Photodissociation of nitric oxide from nitrosyl metalloporphyrins in micellar solutions

Article abstract of DOI:10.1021/jp002515i

Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)Fe^IIHem (Hem = protoporphyrin IX), (NO)-Co^IIOEP (OEP = octaethylporphyrin), and (NO)Mn^IITPP (TPP = tetraphenylporphyrin), in aqueous ionic micellar solutions were carried out. The nitrosyl porphyrins in the micellar solutions readily photodissociate NO, leaving the MP in micelles: the quantum yields are 0.15 (±0.1) for (NO)Fe^IIHem, 0.55 (±0.05) for (NO)Co^IIOEP, and 0.21 (±0.02) for (NO)Mn^IITPP. The MP thus produced recombine with NO to regenerate the parent nitrosyl porphyrins. The decay of MP in the absence of the excess NO follows second-order kinetics with the rate constant k_MNO(second) (MNO = FeNO, CoNO, and MnNO). In the presence of excess NO, the decay of MP follows pseudo-first-order kinetics with the rate constant k_M (M = Fe, Co, and Mn). The value of k_M was measured as a function of [NO]. For Fe^IIHem and Co^IIOEP, the plots of k_Co vs [NO] and k_Fe vs [NO] gave straight lines. The slopes of the lines obtained with Fe^IHem and Co^IIOEP afford the bimolecular rate constants k_FeNO(pseudo) and k_CoNO(pseudo), respectively. It is found that k_FeNO(pseudo) > k_FeNO(second) and k_CoNO(pseudo) > k_CoNO(second). The differences between k_MNO(pseudo) and k_MNO(second) observed for Fe^IIHem and Co^IIIOEP are interpreted by assuming that (1) NO molecules in the micellar solutions are dissolved in both micelles and the aqueous phase and (2) NO molecules trapped in micelles hardly react with MP because of the electrostatic repulsion between ionic micelles. In the case of Mn^IITPP, the pseudo-first-order rate constant, k_Mn, is found to asymptotically increase with an increase in [NO] to a limiting value. The reaction mechanisms for the nitrosylation of MP in micellar solutions are discussed in detail on the basis of the kinetic studies.

Full text of DOI:10.1021/jp002515i

392
J. Phys. Chem. A 2001, 105, 392-398
Photodissociation of Nitric Oxide from Nitrosyl Metalloporphyrins in Micellar Solutions
Haruna Adachi, Hirotaka Sonoki, Mikio Hoshino,* Masanobu Wakasa, and Hisaharu Hayashi
The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan
Yoshio Miyazaki
Department of Chemistry, Faculty of Engineering, Toyo UniVersity, Kujirai, Kawagoe,
Saitama 350-8585, Japan
ReceiVed: July 13, 2000; In Final Form: October 18, 2000
Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)FeIIHem (Hem ) protoporphyrin IX), (NO)-
CoIIOEP (OEP ) octaethylporphyrin), and (NO)MnIITPP (TPP ) tetraphenylporphyrin), in aqueous ionic
micellar solutions were carried out. The nitrosyl porphyrins in the micellar solutions readily photodissociate
NO, leaving the MP in micelles: the quantum yields are 0.15 ((0.1) for (NO)FeIIHem, 0.55 ((0.05) for
(NO)CoIIOEP, and 0.21 ((0.02) for (NO)MnIITPP. The MP thus produced recombine with NO to regenerate
the parent nitrosyl porphyrins. The decay of MP in the absence of the excess NO follows second-order kinetics
with the rate constant kMNO(second) (MNO ) FeNO, CoNO, and MnNO). In the presence of excess NO, the
decay of MP follows pseudo-first-order kinetics with the rate constant kM (M ) Fe, Co, and Mn). The value
of kM was measured as a function of [NO]. For FeIIHem and CoIIOEP, the plots of kCo vs [NO] and kFe vs
[NO] gave straight lines. The slopes of the lines obtained with FeIIHem and CoIIOEP afford the bimolecular
rate constants kFeNO(pseudo) and kCoNO(pseudo), respectively. It is found that kFeNO(pseudo) > kFeNO(second)
and kCoNO(pseudo) > kCoNO(second). The differences between kMNO(pseudo) and kMNO(second) observed for
FeIIHem and CoIIOEP are interpreted by assuming that (1) NO molecules in the micellar solutions are dissolved
in both micelles and the aqueous phase and (2) NO molecules trapped in micelles hardly react with MP
because of the electrostatic repulsion between ionic micelles. In the case of MnIITPP, the pseudo-first-order
rate constant, kMn, is found to asymptotically increase with an increase in [NO] to a limiting value. The
reaction mechanisms for the nitrosylation of MP in micellar solutions are discussed in detail on the basis of
the kinetic studies.
Introduction
The nitrosyl adducts of the synthetic MP dissolved in micellar
solutions are surrounded by surfactant molecules. Thus, like
nitrosyl hemeproteins, the NO adducts of the synthetic MP in
the micellar solutions are located in the hydrophobic phase. The
present paper reports the laser-photolysis studies of the NO
adducts of the synthetic MP with central metals FeII, CoII, and
MnII in micellar solutions.
