Photochemistry of nitrosyl metalloporphyrins: Mechanisms of the photoinduced release and recombination of NO

Article abstract of DOI:10.1021/jp9811169

A series of substituted nitrosylmetalloporphyrins of the type M(NO)(meso-tetra(p-X)phenylporphyrin) (M = Co, Mn; X = H, NO₂, OMe) were prepared and studied by laser flash photolysis. The kinetics of the recombination reactions between the metalloporphyrins and NO in THF were examined. The recombination mechanism appears to be more complicated than the simple second-order process previously reported. Out-of-plane puckering of the porphyrin ring immediately after denitrosylation is one possible explanation. Kinetic experiments and NO transfer studies between manganese and cobalt porphyrins suggest that these compounds exist in equilibrium with NO in solution. Varying the substituents in the phenyl ring of the tetraphenylporphyrin ligand has little effect on rates of recombination with NO.

Full text of DOI:10.1021/jp9811169

J. Phys. Chem. B 1998, 102, 7287-7292
7287
Photochemistry of Nitrosyl Metalloporphyrins: Mechanisms of the Photoinduced Release
and Recombination of NO
Igor S. Zavarine, Abhay D. Kini, Bruce H. Morimoto, and Clifford P. Kubiak*,†
Department of Chemistry, Purdue UniVersity, 1393 Brown Laboratory, West Lafayette, Indiana 47907
ReceiVed: February 12, 1998; In Final Form: June 8, 1998
A series of substituted nitrosylmetalloporphyrins of the type M(NO)(meso-tetra(p-X)phenylporphyrin) (M )
Co, Mn; X ) H, NO , OMe) were prepared and studied by laser flash photolysis. The kinetics of the
2
recombination reactions between the metalloporphyrins and NO in THF were examined. The recombination
mechanism appears to be more complicated than the simple second-order process previously reported. Out-
of-plane puckering of the porphyrin ring immediately after denitrosylation is one possible explanation. Kinetic
experiments and NO transfer studies between manganese and cobalt porphyrins suggest that these compounds
exist in equilibrium with NO in solution. Varying the substituents in the phenyl ring of the tetraphenylporphyrin
ligand has little effect on rates of recombination with NO.
Introduction
SCHEME 1
Nitric oxide (NO) has emerged as an important biological
messenger involved in the regulation of blood pressure, as a
neurotransmitter, and as a cytotoxic agent.1-7 NO is also a
difficult molecule to study because it rapidly oxidizes to form
NO2, and in water, NO2 subsequently dissociates into an acidic
mixture of nitrite and nitrate. The reactive nature of NO has
made it difficult to understand the effect NO has on its receptors.
The availability of molecules that can be triggered to release
NO can help in understanding the biological role of NO and
8
,9
could also be potentially developed as therapeutic agents.
Reagents that can trigger NO release in situ have been sought
by several groups.
Most of the known NO donor systems rely on the formation
of nitric oxide by either simple dissociation or by a complex
1
0
series of reactions, often requiring metabolism in the cell.
There are only a few known cases where NO release is triggered
photolytically. Of the known systems, most suffer from a num-
ber of disadvantages. For example, photolysis of Ru(NO)Cl3
releases not just NO but toxic products as well.11,12 Triazenes
is useful for understanding the structure and reactivities of the
described nitrosylmetalloporphyrins.
The photochemistry of nitrosylmetalloporphyrins has been
1
6-19
studied in some detail.
M. Hoshino and co-workers have
11
are another class of compounds tested as a NO source. How-
ever, release of NO from these compounds requires UV light,
and the quantum yields for these reactions are low (2-5%).
shown that NO complexes of Co(II) and Mn(II) meso-tetraphen-
ylporphyrins, Co(TPP)(NO) (1), and Mn(TPP)(NO) (4) undergo
1
6,17
efficient (Φ ≈ 1) photodenitrosylation; see eq 1.
