Reduction of nitrite to NO in an organised triphasic medium by platinum carbonyl clusters and redox active dyes as electron carriers

Article abstract of DOI:10.1039/b703352d

Artificial electron donors such as leuco methylene blue and leuco safranin O reduce nitrite ion to nitric oxide. The reaction is effected in a U-tube where nitrite ion and dye in two aqueous layers are separated by a layer of dichloromethane (a close model for a biological liquid membrane) that contains the platinum carbonyl cluster ([Bu₄N]₂[Pt ₁₂(CO)₂₄], Chini cluster). On passing dihydrogen an electron transfer chain involving dihydrogen, the dye, the clusters and the nitrite ion is initiated. The cluster catalytically reduces the dye in the presence of dihydrogen, the reduced dye migrates across the phase boundaries and in turn reduces the nitrite ions. The resultant nitric oxide in the effluent gas has been identified by its reactions with cobalamine and myoglobin. When safranin O is the dye, an adduct is formed between the reduced dye and NO. It has been identified by spectroscopic techniques and its probable structure investigated by DFT calculations. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b703352d

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Reduction of nitrite to NO in an organised triphasic medium by platinum
carbonyl clusters and redox active dyes as electron carriers†
Nalinava Sen Gupta,a Susmit Basu,a Pramatha Payra,a Pradeep Mathur,a Sumit Bhaduri*b and
Goutam Kumar Lahiri*a
Received 5th March 2007, Accepted 29th March 2007
First published as an Advance Article on the web 19th April 2007
DOI: 10.1039/b703352d
Artificial electron donors such as leuco methylene blue and leuco safranin O reduce nitrite ion to nitric
oxide. The reaction is effected in a U-tube where nitrite ion and dye in two aqueous layers are separated
by a layer of dichloromethane (a close model for a biological liquid membrane) that contains the
platinum carbonyl cluster ([Bu4N]2[Pt12(CO)24], Chini cluster). On passing dihydrogen an electron
transfer chain involving dihydrogen, the dye, the clusters and the nitrite ion is initiated. The cluster
catalytically reduces the dye in the presence of dihydrogen, the reduced dye migrates across the phase
boundaries and in turn reduces the nitrite ions. The resultant nitric oxide in the effluent gas has been
identified by its reactions with cobalamine and myoglobin. When safranin O is the dye, an adduct is
formed between the reduced dye and NO. It has been identified by spectroscopic techniques and its
probable structure investigated by DFT calculations.
thionitrosyls (RSNO) have been intensively studied as the major
Introduction
carriers of NO, recently it has been shown that N-nitrosation and
heme-nitrosylation are also as ubiquitous as S-nitrosation.5,6
Reductions of nitrite in biological processes such as denitrification
and meat curing have been known for a long time. However,
The work reported here was undertaken with two objectives.
the ability of nitrite as a major bio-available pool of NO and to
First, to construct a model that satisfies all the four conditions
intervene in blood flow regulation by reaction with deoxygenated
hemoglobin (Hb) has only recently come to light.1–3 A recent report
mentioned above. Second to find out if artificial electron donors
such as leuco methylene blue (MBH) or leuco safranin O (SafH)
−
also shows that in the presence of nitrite, the enzyme xanthine
oxidoreductase plays a protective role by reducing nitrite to NO.4
In view of these reports, the design of chemical models for the
biological reduction of nitrite to nitric oxide is important.
To a first approximation, any chemical model for a biological
redox reaction such as eqn (1), should satisfy the following
conditions. As most biological redox reactions involve an electron
could reduce NO2 to NO. The use of these redox active dyes as
artificial electron acceptors and/or donors is well established in
biochemical and biological research.7 In our previous publications
we have shown, that the reduction of a chemical analogue of
biological NAD+ by SafH, is chemo- and regio-selective.8 Also
by using catalytic amounts of Saf+, NAD+, [Pt12(CO)24]2− and the
enzyme lactate dehydrogenase, the enantioselective hydrogenation
of methyl pyruvate to methyl lactate by dihydrogen in a biphasic
system could be achieved.9 We have also shown that in a triphasic
system (two aqueous layers separated by an organic layer),
[Pt12(CO)24]2−, due to its unique redox properties can mimic the
pH driven electron transport of biological systems.10
The model described in this paper utilises the results and insights
gained from our early work. Here we have used a U-tube (Fig. 1)
where an aqueous solution of nitrite and an aqueous solution of
the artificial electron carrier are separated by a water immiscible
organic liquid such as dichloromethane. This type of triphasic
system in a U-tube has been used in the past to model the gross
features of a biological membrane that separates out the inner
and outer aqueous phases of a cell. However, to the best of our
knowledge there have been no reports on the use of such models
for biological redox reactions in general and nitrite reductions in
particular. The NO produced by the reduction of the nitrite ion in
turn reacts with SafH to give SafH–NO. This adduct SafH–NO
has a “N–NO” linkage, is stable enough to be characterised by
spectroscopic techniques. No such adduct formation is observed
with MBH. This difference in behaviour between SafH and MBH
has been rationalised by DFT calculations.