Metalloporphyrins (MP) with simple diatomic molecules at
the axial positions have been extensively studied for the
elucidation of the functions and reactions of hemeproteins in
vivo.1-12 Particular attention has been paid to the dioxygen and
carbon monoxide adducts of MP.1-4,6-9 Recent advances in
medical science reveal that NO plays a key role in vivo as a
bioregulatory molecule in blood pressure control,13,14 neuronal
transmission,15,16 and immune response.17,18 Because NO readily
reacts with metalloenzymes and hemeproteins to give their NO
adducts,19-21 the studies of nitrosyl complexes in chemistry are
increasingly important in relation to the understanding of the
biological functions of NO.
Experimental Section
Chlorocobalt(III) octaethylporphyrin, ClCoIIIOEP (OEP )
octaethylporphyrin), and chloromanganese(III) tetraphenylpor-
phyrin, ClMnIIITPP (TPP ) tetraphenylporphyrin), were syn-
thesized and purified according to the literature.25,26 Reagent-
grade chlorohemin (ClFeIIIHem), cetyltrimethylammonium
bromide (CTAB), and sodium dodecyl sulfate (SDS) were used
as supplied. The concentrations of surfactants used in the present
study are 1.0 × 10-2 M for CTAB and 8 × 10-2 M for SDS
aqueous solutions. The critical micelle concentrations (cmcs)
of CTAB and SDS are respectively 9.2 × 10-4 and 8 × 10-3
M.27 It is confirmed by spectroscopic measurements that the
MP are insoluble in water without surfactants.
The MP were dissolved in 8 × 10-2 M SDS or 1.0 × 10-2
M CTAB aqueous solutions. For preparation of the NO adducts,
the sample solutions were degassed, and then NO gas was
introduced on a vacuum line. The NO concentrations in micellar
solutions are determined from the NO partial pressure and the
NO adducts of synthetic MP with central metals FeII, CoII,
and MnII in organic solutions efficiently undergo photochemical
dissociation of NO to yield the MP,2,5 which readily return to
the nitrosyl adducts by the recombination reaction with NO.
Nitrosyl hemeproteins also dissociate NO by photolysis.22,23
However, the NO dissociation yields and the mechanism for
the NO rebinding of hemeproteins are markedly different from
those of the synthetic MP.11,22-24 The proteins surrounding the
heme control both the NO dissociation yields and the chemical
reaction between NO and the central iron atom in the hemo-
proteins.
* To whom correspondence should be addressed. Fax: 048-462-4668.
Phone: 048-467-9426. E-mail: hoshino@postman.riken.go.jp.
10.1021/jp002515i CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/15/2000

Photodissociation of Nitric Oxide
J. Phys. Chem. A, Vol. 105, No. 2, 2001 393
Figure 2. Absorption spectra of (c) CoIIOEP and (d) (NO)CoIIOEP in
an 8.0 × 10-2 M SDS aqueous micellar solution.
to produce (NO)FeIIHem.31,32
Figure 1. Absorption spectra of (a) FeIIHem and (b) (NO)FeIIHem in
a 1.0 × 10-2 M CTAB micellar solution.
Bunsen coefficient of NO in an aqueous solution (4.71 × 10-2
at 298 K).28 Here we assume that the NO solubility in the
aqueous phase of micellar solutions is the same as that in an
aqueous solution.
ClFeIIIHem + 2NO + H2O f (NO)FeIIHem + HONO +
HCl (2)
Optical absorption and electron spin resonance (ESR) spectra
were recorded on a Hitachi 330 spectrophotometer and Jeol
FE-3A X-band spectrometer, respectively.
ClFeIIIHem was found to be insoluble in SDS micellar
solutions. Neither ClFeIIITPP nor ClFeIIIOEP was soluble in the
CTAB or SDS micellar solutions.
Laser-photolysis studies were carried out with the use of a
Nd:YAG laser (HY 500 from JK Lasers, Ltd.) equipped with
second (532 nm), third (355 nm), and fourth (266 nm) harmonic
generators. The transient spectra were measured by the intensi-
fied charge-coupled device detector (DH 520-18F-01 from
Andor Technology, Ltd.). For monitoring the decay of the
transient absorption, the output from the photomultiplier (R758
from Hamamatsu Photonics) was conducted to the digital storage
oscilloscope (Gould model 630 from Gould Instrument System,
Ltd.).
An SDS micellar solution of ClCoIIIOEP shows absorption
peaks at 409 (ꢀ ) 1.09 × 105 M-1 cm-1), 520, and 555 nm.
When a small amount of sodium dithionite is added into the
degassed SDS micellar solution, ClCoIIIOEP is readily reduced
to yield CoIIOEP. The spectrum of CoIIOEP in the solution has
peaks at 390 (ꢀ ) 6.33 × 104 M-1 cm-1), 510, and 548 nm.
The micellar solution of CoIIOEP was exposed to NO gas at
200 Torr to produce (NO)CoIIOEP.
CoIIOEP + NO f (NO)CoIIOEP
(3)
Results
Figure 2 shows the absorption spectra of CoIIOEP and (NO)-
CoIIOEP in SDS micellar solutions. The spectrum of (NO)CoII-
OEP exhibits absorption peaks at 380 nm (ꢀ ) 5.2 × 104 M-1
cm-1) with a shoulder at 400 nm in the Soret band region and
at 525 and 557 nm in the Q-band region.