S-Nitrosoglutathione releases NO under visible light, but ex-
tremely slowly.13 Recently, a new system, based on N,N′-di-
hν
M(TPP)(NO) y z M(TPP) + NO (M ) Co or Mn) (1)
k-1
methyl-N,N′-dinitroso-p-phenylenediamine and similar com-
pounds, was reported as a high-performance phototriggered NO
The second-order rate constants (k-1) for the recombination
14
source. Although this compound was shown to be an effective
NO donor, its feasibility in biological systems remains to be
determined.
8
-1 -1
reaction were reported to be 7.0 × 10 M
s
for the Co(II)
7
-1 -1
complex and 3.3 × 10 M
s
for the Mn(II) complex.
Bohle and co-workers reported a dramatic effect that different
substituents on the phenyl groups of iron nitrosylmetallopor-
The electronic structure and geometry of the molecules
described in this report can be conveniently compared in Scheme
20
phyrins can have on NO release rates. It was not clear whether
n
1
. Here, {MXO} represents isoelectronic homologues of small
these changes were caused by steric factors (distortion of the
porphyrin core by bulky phenyl groups) or by electronic effects.
The question of what effects substitution of the phenyl rings of
tetraphenyl porphyrin ligands can have on the rates of the
recombination reaction with NO is key in developing suitable
NO release agents. Clearly, slowing the recombination rate
could result in increased efficiencies of NO release.
molecules, XO (X ) C, N, O), bound to metalloporphyrins of
n
15
different d configurations. Mn(TPP)(NO) is linear, isoelec-
tronic, and isolobal to Fe(TPP)(CO), while Co(TPP)(NO) is
bent, isoelectronic, and isolobal to Fe(O2)(TPP). This scheme
*
To whom correspondence should be addressed.
Current Address: Department of Chemistry and Biochemistry, Uni-
†
Here we report a study of the photochemical denitrosylation
and recombination mechanisms of the six different nitrosyl-
versity of California, San Diego, 9500 Gilman Drive, Dept. 0358, La Jolla,
CA 92093-0358.
S1089-5647(98)01116-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/19/1998

7288 J. Phys. Chem. B, Vol. 102, No. 37, 1998
Zavarine et al.
metalloporphyrins: nitrosylcobalt(II) tetraphenylporphyrin (Co-
(TPP)(NO), 1), nitrosylcobalt(II) meso-tetra(p-nitrophenyl)-
porphyrin (CoT(p-NO2P)P(NO), 2), nitrosylcobalt(II) meso-
tetra(p-methoxyphenyl)porphyrin CoT(p-OMeP)P(NO), 3),
nitrosylmanganese(II) meso-tetraphenylporphyrin Mn(TPP)-
(NO), 4), nitrosylmanganese(II) meso-tetra(p-nitrophenyl)por-
phyrin MnT(p-NO2P)P(NO), 5), nitrosylmanganese(II) meso-
tetra(p-methoxyphenyl)porphyrin MnT(p-OMeP)P(NO), 6).
Figure 1. Transient observation and disappearance of the Co(TPP)
species. λexcitation ) 532 nm; λobservation ) 527 nm.
g). This flask was then purged with N2 for 20 min. To this
flask was transferred 60 mL of degassed methanol. The
resulting solution was then stirred under nitrogen until evolution
of hydrogen ceased completely. At this point, NO was bubbled
into the solution for 20 min. Introduction of the NO results in
an immediate change in the color from green to red. The
nitrosylated Mn porphyrin precipitated from the solution, and
this precipitate was then filtered under nitrogen. The yield of
these reactions varied around 65-70%.
Experimental Section
Transient Absorbance Measurements. Our basic transient
absorbance instrument was described in detail previously.31
Second (λ ) 532 nm) or third (λ ) 355 nm) harmonics of the
fundamental of a Nd:YAG laser (Continuum ND60) were used
for all studies, with pulse energies of 20 mJ. The excitation
and the probe beam cross at a right angle. The path length
was 4 mm for the excitation beam and 10 mm for the probe
beam. Since the photolysis of the porphyrins leads to their slow
photodecomposition, the cell was refreshed after each laser shot
with a syringe pump. A second monochromator was placed
between the arc lamp and the flow cell in order to minimize
photolysis of the sample by the probe lamp. The transient
decays were digitized (LeCroy 9400A) and transferred to a
computer, and kinetics fits were performed using SigmaPlot and
QuattroPro spreadsheet software.