NO2 + 2H+ + e− → NO + H2O
(1)
−
transfer chain and more than one electron carrier, i.e. a redox
catalyst, the model reaction should mimic this feature. Second, the
specific carrier that transfers the electron to nitrite must be active
as an artificial electron donor/acceptor in an actual biological
system. Third, while the electron and proton transfer to nitrite
may take place in an aqueous medium, the electron transport chain
like that in mitochondria, must operate across a model membrane.
Finally, in the model, if there is an in situ formation of an “A–NO”
bond (A = S or N) from the liberated NO and the electron carrier,
it would be an additional attractive feature. This is because while
aDepartment of Chemistry, Indian Institute of Technology-Bombay, Powai,
Mumbai, 400076, India. E-mail: lahiri@chem.iitb.ac.in; Fax: +91 22
25763480; Tel: +91 22 25767159
bReliance Industries Limited, Swastik Mills Compound, V.N. Purav
Marg, Chembur, Mumbai, 400071, India. E-mail: sumit_bhaduri@ril.com;
Fax: +91 22 67677380; Tel: +91 22 67677324
† Electronic supplementary information (ESI) available: ESR (Fig. S1) and
IR (Fig. S2) spectra of [Co(Salen)]/[(NO)Co(Salen)]; UV-Visible spectral
changes of nitrite reduction to NO by SafH (Fig. S3).
2594 | Dalton Trans., 2007, 2594–2598
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Scheme 1 Reduction of nitrite by Dyered in a U-tube. 1 catalyses the
reduction of Dyeox to Dyered by dihydrogen. The comparative lengths of the
half arrows in the equilibria are indicative of the solubilities of Dyeox/Dyered
in dichloromethane and water.
Fig. 1 U-tube reaction setup for the reduction of nitrite and subsequent
trapping of the effluent gas. The whole setup was degassed by purging
nitrogen prior to start the reaction. (a) oil bubbler, (b) inlet needle,
(c) aqueous layer containing NaNO2 solution, (d) dichloromethane layer
containing catalyst [Bu4N]2[Pt12(CO)24], (e) aqueous layer containing dye
solution, (f) magnetic stirrer, (g) magnetic bar, (h) rubber septum, (i) outlet
needle (vent), (j) solution of trapping agent for the effluent NO gas through
the vent.
The reduction of Dyeox is rapid, but the reduction of nitrite to NO
in the other aqueous layer, i.e.
Dyeox + H2 → Dyered + H+
(4)
eqn (5), commences only after an induction time. The induction
time depends on the rate of
Results and discussion
Dyered + H+ + 2NO2 → Dyeox + 2NO + 2HO−
(5)
−
The model arrangement (U-tube set up, Fig. 1) for the reduc-
tion of nitrite to nitric oxide described here consists of two
aqueous phases, one containing nitrite and the other containing
the oxidised dye (Dyeox = Saf+ or MB+). The two aqueous
layers are separated by a layer of dichloromethane that contains
[Bu4N]2[Pt12(CO)24], 1, which is insoluble in water but soluble in
dichloromethane. The rationale behind using a triphasic rather
than a bipasic system is as follows. Separating out the nitrite layer
from that of the Dyeox ensures that there is minimum interference
from organic species during spectrophotometric monitoring of the
left hand side aqueous layer and also quantitative turnover number
measurements by IR (see later).
By a control experiment it was first established that in the ab-
sence of Dyeox, 1 by itself could not reduce nitrite ions. Under these
conditions the literature reported reduction of [Pt12(CO)24]2−to
[Pt9(CO)18]2− i.e., eqn (2), does occur but there is no observable
reduction
stirring and hydrogen bubbling, i.e. the rate of diffusion of Dyered
across the two phase boundaries. It may be noted that combination
of eqn (4) and (5) gives eqn (6), which is the net catalytic reaction.