Optical Absorption and ESR Spectra. ClFeIIIHem was
found to be soluble in a CTAB micellar solution. The spectrum
exhibits an absorption band at 398 nm with a shoulder at 365
nm and a broad band centered at 580 nm. The molar absorption
coefficient (ꢀ) at 398 nm is determined to be 6.69 × 104 M-1
The degassed micellar solution of ClCoIIIOEP was exposed
to NO gas at 200 Torr. The absorption spectrum measured after
exposure to NO gas is found to be identical with that of (NO)-
CoIIOEP. Thus, as in the case of ClFeIIIHem, ClCoIIIOEP reacts
with NO to give (NO)CoIIOEP by the reductive nitrosylation
reaction.12
cm-1
.
ClFeIIIHem in the degassed CTAB micellar solution was
reduced by sodium dithionite. FeIIHem, thus obtained, shows
the absorption peak at 400 nm in the Soret band region. The
micellar solution of FeIIHem was exposed to NO at 200 Torr
to make the nitrosyl adducts.5
ClCoIIIOEP + 2NO + H2O f (NO)CoIIOEP + HONO +
FeIIHem + NO f (NO)FeIIHem
(1)
HCl (4)
Figure 1 shows the absorption spectra of FeIIHem and (NO)-
FeIIHem in CTAB micellar solutions. The spectrum of (NO)-
FeIIHem has a sharp absorption peak at 396 nm (ꢀ ) 6.08 ×
104 M-1 cm-1). The ESR spectrum of (NO)FeIIHem in the
micellar solution taken at 77 K was found to exhibit three well-
resolved superhyperfine lines due to the nitrogen atom of the
axial NO.29,30
ClCoIIIOEP was found to be insoluble in cationic CTAB
micellar solutions. ClCoIIITPP was insoluble in both anionic
SDS and cationic CTAB micellar solutions.
ClMnIIITPP dissolved in an SDS micellar solution has
absorption peaks at 385 (ꢀ ) 5.51 × 104 M-1 cm-1), 479, 570,
and 605 nm. MnIITPP obtained by reduction of ClMnIIITPP with
sodium dithionite shows absorption peaks at 433 (ꢀ ) 4.12 ×
105 M-1 cm-1), 570, and 608 nm. When the solution of MnII-
TPP is exposed to NO gas at 200 Torr, (NO)MnIITPP is
produced.33
When the degassed micellar solution of ClFeIIIHem was
exposed to NO gas at 200 Torr, a new spectrum with a
maximum peak at 396 nm appeared. The absorption spectrum
was completely identical with that of (NO)FeIIHem produced
by the nitrosylation of FeIIHem. The ESR spectra of the NO-
exposed solutions of both ClFeIIIHem and FeIIHem at 77 K were
the same. These results indicate that ClFeIIIHem in a CTAB
micellar solution readily undergoes the reductive nitrosylation
MnIITPP + NO f (NO)MnIITPP
(5)
Figure 3 shows the absorption spectra of MnIITPP and (NO)-
MnIITPP in SDS micellar solutions. The absorption spectrum

394 J. Phys. Chem. A, Vol. 105, No. 2, 2001
Adachi et al.
Figure 3. Absorption spectra of (e) MnIITPP and (f) (NO)MnIITPP in
an 8.0 × 10-2 M SDS aqueous micellar solution.
Figure 5. Transient absorption spectrum observed for (NO)CoIIOEP
in a 8.0 × 10-2 M SDS aqueous micellar solution at 20 ns after the
355 nm laser pulse. The inset exhibits the plot of the pseudo-first-
order rate constant, kCo, represented as a function of [NOaq].
Figure 4. Transient absorption spectrum observed for (NO)FeIIHem
in a 1.0 × 10-2 M CTAB micellar solution at 20 ns after the 355 nm
laser pulse. The inset shows the plot of the pseudo-first-order rate
constant, kFe, represented as a function of [NOaq].
Figure 6. Transient absorption spectrum observed for (NO)MnIITPP
in a 8.0 × 10-2 M SDS aqueous micellar solution at 20 ns after the
355 nm laser pulse. The inset exhibits the plot of the pseudo-first-
order rate constant, kMn, represented as a function of [NOaq].
of (NO)MnIITPP exhibits absorption peaks at 432 (ꢀ ) 2.00 ×
105 M-1 cm-1), 543, 575, and 608 nm.
is identical with the difference spectrum (CoIIOEP minus (NO)-
CoIIOEP), indicating that NO is photodissociated from (NO)-
CoIIOEP.
ClMnIIITPP in the CTAB micellar solution exhibits no
absorption spectral changes by exposure to NO gas, suggesting
that, unlike ClFeIIIHem and ClCoIIIOEP, ClMnIIITPP does not
react with NO.
(NO)CoIIOEP + hν f NO + CoIIOEP
(8)
Laser Photolysis of Nitrosyl Porphyrins in Micellar
Solutions. Figure 4 shows the transient absorption spectrum
observed for (NO)FeIIHem in a 1.0 × 10-2 M CTAB micellar
solution at 20 ns after the 355 nm laser pulse. The transient
spectrum is in good agreement with the difference spectrum
obtained by subtracting the spectrum of (NO)FeIIHem from that
of FeIIHem. This result indicates that NO is photodissociated
from (NO)FeIIHem by the 355 nm laser pulse.