Materials. Dichloromethane (CH2Cl2) was distilled over
calcium hydride. THF was distilled from sodium benzophenone
ketyl. Nitric oxide was purchased from Matheson and passed
through a column of KOH pellets to remove higher oxides of
nitrogen. All moisture sensitive reactions were carried out under
nitrogen or argon in oven-dried glassware. All solutions for
flash photolysis were prepared in a drybox using freshly distilled
and dried solvents. Ultraviolet-visible spectra were obtained
using a Hitachi U-2000 spectrophotometer. Infrared spectra for
all compounds were obtained on a Mattson Research Series 1
spectrometer as films in dichloromethane (CH2Cl2) unless
otherwise specified. Flash column chromatography was per-
formed using silica gel 60 G 230-400 mesh (EM Science) or
alumina 80-200 mesh (Fisher Scientific). The nitric oxide trap
PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)
was purchased from Alexis Corp.
Some transient absorbance data were obtained using a lens-
intensified charge-coupled device (ICCD) as a detector. A
Princeton Instruments CCD detector with a Princeton Instru-
ments programmable pulse generator (PG-200) and an Action
Research Corp. Spectra Pro-275 spectrograph were used. The
Synthesis of Nitrosyl Porphyrins. The syntheses of T(p-
X)PPH2 (X ) H, OMe, NO2) were adapted from published
procedures.2
1,22
The cobalt
23-25
and manganese
26-28
metal-
3
00-900 nm region of the spectrum could be observed with
loporphyrins were either purchased from Midcentury Chemicals
or synthesized by adaptations of literature procedures.
Synthesis of CoT(p-X)PP(NO) (X ) H, 1; OMe, 2; NO2,
this instrument with the time resolution of 5 ns. With this
detector, entire transient absorbance spectra were obtained at a
specific delay time and then the delay time was scanned
incrementally to obtain full spectra as a function of time.
NO Concentration Determination. The concentration of
NO in the THF solutions was measured by gas chromatography
3
). The nitrosylated cobalt porphyrins CoT(p-X)PP(NO) (X )
H, 1; OMe, 2; NO2, 3) were synthesized by adapting a procedure
described by Scheidt and Hoard.28 The cobalt porphyrins (CoT-
(p-X)PP; 200 mg) were dissolved in dichloromethane and
(HP Series II 5890) at 27 °C. A saturated solution of NO in
carefully degassed. Dry piperidine (1.5 mL) was added, and
NO was bubbled through a potassium hydroxide (KOH)
scrubber and into the solution as a steady stream for 0.5 h. The
solution was purged with nitrogen to remove excess NO and
subsequently heated to boiling. The nitrosyl porphyrin pre-
cipitated after addition of degassed methanol. The products 1-3
were filtered and dried under vacuum. The yields of the
nitrosylated metalloporphyrins 1-3 were ca. 50%.
benzene (with a published value of NO mole fraction )
3
2
0
.00109) was used for calibration.
Results and Discussion
Cobalt Porphyrins. In general, the mechanism of photo-
denitrosylation and NO recombination is believed to be a simple
1
6,17
reversible process; see eq 1.
Our initial study focused on
General Method for the Synthesis of Nitrosyl Derivatives
of MnT(p-X)PP. The nitrosylated manganese porphyrins, Mn-
the kinetics of eq 1 as observed by transient UV-vis detection
in the 350-630 nm region. Figure 1 shows a typical decay of
photogenerated Co(TPP), monitored at 527 nm. The decay of
Co(TPP) is kinetically coupled with the reformation of Co(TPP)-
(NO) (monitored at 540 nm), which has the same rate constant.
(
Cl)T(p-X)PP, were synthesized by adapting a procedure of
26,29,30
Wayland and co-workers.