This reaction is thermodynamically slightly unfavourable and in
the absence of catalysts it does not take place.
H2 + 2NO2 → 2NO + 2HO−
(6)
−
All the reactions have been carried out at neutral pH and no
attempt was made to study the effect of pH on the overall
reaction. This is because the carbonyl clusters undergo facile
reactions with H+ and HO−. From a practical point of view
this limits the productive use of the catalytic system for any
denitrification reaction. Furthermore, apart from the reactions
shown in Scheme 1 more complex reactions are possible and
indeed do take place when the model is operated for a long
(≥ 3 h) time. Under these conditions the evolved nitric oxide reacts
with the carbonyl clusters and produces complexes with spectral
signatures different from that of the original clusters. No attempt
was made to identify these complexes.
During the induction time spectroscopic monitoring shows no
change in the nitrite solution, but in the dichloromethane and
the Dyeox aqueous layers, changes are consistent with reactions
in eqn (2)–(4). The formation of NO commences after the
induction time, and its presence in the effluent hydrogen is
evidenced by its reactions separately with Co(Salen) [Salen =
N,Nꢀ-ethylenebis(salicylideneiminato)], cobalamine/glutathione
and also myoglobin. It is important to note that the overall electron
transfer chain that ends with the reduction of nitrite, critically
depends on the selective reduction of Dyeox to Dyered and therefore
cannot be achieved by conventional heterogeneous catalysts such
as platinum on carbon which over reduces and/or decomposes
Dyeox.9
3[Pt12(CO)24]2− + H2 → 4[Pt9(CO)18]2− + 2H+
(2)
of nitrite. In the presence of Dyeox in the right hand side aqueous
layer, selective reduction of Dyeox to Dyered (Dyered = SafH and
MBH) by dihydrogen and catalysed by 1, is observed.8,9 As
indicated in Scheme 1 both Dyeox and Dyered have measurable
solubilities in water and dichloromethane, and the partition
coefficients of Saf+ and SafH in a 1 : 1 mixture of water
and dichloromethane are 6.8 and 0.14, respectively.10 Thus the
solubility of Dyered in both organic and aqueous media enables
it to act as a shuttle carrier, and to transfer the required redox
equivalents across the phase boundary (Scheme 1).
The ability of 1 to act as a selective redox catalyst for the
reduction of Dyeox is due to reactions as stated in eqn (2) and
(3) and slight solubility of Dyeox in dichloromethane.9–12
4[Pt9(CO)18]2− + Dyeox + H+ → 3[Pt12(CO)24]2− + Dyered
Combination of these two reactions gives the overall reaction,
eqn (4), and on passing hydrogen through the dichloromethane
layer, the electron transfer chains shown in Scheme 1 are initiated.
(3)
The black crystalline product isolated from the reaction of the
effluent gas with Co(Salen) is identified as [Co(Salen)(NO)] on
the basis of analytical data and spectroscopic comparison with an
authentic sample (Fig. S1 and S2†).13 The reaction of NO with
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cobalamine in the presence of glutathione (GSH) is known to
exhibit characteristic spectrophotometric changes.14 In our system
too, the effluent gas on reaction with cobalamine and glutathione,
brings about all the essential features of the literature reported
spectrophotometric changes (Fig. 2). Thus isobestic points close
to 500, 400 and 300 nm are observed and while the absorption
above 500 nm decreases in intensity, there is a gradual increase in
the intensity in absorption between 400–500 nm. The reaction of
myoglobin (Mb) with NO has recently been much studied.15 On
passing the effluent gas through a freshly prepared solution of Mb,
the characteristic band of Mb–NO is also observed (Fig. 3).
Fig. 4 IR spectrum (in CHCl3) of SafHNO. The peak at 1428 cm−1
=
corresponds to N-nitroso (N–N O) functionality.
Fig. 2 Changes in the UV-Visible spectra on passing the effluent gas
through a GS–Cbl(III) solution in water.
Fig. 5 ESR spectrum (in CH2Cl2 at 77 K) of (a) before passing the effluent
gas in Saf+Cl−, (b) after passing the effluent gas coming out from the vent
into Saf+Cl−.
Fig. 3 Changes in the UV-Visible spectra on passing the effluent gas
through a freshly prepared myoglobin solution in water.