CoIIOEP thus produced reacts with NO according to eq 3,
returning to (NO)CoIIOEP. The decay of the transient spectrum
follows pseudo-first-order kinetics in the presence of excess NO.
The inset of Figure 5 shows the plot of the pseudo-first-order
rate constant kCo represented as a function of [NO]. The slope
of the line gives 2.82 × 109 M-1 s-1 as a bimolecular rate
constant, kCoNO(pseudo), for the reaction between CoIIOEP and
NO.
Figure 6 shows the transient spectrum observed at 20 ns for
an 8 × 10-2 M SDS micellar solution of (NO)MnIITPP after
the 355 nm laser pulse. The transient spectrum is identical with
the difference spectrum (MnIITPP minus (NO)MnIITPP).
(NO)FeIIHem + hν f NO + FeIIHem
(6)
The transient uniformly decays according to eq 1 in the whole
wavelength region studied.
The decay of the transient FeIIHem follows pseudo-first-order
kinetics in the NO partial-pressure range of ca. 10-610 Torr.
As shown in the inset of Figure 4, the plot of the pseudo-first-
order rate constant kFe vs [NOaq] gives a straight line with an
intercept at the origin.
(NO)MnIITPP + hν f NO + MnIITPP
(9)
The transient MnIITPP reacts with excess NO (eq 5) according
to pseudo-first-order kinetics. The inset of Figure 6 shows the
plot of the pseudo-first-order decay rate constant kMn represented
as a function of [NO]. The rate constant kMn increases with an
increase in [NO] and asymptotically approaches a limiting value.
The apparent bimolecular rate constant kMnNO(pseudo) for the
reaction of MnIITPP with NO is obtained as 1.56 × 107 M-1
s-1 from the initial slope of the plot of kMn vs [NO] obtained at
[NO] < 6.0 × 10-4 M.
kFe ) kFeNO(pseudo) [NOaq]
(7)
The slope of the line gives the bimolecular rate constant
FeNO(pseudo) for the reaction between FeIIHem and NO:
k
k
FeNO(pseudo) ) 5.7 × 108 M-1 s-1
.
Figure 5 shows the transient absorption spectrum observed
at 20 ns for the 8.0 × 10-2 M SDS micellar solution of (NO)-
Bimolecular Rate Constants in the Absence of Excess NO.
CoIIOEP after the 355 nm laser pulse. The transient spectrum
As mentioned above, the second-order rate constants kMNO-

Photodissociation of Nitric Oxide
J. Phys. Chem. A, Vol. 105, No. 2, 2001 395
TABLE 1: Bimolecular Rate Constants kMNO(pseudo) and
kMNO(second) for the Nitrosylation Reaction of MP
TABLE 2: Quantum Yields for the Photodissociation of NO
from Nitrosyl Porphyrins in Micellar and Toluene Solutions
Determined by 355 nm Laser Photolysis
CoIIOEP,a
M-1 s-1
FeIIHem,b
M-1 s-1
MnIITPP,a
M-1 s-1
(NO)CoIIOEP
(NO)FeIIHem
(NO)MnIITPP
2.82 × 109
5.7 × 108
1.56 × 107
micelle
toluene
0.55 ((0.05)
1.0 ((0.05)a
0.15 ((0.01)
0.5 ((0.05)a,b
0.21 ((0.02)
0.78 ((0.05)a
kMNO(pseudo)
kMNO(second)
1.52 × 109
2.26 × 108
5.0 × 107c
a Reference 5. b (NO)FeIIHem is insoluble in toluene. For a com-
parison with others, the quantum yield for photoinduced denitrosylation
of (NO)FeIITPP in toluene5 is given in the table.
a 8 × 10-2 M SDS micellar solution. b 1.0 × 10-2 M CATB micellar
solution. c This value is roughly estimated from the slope of the plot
k
MnNO(pseudo) vs [NO] in the range of 0 < [NO] < 3.0 × 10-4 M.
Similarly, the plot of kCoNO(second) vs [SDS] demonstrates
that kCoNO(second) increases with a decrease in [SDS] and
approaches kCoNO(pseudo) on going to low concentration of SDS.
Quantum Yield Measurements. The quantum yields for the
photodissociation of NO from nitrosyl MP, (NO)MP, in micellar
solutions were determined with the use of the 355 nm laser-
photolysis method. The details are described previously.2
Actinometry was carried out with the use of a benzene solution
of benzophenone. The laser photolysis of benzophenone at 355
nm gives the triplet benzophenone with the quantum yield of
unity, φT ) 1.0.34 The molar absorption coefficient of the triplet
benzophenone in benzene has been determined as ꢀT ) 7.6 ×
103 M-1 cm-1 at 530 nm.34 The micellar solution of (NO)MP
is adjusted to have an absorbance identical with that of the
benzophenone solution at 355 nm. When the micellar solution
is subjected to a 355 nm laser pulse, the absorbance change
∆DMP(λ) monitored at λ for the formation of MP is detected at
20 ns after the pulse. Similarly, the benzene solution of
benzophenone gives the triplet benzophenone with the absor-
bance at 530 nm, ∆DBP(530 nm), after the pulse. Then, the
quantum yield, φNO, for photoinduced denitrosylation of (NO)-
MP is represented as
Figure 7. Second-order rate constant, kCoNO(second), represented as a
function of [CTAB]. The inset shows the plot of kCoNO(pseudo)/kCoNO
-
(second) vs [micelle]CTAB (see text).