Mn(Cl)T(p-X)PP (200 mg) was
added to a 100 mL round-bottom flask containing NaBH4 (0.3

Photochemistry of Nitrosyl Metalloporphyrins
J. Phys. Chem. B, Vol. 102, No. 37, 1998 7289
Figure 2. Full transient absorption spectrum of the CoTPPNO, Co-
(
(
T(p-NO
2
)PP)NO, Co(T(p-OMe)TPP)NO system. λexcitation ) 532 nm.
For Figures 2 and 6 data were not obtained in the 425 nm region due
Figure 3. Transient formation and disappearance of “ruffled” CoT-
to a very intense Soret band.)
(p-NO
2
)PP(NO) monitored at 540 nm. Solid line corresponds to the
biexponential fitting of the decay.
TABLE 1: Rate Constants of the Recombination Reaction 1
in Different Solvents
0
.094 Å, as estimated by Scheidt and Hoard, and the NO ligand
THF (this
28
is bent. In the liquid phase, the geometries of both of these
molecules are probably less rigid and a manifold of states
puckered (or ruffled) to different degrees may contribute to the
observed electronic spectra. Photodenitrosylation thus can
change the geometry of the porphyrin from puckered (NO
adduct) to planar (free metalloporphyrin). However, the avail-
able kinetic data do not preclude other processes contributing
to the complex NO recombination behavior, such as M-NO
geometry isomerism. The NO ligand is known to bind to metals
in both linear and bent geometries.
2
-MTHF16 communication) benzene18,19
rate constant, M- s-1
1
7 × 10
8
2.2 × 10
9
7.9 × 10
9
The full transient absorption (TA) spectrum is shown in Figure
2
and it is very close to the difference of the spectra of Co-
(TPP)(NO) and Co(TPP). Table 1 compares the recombination
kinetics in 2-methyl-THF, THF (2-MTHF), and benzene. The
NO recombination reaction is markedly solvent dependent and
is found to be significantly slower in better coordinating solvents
such as 2-methyl-THF and THF.
To see the effects of porphyrin ring substituents on the rate
of NO recombination, we studied the kinetics of the recombina-
tion between these substituted Co porphyrins and NO. The
corresponding rates of recombination (at carefully selected
wavelengths where the decay curves fit a second-order rate law)
are given in Table 2. Although the effects are small, the
electron-withdrawing NO2 group does decrease the recombina-
tion rate, while the rate for CoT(p-OMeP)P(NO) recombination
is the same within experimental error as that for Co(TPP)(NO).
The NO2 group has a significantly higher Hammet’s σ value
Previous studies assumed simple second-order recombination
between NO radical and CoTPP.16,17 We observe, however, a
somewhat more complicated picture in THF. The shape of the
decay curve is highly wavelength dependent, and at few selected
wavelengths (527 and 540 nm, for example), decays can be fit
to a second-order rate law. However at other wavelengths, the
decays could not be fit to either a simple first- or second-order
rate law. The behavior of the recombination reaction is even
more complicated for the substituted porphyrins. For CoT(p-
OMeP)P(NO) (3) at λ ) 510 nm, a different reaction on a much
longer time scale dominates the transient absorbance signal. For
the CoT(p-NO2P)P(NO), a complex picture emerges suggesting
that at least two parallel reactions are occurring: (1) a fast
process with the negative absorbance change and (2) a slower
process with the positive absorbance change as observed at λ
(
0.81) than does a methoxy group (-0.28), consistent with these
36
observations. Previously, oxidation potentials, frequencies of
IR ν(NO) bands, and spectroelectrochemical behavior were
found to be very similar for 1 and 3 but different from 2.
It is pertinent to compare our data with recent studies on the
rates of ligand-induced NO dissociation from iron porphyrins.
37
20
)
540 nm; see Figure 3.
This mixed kinetics behavior could be explained by formation
At equilibrium, in eq 2,
of a solvento species. However, the shape of the recombination
curves remains similarly complex for both Co and Mn porphy-
rins (vide infra), and for Mn porphyrins the shape of the
recombination curve is the same in benzene and THF. There-
fore, competitive solvent coordination to the metal does not
completely explain this complex behavior. One alternative
explanation involves puckering of the porphyrin ring in solution.