Fig. 6 Electrospray mass spectrum of SafHNO (in methanol). The peak at
346.166 authenticates the formation of SafHNO. The peaks at 173.586 and
116.060 correspond to dicationic [MH]2+ and tricationic [MH2]3+ species,
respectively
−
The turnover numbers for the reduction of NO2 to NO for
Saf+ and MB+ over a period of one hour are 18
2 and
2, respectively. During the induction time, isobesticities
10
and tricationic [MH2]3+ species are also observed at 173.586
and 116.060, respectively). As the amount of material formed
was exceedingly small (< 0.5 mg), no attempts were made for
microanalysis or yield measurements. However, the spectroscopic
observations taken together convincingly suggest its formulation
to be SafH–NO. In contrast to the case of MB+, the organic
solid which extracted from the nitrite layer has been identified
spectroscopically as simply MB+.
corresponding to the conversion of Saf+ to SafH is observed
by spectrophotometry (Fig. S3†). However, at the end of the
induction time the commencement of NO formation is found to
be accompanied by the loss of isobesticity, and the formation
of an absorbing species that is ESR active. At the end of an
experimental run trace quantities of a dichloromethane soluble
solid could be extracted from the nitrite layer. This solid shows an
IR band at 1428 cm−1 (Fig. 4) assignable to the N–NO linkage,
an ESR signal at g ∼2.0 (77 K) (Fig. 5) and a molecular ion
peak at 346.166 (m/z) (Fig. 6, the peaks for dicationic [MH]2+
To provide a rationale for the difference in reactivity between
MBH and SafH towards NO, and the stability of SafH–NO
that allows its isolation to be possible, DFT (B3LYP/6-31G*)
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Table 1 Zero point energy (ZPE) corrected relative energies (kcal mol−1
)
Experimental
of optimised MBHNO and SafHNO adducts at B3LYP/6–31G* level and
corresponding “N–NO” bond lengthsa
General
˚
Adducts
Establb/kcal mol−1
Bond length of N–NO/A
All reactions and manipulations were performed under an inert
atmosphere. Solvents were dried, distilled and degassed appropri-
ately. For taking out aliquots from the aqueous layer maintaining
an inert atmosphere, a cannula technique was used. An argon
flow throughout the system and a vacuum inside the cuvette were
applied to transfer the part of the aqueous layer through the
cannula into the cuvette. Sodium nitrite, safranin O, methylene
blue, tetra-n-butylammonium bromide, met-myoglobin, were ob-
tained from Sigma-Aldrich. Hexachloroplatinic acid was obtained
from Johnson-Mathey. Cobalamine and glutathione were pur-
chased from Fluka and Lancaster, respectively. Salicylaldehyde,
ethylenediamine, sodium acetate, cobalt(II) chloride, ethanol and
chloroform were purchased from SDfine Chemicals (India). Other
solvents were obtained from Merck. The platinum carbonyl cluster
[Bu4N]2[Pt12(CO)24] was prepared according to the literature
reported procedure.11 Double distilled deionised water of pH ∼6.5
was used for preparing the nitrite solution.
Infrared spectra were recorded on a Nicolet Impact 400 FTIR
spectrophotometer, as solutions in a 0.1 mm path length and CsCl
disks for Nujol mull and KBr pellets for solid state according to the
purpose. ESR spectra were monitored with a Varian model and
calibrated by using the tetracyanoethylene radical (g = 2.0023).
Mass spectra were taken in a Q-TOF (YA-105) micro mass
spectrometer. UV-Visible spectra were recorded on a Perkin Elmer
Lambda 950 spectrophotometer.
MBH–NO (1N)
MBH–NO (2N)
MBH–NO (S)
SafH–NO (2N)
SafH–NO (3N)
SafH–NO (1N)
−1.25
−1.48
−0.66
−3.03
−1.12
−1.88
2.692
2.736
3.164
2.538
2.954
2.675
a Total ZPE corrected energies (electronic + ZPE in a.u.) calculated at
B3LYP/6–31G* level of MBH, SafH and NO are: MBH = −1261.822356;
SafH = −992.825202; NO = −129.883621. The atom to which NO binds
is given in parentheses and Scheme 2. b
Establ is relative stabilization energy
(Establ = E(Adduct) − [E(MBH/SafH) + E(NO)]) in kcal mol−1
.
calculations have been carried out. Six possible structures, three
for each dye, with “N–NO” and “S–NO” (for MBH) linkages have
been considered. The relative stabilisation energies of Dyered–NO
with respect to the total energy of Dyered and NO, are given in
Table 1. From the data it is observed that for both the dyes the
adducts with NO linked to the ring ‘NH’ i.e., N(2), are the most
stabilised (Scheme 2).