(pseudo) (M ) Fe, Co, and Mn) for the reaction between NO
and metalloporphyrin can be determined from the pseudo-first-
order rate constants, kM (M ) Fe, Co, and Mn), obtained in the
presence of excess NO: the slope of the plot of kM vs [NO]
gives kMNO(pseudo). The second-order rate constants can also
be determined with laser photolysis of nitrosyl porphyrins in
the absence of excess NO. The decay of the MP produced by
laser photolysis was found to follow second-order kinetics with
the rate constants kMNO(second) (M ) FeII, CoII, and MnII).
φNO ) ꢀT∆DMP(λ)/∆ꢀT(λ) ∆DBP(530 nm)
(11)
Here ∆ꢀT(λ) is the difference in the molar absorption coefficient
between MP and (NO)MP at the wavelength λ in the micellar
solution. The quantum yields, φNO, thus obtained are 0.15
((0.01) for (NO)FeIIHem, 0.55 ((0.05) for (NO)CoIIOEP, and
0.21((0.02) for (NO)MnIITPP. It is found that the quantum
yields are independent of the NO pressures in the range 0-600
Torr.
In Table 2 are listed the quantum yields for the photodisso-
ciation of NO from nitrosyl porphyrins in both micellar solutions
and toluene. The dissociation yields in toluene solutions are 2-4
times larger than those in micellar solutions.
0 -1
(∆Dλ)-1 ) (∆Dλ ) [-kMNO(second)/∆ꢀλ]t
(10)
Here, ∆Dλ0 and ∆Dλ (λ ) monitoring wavelength) are respec-
tively the absorbance changes of the transient spectrum at times
zero and t after the laser pulse. ∆ꢀλ is the difference in the molar
absorption coefficient of the metalloporphyrin and the nitrosyl
metalloporphyrin.
In Table 1 are listed the bimolecular rate constants kMNO
-
(pseudo) and kMNO(second) (M ) FeII, CoII, and MnII) obtained
for the reaction between MP and NO in 1.0 × 10-2 M CTAB
and 8.0 × 10-2 M SDS micellar solutions. It is noteworthy that
the values of kMNO(pseudo) are significantly different from those
of kMNO(second).
Discussion
Methemoglobin, metmyoglobin, and ferric cytochrome c
undergo reductive nitrosylation to yield their NO adducts of
the reduced forms.31 Such reductive nitrosylation takes place
when the synthetic FeIII and CoIII porphyrins in alcohol are
exposed to NO gas.2,32 The present study shows that FeIII and
CoIII porphyrins undergo reductive nitrosylation even in micellar
solutions to give the nitrosyl adducts of FeII and CoII porphyrins.
The present laser-photolysis studies demonstrate that (NO)-
MP photodissociate NO in micellar solutions. The quantum
yields for full photodissociation of NO in micellar solutions
are larger than those of nitrosyl hemoproteins.11 Probably, unlike
the proteins of hemoglobin and myoglobin, micelles have no
trapping site of NO: the photodissociated NO freely diffuses
into the aqueous phase.
For CoIIOEP and FeIIHem, kMNO(pseudo) is much larger than
k
MNO(second). However, MnIITPP gives kMnNO(pseudo) < kMnNO
-
(second). These results are discussed later on the basis of the
reaction mechanism for the nitrosylation of MP in micellar
solutions: the nitrosylation mechanism of MnIITPP is very
different from those of FeIITPP and CoIITPP.
The rate constants kFeNO(second) and kCoNO(second) were
measured by changing the concentrations of CTAB and SDS
in aqueous solutions. Figure 7 shows the plot of kFeNO(second)
represented as a function of [CTAB]. With a decrease in
[CTAB], kFeNO(second) becomes larger and approaches the
limiting value, kFeNO(pseudo). As will be shown later, this result
suggests that the “effective” [NO] concentration responsible for
the nitrosylation decreases with an increase in the micelle
concentrations.
The kinetic studies on the nitrosylation of MP in solutions
have been extensively studied by means of a laser-flash-
photolysis technique. In Table 3 are listed the bimolecular rate

396 J. Phys. Chem. A, Vol. 105, No. 2, 2001
Adachi et al.
TABLE 3: Bimolecular Rate Constants kMNO(pseudo) for
the Nitrosylation Reaction of MP in Benzene, Ethanol, and
Micellar Solutions
repulsion between micelles. Equation 16, thus, is represented
as
kMNO(second) ) kaq,M/(1 + K[micelle])
(17)
micelle
benzene,
M-1 s-1
ethanol,
M-1 s-1
k
MNO(pseudo),
M-1 s-1
In the presence of excess NO, the pseudo-first-order rate
CoIIOEP
FeIIHem
MnIITPP
2.3 × 109 a
5.3 × 108 a
2.98 × 109
1.42 × 109
4.90 × 105 b
2.82 × 109
5.7 × 108
1.56 × 107
constant kM is derived from eqs 14 and 15.
insoluble
kM ) kaq,M[NOaq] + kmicelle,M[NOmicelle
]
(18)
a Reference 5. b Reference 2.