It has frequently been suggested that nonplanar conformations
of porphyrins may play a critical role in modulating their
properties in vivo. There are reports in the literature of
k1
Fe(por)(NO)L y z Fe(por)L + NO
(2)
-1
k
6
where L ) pyridine or N-methylimidazole, a 10 -fold variation
in the value of k1 in the series of tetraarylporphyrins (p-tolyl,
2,6-difluorophenyl, mesityl, 2,6-dichlorophenyl, and pentafluo-
rophenyl) was reported. Our results are in sharp contrast to
these observations. This could originate from the significantly
distorted conformation of the porphyrins in the iron series. All
of our meso para-substituted porphyrins presumably have the
same (planar) conformation of the porphyrin ring and, therefore,
do not significantly differ in their rates of NO recombination.
Mn Porphyrins. We have also investigated the NO recom-
bination kinetics of substituted manganese nitrosyl porphyrins.
Two major differences between the kinetic behavior of the Co
and Mn porphyrins emerge from these studies. First, the rate
of recombination between Mn porphyrins and NO is ca. 1000
times slower than those of Co porphyrins. Second, at any
“
ruffling” of the porphyrin core and on its effects on reactivity
33,34
and electronic and redox properties.
NO can influence the conformation of the porphyrin ring.
Axial ligands such as
3
3,34
The crystal structure data of Co(TPP) at 100 K show that the
molecule is essentially planar, with a deviation of the Co atom
from the porphyrin plane of only 0.002 Å.35 The crystal
structure of the Co(TPP)(NO), however, is not planar. The Co
atom is displaced from the plane of the porphyrin ligand by

7290 J. Phys. Chem. B, Vol. 102, No. 37, 1998
Zavarine et al.
TABLE 2: Rate Constants (in THF) of the Recombination Reaction 1 for Porphyrins with Different Substituents in the Phenyl
Ringa
CoTPPNO (1)
CoT(p-OMe)PP(NO) (3)
CoT(p-NO
2
)PP(NO) (2)
rate constant, M- s-1
1
2.2 × 10
9
2.3 × 10
9
1.5 × 10
9
a
Estimated errors are less than 30%.
Figure 7. Full transient absorption spectrum of the Mn(TPP)(NO)/
MnTPP system.
Figure 4. Transient formation and disappearance of ruffled Mn(TPP)
species; λexcitation ) 532 nm.
SCHEME 2
while absorptions at λ < 500 nm are intraligand (Soret) bands.
TA spectra are virtually the same above 500 nm, except for
small variations in intensities, but the bands below 500 nm are
somewhat different. The bands for 4 and 5 are very similar,
while the intraligand band for 6 appears to be blue-shifted by
about 25 nm. It is interesting to compare the results obtained
approximately 1 ms after the laser pulse with the TA spectrum
Figure 5. Transient photobleaching and recovery of the ruffled Mn-
(TPP)(NO) species; λexcitation ) 532 nm.
(Figure 7) for 4 obtained with the ICCD detector 100 ns after
the laser pulse. The regions above 500 nm are essentially
identical at both times, but the intraligand absorptions for 4
below 500 nm are different at different time delays. These data
are consistent with a very fast photodenitrosylation process and
a relatively slow relaxation of the porphyrin ring to the non-
planar equilibrium structure found in 4.
Analysis of the kinetics supports this view. The rate of
recombination between Mn porphyrins and NO is significantly
slower than the corresponding rates for the Co porphyrin. The
shape of the decay curve (see Figure 4 and Figure 5) indicates
that at least two reactions are taking place at the same time. As
with Co porphyrins, the shape of the kinetic traces varies with
wavelength. At a number of wavelengths, kinetic data were
simply biexponential, consistent with two parallel first-order
recombination reactions. This biexponential kinetics can be
explained in several ways. Perhaps, the most plausible model
is a multistage equilibrium illustrated in Scheme 2, where
MnTPP* represents the puckered isomer of planar MnTPP.