Experimental set up for nitrite reduction
For the reduction of nitrite, the U-tube reaction set up was used
(Fig. 1). The catalyst ([Bu4N]2[Pt12(CO)24], 0.1 g, 0.0285 mmol)
was dissolved in an organic layer (25 cm3 of dichloromethane). An
aqueous solution (15 cm3) of sodium nitrite (0.1 g, 1.449 mmol)
was kept in one arm and in the other arm a safranin O (0.002 g,
0.0056 mmol) solution (15 cm3) was kept. The ratio of safranin
O to cluster (∼0.2) was deliberately kept at a low level to avoid
interference by the dye in spectroscopic measurements. The two
aqueous layers were separated by the organic layer containing
catalyst solution in dichloromethane. Both the openings of the
U-tube arms were capped with rubber septums. During the
course of the reaction dihydrogen gas was bubbled through the
dichloromethane solution using a needle without allowing any
mixing between the two aqueous layers. The organic layer was
stirred vigorously with a magnetic stirrer.
Scheme 2 Most stable structures of SafH–NO according to DFT
(B3LYP/6–31G*) calculations.
Furthermore, the SafH–NO adduct is about 1.2 kcal mol−1
more stable than the MBH–NO adduct and has the shortest
‘N–N’ bond length. The calculation results are thus consistent
with the observed stability of SafH–NO. Among the MBH–NO
adducts, the one with NO linked to the sulfur atom is the least
stabilised. This is not surprising since in MBH the sulfur atom is
sp2 hybridised, but biological NO carriers that have thionitrosyl
functionalities, have an sp3 hybridised sulfur atom. It should be
noted that the DFT methods at this basis set are not expected to
provide an accurate quantitative measure of the stabilities of the
adducts. However they are sensitive enough to predict the trend
in the relative stabilities of the adducts. Our future work will be
directed towards providing experimental evidence for the structure
shown above.
The nitrite reduction reaction using a U-tube set up was
monitored by IR spectroscopy using sodium acetate as an internal
−
standard. The change in concentration of NO2 in the aqueous
layer was determined by following the change in ratio of IR peak
areas (the area under the peak was calculated by measuring the
half width and the height of the IR peak) of internal standard
acetate (macetate = 1644 cm−1) and nitrite (mnitrite = 1271 cm−1).
Conclusions
In conclusion, the present article demonstrates the design of
a unique liquid triphasic chemical model close to the liquid
membrane based biologically relevant catalytic reduction of
nitrite to NO in the presence of electron carriers, [Pt12(CO)24]2−/
[Pt9(CO)18]2− and dyes (safranin O/methylene blue).
Identification of NO in the effluent gas
The effluent gas coming out from the vent (Fig. 1) was passed
through a cannula to a chloroform solution of Co(Salen).
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The black crystalline compound N,Nꢀ-ethylenebis(salicylidene-
iminato)nitrosylcobalt [(NO)Co(Salen)] was obtained and its ESR
(Fig. S1†) and IR (Fig. S2†) spectra were compared with those of
an authentic sample.
Dr S. Mukhopadhyay and Dr S. Kulkarni (Vlife Sciences Tech-
nologies Pvt. Ltd., Pune, India) for their contributions to the DFT
calculations.
In a separate reaction the effluent gas coming out from the vent
was passed through a cannula into a freshly prepared aqueous
solution of glutathione–cobalamine (0.4 mmol cobalamine and
4 mmol glutathione were mixed in 20 cm3 distilled water and
phosphate buffer was used to make the solution of pH = 7). A
colour change from violet to brown was observed indicating the
cobalamine–NO adduct formation which was further followed by
UV-Visible spectral changes (Fig. 2).
Similarly the effluent gas coming out from the vent was also
passed through a cannula to a freshly prepared aqueous solution
of myoglobin (Mb). The formation of the Mb–NO adduct was
confirmed by UV-Visible spectroscopy (Fig. 3).
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Acknowledgements
Financial support received from Reliance Industries Lim-
ited, Mumbai, India is gratefully acknowledged. We thank
2598 | Dalton Trans., 2007, 2594–2598
This journal is
The Royal Society of Chemistry 2007
©