Here again, we assume that kmicelle,M is negligibly small, owing
to the electrostatic repulsion between micelles. Thus, eq 18 is
readily transformed to
constants for the nitrosylation of MP in benzene, ethanol, and
micellar solutions.
The bimolecular rate constant for the nitrosylation of
CoIIOEP in the SDS micellar solution is almost identical with
those in toluene and ethanol. In the case of MnIITPP, the smallest
rate constant is given in ethanol. This has been interpreted in
terms of the strong coordination of solvent ethanol molecules
to the axial positions of MnIITPP.2
kM ) kaq,M[NOaq]
(19)
where [NOaq] is calculated from the Bunsen coefficient of NO
and the partial pressure of NO. In fact, as shown in the insets
of Figures 4 and 5, the plot of kM vs [NOaq] gives a straight
line for FeIIHem and CoIIOEP. From the above consideration,
the bimolecular rate constant kMNO(pseudo) obtained in the
presence of excess NO is expressed as
An interesting finding in the present study is that the values
of kMNO(pseudo) are significantly different from those of kMNO
-
(second). The (NO)MP dissolved in micelles dissociate NO by
laser flash photolysis, leaving the MP in micelles. The plots of
both kFe vs [NO] and kCo vs [NO] give straight lines. However,
the plot of kMn vs [NO] tends to level off at a higher
concentration of NO. This result indicates that the nitrosylation
mechanism of MnIITPP is very different from those of CoII-
OEP and FeIIHem. We, thus, initially discuss the nitrosylation
mechanism of CoIIOEP and FeIIHem.
kMNO(pseudo) ) kaq,M
(20)
Equations 17 and 20 lead to the following equation:
kMNO(second) ) kMNO(pseudo)/(1 + K[micelle]) (21)
Because K > 0, eq 21 clearly indicates that kMNO(pseudo) >
MNO(second), which is in agreement with the present experi-
k
NO molecules are soluble in both aqueous and hydrocarbon
solutions. In micellar solutions, NO molecules in the aqueous
phase, NOaq, are in equilibrium with those trapped in micelles,
mental data for (NO)FeIIHem and (NO)CoIIOEP.
Equation 21 also explains well the plot of kFeNO(second) vs
[CTAB] shown in Figure 7. The molar concentration of the
micelle, [micelle]CTAB, of CTAB is assumed to be expressed
as28
NOmicelle
.
NOaq + micelle {K
K ) ([NOmicelle]/[NOaq])[micelle]
\
} NOmicelle
(12)
(13)
[micelle]CTAB ) ([CTAB] - [CTAB]cmc)/N
(22)
where [CTAB]cmc and N are the cmc concentration and the
number of aggregations of CTAB, respectively. Because N )
60 and [CTAB]cmc ) 9.2 × 10-4 M for CTAB,28 we obtain
The MP, MIIPmicelle, in micellar solutions produced by photolysis
of (NO)MIIPmicelle react with both NOaq and NOmicelle. Thus, the
nitrosylation reaction of MIIPmicelle is represented by
kFeNO(pseudo)/kFeNO(second) ) 1 + K([CTAB] -
kaq,M
MIIPmicelle + NOaq
8 (NO)MIIPmicelle
(14)
[CTAB]cmc)/60 (23)
As shown in the inset of Figure 7, the plot of kFeNO(pseudo)/
kmicelle,M
MIIPmicelle + NOmicelle
8 (NO)MIIPmicelle (15)
k
FeNO(second) vs [micelle]CTAB gives a straight line. From the
slope of the line, K is determined to be 7.6 × 103 M-1 for
CTAB. Similarly, the K values of SDS are obtained as 9.2 ×
102 M-1 from the laser photolysis of (NO)CoIIOEP in an
aqueous SDS solution: the plot of kCoNO(pseudo)/kCoNO(second)
vs [micelle]SDS gives a straight line (N ) 62 for SDS). This
value is 1 order of magnitude smaller than that obtained from
CTAB.