The kinetic behavior of these multiequilibrium systems is
Figure 6. Transient absorption spectra of Mn(TPP)NO, Mn(T(p-NO
PP)NO, and Mn(T(p-OMe)TPP)NO.
2
)-
wavelength, all three Mn porphyrins in this study did not exhibit
the simple second-order recombination behavior previously
reported. Typical decay and restoration curves show complex
kinetics in both formation/decay of the Mn(TPP) species (Figure
38
4) and the bleaching/reappearance of Mn(TPP)(NO) (Figure 5)
well-known. For a system with three different states, behavior
(
though the shape of the slow components on these figures could
is always biexponential, and for n states, there will be n - 1
38
be somewhat distorted due to the limitations of our instrument).
Full TA spectra of the compounds 4, 5, and 6 acquired 1 ms
after the laser pulse are shown in Figure 6. Positive bands at
exponential terms. Of course, the problem is underdetermined,
and one cannot evaluate all six rate constants in Scheme 1.
One test of the model is to conduct flash photolysis of Mn-
(TPP)(NO) in the presence of a large and known excess of NO
in the solution. Under these conditions, kinetic traces at all
wavelengths become clean monoexponentials (Figure 8). This
result confirms that it is a process competing with NO re-
6
10, 570, and 420 nm correspond to the transient formation of
the Mn(TPP) species, while the negative band at 540 nm
corresponds to the photobleaching of the parent Mn(TPP)(NO)
molecules. Absorptions at λ > 500 nm involve metal d orbitals,

Photochemistry of Nitrosyl Metalloporphyrins
J. Phys. Chem. B, Vol. 102, No. 37, 1998 7291
and 6a are not known accurately and experimental rate constant
determination requires these values.)
NO Transfer Studies. Kadish and co-workers showed that
for the NO transfer reaction between two different metallopor-
phyrins
Co(TPP)(NO) + Fe(OEP) h Co(TPP) + Fe(OEP)(NO)
(4)
occurs on the electrochemical time scale.39 They further suggest
that the driving force for the NO transfer reactions is a stronger
Fe-N (1.717 Å bond length) bond as compared to the Co-N
bond (1.833 Å bond length). Extending these arguments to Mn
porphyrins (MnTPP has even shorter bond length), they sug-
gested and attempted (unsuccessfully) the following NO transfer
reactions:
Figure 8. Clean pseudo-first-order photobleaching and recovery of
Mn(TPP)(NO), NO in big excess.
Fe(OEP)(NO) + Mn(TPP) f Fe(OEP) + Mn(TPP)(NO)
(5)
Co(TPP)(NO) + Mn(TPP) f Co(TPP) + Mn(TPP)(NO)
(6)
They concluded that neither the bond strength nor the linearity
of the M-N-O bond is the driving force of the transfer reaction.
Upon consideration of the results, described herein, that indicate
that NO binds with Co porphyrins about 100 times faster than
Mn porphyrins do, this result is not surprising. In fact, our
kinetic study suggests that the opposite of the reaction 6 should
occur.
Figure 9. Dependence of the apparent pseudo-first-order rate constant
of the Mn(TPP) recombination, with NO on the concentration of the
excess NO in THF.
combination that causes the recombination plots to deviate from
a simple second-order rate law. In the presence of the large
excess of NO, the rate constants k1 and k2 from Scheme 1
become much higher than k3 as only one isomer (either MnTPP
or MnTPP*) recombines with NO. In this case, the apparent
rate constant kapp is given by eq 3
When a 2 mM solution of the Mn(TPP)(NO) (or 5 or 6) is
mixed with a 2 mM solution of Co(TPP), the NO transfer from
Mn(TPP)(NO) to CoTPPP occurs (eq 7).
