2-yloxyl-5-ethynyl)-10-(phenylethynyl)anthracene (1, 10 mm in
DMSO) was sonicated in an Elma T460H ultrasonic bath and the
fluorescence was measured every 5 min by using a spectrofluorime-
ter. After 60 min, the fluorescence no longer increased and the reac-
tion mixture was analyzed by HPLC/MS.
Hydrogen Peroxide: Hydrogen peroxide (1.0 mL, 70% aqueous
solution) was added to a solution of nitroxide 1 (10 mm in DMSO,
10 mL). The solution was stirred at room temperature for 30 min
and then analyzed by HPLC/MS.
2,2Ј-Azobis(2-methylpropionamidine) Dihydrochloride (AAPH)
Aerobic: 2,2Ј-Azobis(2-methylpropionamidine) dihydrochloride
(0.45 g, 1.66 mmol) was added to a quartz cuvette containing a
solution of nitroxide 1 (10 mm in DMSO, 15 mL). The cuvette was
heated in thermostatted water at 38.5Ϯ0.5 °C for 30 min and then
the solution was analyzed by HPLC/MS.
Figure 3. Photoionization (+ mode) mass spectrum of the major
HPLC component (at 5.16 min) from the reaction of nitroxide 1
(4 μm in DMSO) with particulate matter derived from a compres-
sion ignition engine employing biodiesel.
The species giving rise to the observed fluorescence in-
crease upon exposure of profluorescent nitroxide 1 to PM
derived from a compression ignition engine fuelled with
biodiesel has now been identified as methanesulfonamide 3.
Presumably, the free radicals and related ROS associated
with the generated PM can fragment DMSO in a similar
manner to that observed during sonication, although the
mechanism by which this occurs is currently unclear. In this
case, however, the temperature increases associated with
sonication are not a factor, as the engine exhaust is cooled
to ambient temperature before exposure to nitroxide 1 in
DMSO. Moreover, engine exhaust with particles removed
does not increase the fluorescence of the DMSO solution
of nitroxide 1.
Anaerobic: 2,2Ј-Azobis(2-methylpropionamidine) dihydrochloride
(0.45 g, 1.66 mmol) was added to an argon-filled quartz cuvette
containing a degassed solution of nitroxide 1 (10 mm in DMSO,
15 mL). The solution was heated in thermostatted water at
38.5Ϯ0.5 °C for 30 min and bubbled with argon for the duration
of heating. After cooling to room temperature, the reaction mixture
was analyzed by HPLC/MS.
Procedure for Biodiesel Sampling: The sample was collected by bub-
bling the aerosol through an impinger containing a solution of ni-
troxide 1 (4 μm in DMSO, 20 mL). Custom-made impingers were
used, with sintered porosity grade 1 (pore size of 100–160 μm) and
a Quickfit dreschel bottle head was modified to fit a Quickfit 75-
mL test tube. Experiments were conducted on a common rail 6-
cylinder Cummins Euro 3 diesel engine at the QUT Biofuel Engine
Research Facility (BERF). An aerosol stream was generated by em-
ploying 100% soy diesel at half-load at 1.0 L/min for 60 min. Im-
pingers were placed after the two-stage dilution system (see Sup-
porting Information for set-up). Fluorescence measurements were
taken after 20, 40, and 60-min intervals from both the test sample
and a HEPA-filtered control sample. After 60 min, the samples
were analyzed by HPLC/MS. An increase in fluorescence upon ex-
posure to biodiesel was normalized with respect to particulate mat-
ter (PM) mass. PM mass was measured by using a TSI 8520 (see
Supporting Information). The actual mass readings from the Dust-
Trak were converted into a gravimetric measurement by using the
tapered element oscillating microbalance to DustTrak correlation
for DPM obtained by Jamriska.[26] The amount of particles re-
maining in the impingers post-sampling was measured, taking into
account the impinger collection efficiency correction factor.[17]
Conclusions
We have identified DMSO adduct methanesulfonamide
3 as the main fluorescent species resulting from the reaction
of aerosol-derived biodiesel exhaust with a solution of 1
in DMSO by using HPLC/MS. The fluorescence increase
previously observed in our work from the exposure of ni-
troxide 1 to other pollution sources presumably results from
the presence of DMSO-related adducts such as methane-
sulfonamide 3. The mechanism by which biodiesel-derived
PM interacts with DMSO is currently unclear; however, it
is interesting to note that the same product was generated
upon exposure of nitroxide 1 to high concentrations of hy-
drogen peroxide and peroxyl radicals and upon sonication
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and characterization data for com-
1
of nitroxide 1 in DMSO. This is also the first example of pounds 3, 5, and 7; H NMR and 13C NMR spectra and HPLC
4
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Nitroxide Probe to Capture Free Radicals in Particulate Pollution
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Acknowledgments
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Received: July 9, 2012
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K. E. Fairfull-Smith et al.
SHORT COMMUNICATION
Biodiesel Radical Production
S. Stevanovic, B. Miljevic,
G. K. Eaglesham, S. E. Bottle,
Z. D. Ristovski,
K. E. Fairfull-Smith* ......................... 1–6
The Use of a Nitroxide Probe in DMSO to
Capture Free Radicals in Particulate Pol-
lution
The oxidative potential of particulate pol-
lution derived from a biodiesel engine ex-
haust stream was evaluated by using a pro-
amide adduct arising from the reaction of
the nitroxide with DMSO-derived sulfoxyl
radicals was identified as the main fluo-
rescent product.
fluorescent nitroxide.
A methanesulfon-
Keywords: Atmospheric chemistry / Rad-
icals / Fluorescence / Nitroxides / Environ-
mental chemistry
6
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