The concentration of NO produced by photolysis of (NO)-
MIIPmicelle in the absence of excess NO is very low (<10-5 M)
in comparison with the concentration of micelles. Presumably,
one micelle traps one NO molecule photodissociated. From eqs
13-15, the bimolecular rate constants kMNO(second) obtained
by the second-order decay of the transient porphyrins are
expressed as
The nitrosylation mechanism of MnIITPP differs markedly
from those of CoIIOEP and FeIIHem. For interpretation of the
leveling-off of kMn at high concentrations of [NOaq], we assume
the following reaction mechanism in micellar solutions:
kMNO(second) ) kaq,M/(1 + K[micelle]) +
kmicelle,MK[micelle]/(1 + K[micelle]) (16)
kd
MnIITPP(H2O)2,micelle
{
} MnIITPP(H2O)micelle + H2O
Equation 16 implies that the rate constant kMNO(second) is
ka[H2O]
composed of two terms: the first term in eq 16 is the rate
(24)
constant for the reaction between MPII
and NO in the
micelle
kaq,Mn
MnIITPP(H2O)micelle + NOaq
{
}
aqueous phase, and the second term, the rate constant between
MPII
and NOmicelle. Here we assume that kmicelle,M is very
micelle
(NO)MnIITPP(H2O)micelle (25)
small in comparison with kaq,M because of the electrostatic

Photodissociation of Nitric Oxide
J. Phys. Chem. A, Vol. 105, No. 2, 2001 397
kmicelle,Mn
k
Mn(pseudo)/kMn(second) ) kd(1 + K[micelle]SDS)/
(ka[H2O] + kd) (33)
MNIITPP(H2O)micelle + NOmicelle
{
}
(NO)MnIITPP(H2O)micelle (26)
By using kd, K, and ka[H2O] obtained above, kMn(pseudo)/
Mn(second) is determined as 0.27. This value is in moderate
MnIITPP yielded by photolysis of (NO)MnTPP readily reacts
with water to give MnIITPP(H2O)2, which is in equilibrium with
MnIITPP(H2O) (eq 24). NOaq is assumed to react solely with
MnIITPP(H2O), because if the two axial positions are occupied
by water molecules, MnIITPP(H2O)2 is unable to react with NO.
This is further supported by an earlier observation that the rate
constant for the nitrosylation of MnIITPP in ethanol is 3 orders
of magnitude smaller than that in noncoordinating solvent
toluene.2
k
agreement with that (0.31) calculated from Table 1. From these
considerations, it is concluded that the leveling-off observed
for kMn at higher concentrations of NO is not ascribed to the
effects of the micelle on the nitrosylation reaction of MnIITPP.
The leveling-off originates from the reaction mechanism for the
nitrosylation of MnIITPP: the rate-determining step for the
nitrosylation reaction is the dissociation of an axial water from
MnIITPP(H2O)2.
The rate constants kMnNO(second) and kMn are obtained from
the combination of eqs 13 and 24-26. In the absence of excess
NO, the equilibrium reaction shown in eq 23 is assumed to hold
during the course of the nitrosylation reaction because of the
low concentration of NO photodissociated. Then, the rate
constant kMnNO(second) is expressed as
Summary
The laser-photolysis studies of nitrosyl porphyrins revealed
that the quantum yields for the photodissociation of NO in
micellar solutions are smaller than those in toluene solutions,
suggesting that the geminate recombination between NO and
MP in the micelles occur more efficiently than in toluene.
The reaction mechanism for the nitrosylation of MnIITPP is
different from that of CoIIOEP and FeIIHem. For the former,
the major species in micellar solution is the six-coordinate
species, MnIITPP(H2O)2,micelle, which is unable to react with NO.
The kinetic study reveals that the rate-determining step for the
nitrosylation is the dissociation of the axial water from MnII-
TPP(H2O)2,micelle. The five-coordinate species, MnIITPP(H2O),
thus produced reacts with NO to yield (NO)MnTPP. On the
other hand, the major species of CoIIOEP and FeIIHem in
micellar solutions are considered to be the four- or five-
coordinate species which readily react with NO.
The bimolecular rate constants kM(pseudo) (M ) Co and Fe)
obtained from the plot of kM vs [NO] markedly differ from kM-
(second) determined from the second-order decay of MPmicelle
in the absence of excess NO. This result is well-explained by
assuming that (1) NO molecules trapped in micelles are in
equilibrium with those in the aqueous phase, (2) MP reacts with
NO in the aqueous phase, and (3) NO molecules in micelles
are unable to react with MPmicelle because of the electrostatic
repulsion between micelles.
kMnNO(second) )
kaq,Mn + kmicelle,MnK[micelle]SDS ka[H2O] + kd
(27)
1 + K[micelle]SDS
ka[H2O]
In the presence of excess NO, the equilibrium in eq 23 does
not hold during the nitrosylation. By using a steady-state
approximation with regard to MnIITPP(H2O), the pseudo-first-
order rate constant, kMn, is expressed as
kd(kaq,Mn + kmicelle,MnK[micelle]SDS)[NOaq]
kMn
)
ka[H2O] + (kaq,Mn + kmicelle,MnK[micelle]SDS)[NOaq]
(28)
Equation 28 explains well the plot of kMn vs [NOaq] shown in
Figure 6: the leveling-off of kMn at a higher concentration of
NO is interpreted by assuming that the rate-determining step
for the nitrosylation of MnIITPP(H2O)2 is the dissociation of
the axial H2O.
Transformation of eq 28 gives
kMn-1 ){ka[H2O]/kd(kaq,Mn
+
kmicelle,MnK[micelle]SDS)}[NOaq]-1 + kd (29)
-1
References and Notes
(1) James, B. R. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. V, Chapter 6.
(2) Hoshino, M.; Nagashima, Y.; Seki, H.; De Leo, M.; Ford, P. C.
Inorg. Chem. 1998, 37, 2464-2469.
(3) Hoffman, B. M.; Weschler, C. J.; Basolo, F. J. Am. Chem. Soc.
1976, 98, 5473-5482.
(4) Collman, J. P.; Gagne, R. R.; Reed, C. A.; Halbert, J. R.; Lang,
G.; Robinson, W. T. J. Am. Chem. Soc. 1975, 97, 1427-1439.
(5) Hoshino, M.; Kogure, M. J. Phys. Chem. 1989, 93, 5478-5484.