Co(TPP) + Mn(TPP)(NO) f Co(TPP)(NO) + Mn(TPP)
(7)
This was determined by the complete disappearance of the IR
kapp ) 2k [NO] + kn
(3)
-n
-
1
ν(NO) absorption of the Mn(TPP)(NO) at 1750 cm and the
-
1
appearance of the ν(NO) band at 1684 cm , corresponding to
the Co(TPP)(NO). Hence, the nitrosyl ligand-transfer reaction
from a nitrosyl cobalt porphyrin to a free manganese porphyrin
could not be triggered photochemically, but it occurs readily in
the dark. When Mn(TPP)(NO) is mixed with nitric oxide trap
where n ) 1 if k1 . k2 and n ) 2 if k1 , k2. Furthermore, the
plot of kapp vs [NO] should be linear, with the slope being equal
to k-n and the intercept being equal to kn. The plot (see Figure
) is a straight line, but the intercept is indistinguishable from
zero. Hence, the plot cannot really offer a proof of the
puckering preequilibrium. The second-order recombination
9
-1
(
PTIO), the band of the Mn(TPP)(NO) at 1750 cm disappears
7
-1 -1
in the IR spectrum and the Mn(TPP) stretches at 568 and 610
nm in the UV-vis spectrum appear. Hence, the thermal
reactivity of Mn(TPP)(NO) together with its kinetic behavior
suggests that Mn(TPP) is in equilibrium with NO in solution.
In view of these results, it is not necessary to invoke the
intermediacy of a bridged Mn-(NO)-Co species as suggested
constant k-n determined from this plot is 1.6 × 10 M
s
for Mn(TPP)(NO) (4) in THF. This value could be compared
7
-1 -1
with the 3.3 × 10 M
s
value reported for this compound
16
by Hoshino and co-workers in MTHF. It could be argued
that the observed slow reaction in our studies is simply a
macrodiffusion of photoproducts from the observation region
in the cell in the TA experiment. However, the magnitude of
this second slower component does not change while the
temperature varies from 5 to 30 °C, and we do not see this
effect in flash photolysis of other systems. Both fast and slow
features are present in benzene, and therefore, this slower
reaction cannot realistically be attributed to the formation of a
solvento species. However, the rate of the reaction is higher in
benzene than it is in THF, suggesting some solvation of the
Mn(TPP) species. The rate of the recombination reaction for
substituted Mn porphyrins MnT(p-NO2P)P(NO) (5) and MnT-
3
9
by Kadish and co-workers, which would require the distance
between the two big porphyrin rings in the intermediate to be
unreasonably small. Existence of equilibrium between the
Fe(III) porphyrins (or ferrihemoproteins) and NO in the solution
4
0,41
was reported in the literature.
Conclusions
The mechanisms of recombination between NO and metal
porphyrins are more complicated than simple second-order
behavior. While Co(TPP)(NO) recombination behavior in many
cases could be described as second-order recombination, Mn-
(TPP)(NO) recombination and reactivity suggests that Mn(TPP)
exists in equilibrium with NO in solution. Kinetic evidence is
presented for the formation of “ruffled” conformations of the
porphyrin molecules after denitrosylation.
(
p-OMeP)P(NO) (6) follows the same trend observed for the
substituted Co porphyrins: the rate constant for 5 is about 50%
higher than for MnTPP(NO) (4), and the rate for 6 is essentially
the same as that for 6 and 4. (Estimates only can be given for
the Mn case, because the extinction coefficients for 5, 5a, 6,

7292 J. Phys. Chem. B, Vol. 102, No. 37, 1998
Zavarine et al.
Photochemical NO transfer from Co(TPP)(NO) to Mn(TPP)
(18) Morlino, E. A.; Walker, L. A. I.; Sension, R. J.; Rogers, M. A. J.
J. Am. Chem. Soc. 1995, 117, 4429-4430.
is thermodynamically unfeasible. The NO transfer in the op-
posite direction, from Mn(TPP)(NO) to Co(TPP), however,
occurs thermally. The relative thermodynamic stabilities of
Mn(TPP)(NO) and Co(TPP)(NO) have been determined in this
study, and a new mechanism for the NO transfer between two
different metalloporphyrins was suggested. The effect of the
substitution in the meso para position is small in comparison
to substitution in other positions.
(
19) Morlino, E. A.; Rodgers, M. A. J. J. Am. Chem. Soc. 1996, 118,
1798-11804.
20) Bohle, S. D.; Hung, C.-H. J. Am. Chem. Soc. 1995, 117, 9584-
9585.