(6) White, D. K.; Cannon, J. B.; Traylor, T. G. J. Am. Chem. Soc.
1979, 101, 2443-2454.
(7) Hoffman, B. M.; Gibson, Q. H. Proc. Natl. Acad. Sci. U.S.A. 1978,
75, 21-25.
The plot of kMn vs [NOaq]-1 gives a straight line. The slope
-1
and the intercept of the line respectively give
ka[H2O]/kd(kaq,Mn + kmicelle,MnK[micelle]SDS) )
6.4 × 10-8 M s (30)
and kd ) 2.06 × 104 s-1. Equations 27 and 30 lead to
6.4 × 10-8×kMn(second) ) (ka[H2O] + kd)/
(8) Hoshino, M.; Ueda, K.; Takahashi, M.; Yamaji, M.; Hama, Y.;
Miyazaki, Y. J. Phys. Chem. 1992, 96, 8894-8869.
(9) Hoshino, M.; Baba, T. J. Am. Chem. Soc. 1998, 120, 6820-6821.
(10) Parkhurst, L. J.; Gross, D. J. Biochemistry 1984, 23, 2180-2186.
(11) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 9568-9575.
kd(1 + K[micelle]SDS) (31)
With the use of kMn(second) ) 5.7 × 107 s-1, kd ) 2.06 × 104
s-1, [micelle]SDS ) 1.16 × 10-3 M, and K ) 9.2 × 102 M-1
obtained from CoIIOEP in SDS micellar solutions, ka[H2O] is
calculated as 1.4 × 105 s-1. Equation 28 gives the apparent
pseudo-first-order rate constant kMn(pseudo).
(12) Hoshino, M.; Laverman, L.; Ford, P. C. Coord. Chem. ReV. 1999,
187, 75-102.
(13) Ignarro, L. J. A. ReV. Pharmacol. Toxicol. 1990, 30, 535-560.
(14) Cullota, E.; Koshland, D. E. Science 1992, 258, 1862-1865.
(15) Klatt, P.; Schmidt, K.; Uray, G.; Mayer, B. J. Biol. Chem. 1993,
268, 14781-14787.
kMn(pseudo) ) kd(kaq,Mn + kmicelle,MnK[micelle]SDS)/ka[H2O]
(16) Shibuki, K.; Okada, D. Nature 1991, 349, 326-328.
(17) Hibbs, J. B., Jr.; Vavrin, Z.; Taintor, J. J. Immunol. 1987, 138,
550-565.
(32)
From eqs 27 and 32, we obtain

398 J. Phys. Chem. A, Vol. 105, No. 2, 2001
Adachi et al.
(18) Heck, D. E.; Laskin, D. L.; Gardner, C. R. J. Biol. Chem. 1992,
267, 21277-21280.
(19) Odaka, M.; Fujii, K.; Hoshino, M.; Noguchi, T.; Tsujimura, M.;
Nagashima, S.; Yohda, M.; Nagamune, T.; Inoue, Y.; Endo, I. J. Am. Chem.
Soc. 1997, 119, 3785-3791.
(26) Boucher, L. J. Am. Chem. Soc. 1968, 90, 6640-6645.
(27) Kalyanasundaram, K. Photochemistry in Microheterogeneous Sys-
tems; Academic Press: New York, 1987.
(28) Chihara, H. In Kagakubinran; Japan Chemical Society, Ed.;
Maruzen: Tokyo, 1984; Vol. II, p 158.
(20) Cleeter, M. W. J.; Cooper, J. M.; Darley-Usmar, V. M.; Moncada,
S.; Scapira, A. H. V. FEBS Lett. 1994, 395, 50-54.
(21) Brown, G. C. Eur. J. Biochem. 1995, 232, 188-191.
(22) Petrich, J. W.; Lambry, J.-C.; Kuczera, K.; Karplus, M.; Poyart,
C.; Martin, J. L. Biochemistry 1991, 30, 3975-3987.
(23) Cornelius, P. A.; Hochstrasser, R. M.; Steel, A. W. J. Mol. Biol.
1983, 163, 119-128.
(29) Yonetani, T.; Yamamoto, H.; Erman, J. E.; Leigh, J. S., Jr.; Reed,
G. H. J. Biol. Chem. 1972, 247, 2447-2455.
(30) Kon, H.; Kataoka, N. Biochemistry 1969, 8, 4757-4762.
(31) Hoshino, M.; Maeda, M.; Konishi, R.; Seki, H.; Ford, P. C. J. Am.
Chem. Soc. 1996, 118, 5702-5707.
(32) Wayland, B. B.; Olson, L. W. J. Chem. Soc., Chem. Commun. 1973,
897-898.
(33) Hoshino, M.; Konishi, S. Chem. Phys. Lett. 1985, 115, 511-514.
(34) Bensasson, R.; Land, E. J. Trans. Faraday Soc. 1971, 67, 1904-
1915.
(24) Morlino, E. A.; Rodgers, M. A. J. J. Am. Chem. Soc. 1996, 118,
11798-11804.
(25) Sakurai, T.; Yamamoto, K.; Naito, H.; Nakamoto, N. Bull. Chem.
Soc. Jpn. 1975, 97, 6461-6466.