(21) Semeikin, A. S. K.; Berezin, B. D. Khim. Geterotsikl. Soedin. 1982,
0, 1354-1355, 1046-1047.
22) Bettelheim, A. W.; White, B. A.; Raybuck, S. A.; Murray, R. W.
Inorg. Chem. 1987, 26, 1009-1017.
23) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976,
1
(
1
(
(
98, 3484-3489.
Acknowledgment. We acknowledge Dr. Kenneth Haber and
the continued support of the Purdue Chemistry Laser Facility.
This work was supported by the National Science Foundation
(24) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim., J. J. Inorg. Nucl.
Chem. 1970, 32, 2443-2445.
25) Buchler, J. W. Synthesis and Properties of Metalloporphyrins;
Academic Press: New York, 1978; Vol. I, part A.
26) Scheidt, W. R.; Hatano, K.; Rupprecht, G. A.; Piciculo, P. L. Inorg.
(
(CHE-9319173 and CHE-9615886), by the American Heart
(
Association, Indiana affiliate, and by funding to B.H.M. as a
Cottrell Scholar of the Research Corporation.
Chem. 1978, 17, 292-299.
(27) Jones, R. D.; Summerville, D. A.; Basolo, F. J. J. Am. Chem. Soc.
1
978, 100, 4416-4424.
28) Scheidt, W. R.; Hoard, J. L. J. Am. Chem. Soc. 1973, 95, 8281-
288.
(29) Wayland, B. B.; Olson, L. W. Inorg. Chim. Acta 1974, 11, L23-
L24.
(
References and Notes
8
(
1) Feldman, P. L.; Griffith, O. W.; Stuehr, D. J. Chem. Eng. News
1
993, 71, 26-38.
(
41.
2) Butler, A. R.; Williams, D.; Lyn, H. Chem. Soc. ReV. 1993, 4, 233-
(30) Wayland, B. B.; Olson, L. W.; Siddiqui, Z. U. J. Am. Chem. Soc.
2
1
976, 98, 94-98.
(
(
(
(
(
3) Choi, D. W. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 9741-9743.
(31) Zavarine, I. S.; Kubiak, C. P. Coord. Chem. ReV. in press.
4) Nathan, C. FASEB J. 1992, 6, 3051-3064.
(32) Solubility Data Series. Oxides of Nitrogen; Young, C. L., Ed.;
5) Crossin, K. L. Trends Biochem. Sci. 1991, 16, 81-82.
6) Bredt, D. S.; Synder, S. H. Neuron 1992, 8, 3-11.
7) Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Pharmacol. ReV. 1991,
Pergamon Press: Oxford, 1981; Vol. 8.
(33) Munro, O. K.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, R. W. J. Am. Chem. Soc. 1995, 117, 935-954.
(34) Ravikanth, M.; Reddy, D.; Misra, A.; Chandrashekar, T. K. J. Chem.
Soc., Dalton Trans. 1993, 1137.
(35) Stevens, E. D. J. Am. Chem. Soc. 1981, 103, 5087-95.
4
3, 109-142.
(
8) Porsti, I.; Paakkari, I. Ann. Med. 1995, 27, 407-420.
(
9) Moncada, S.; Higgs, E. A. FASEB J. 1995, 9, 1319-1330.
(
(
10) Butler, A. R.; Glidwell, C. Chem. Soc. ReV. 1987, 16, 361-380.
11) Makings, L. R.; Tsien, R. Y. J. Biol. Chem. 1994, 269, 6282-
(
36) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley-Inter-
science: New York, 1992.
37) Kini, A. D.; Washington, J.; Kubiak, C. P.; Morimoto, B. H. Inorg.
Chem. 1996, 35, 6904-6906.
38) Bernasconi, C. F. Relaxation Kinetics; Academic Press: New York,
976.
6
285.
(
12) Carter, T. D.; Bettache, N.; Ogden, D.; Trentham, D. R. J. Physiol.
London) 1993, 467, 165P.
13) Sexton, D. J.; Muruganandam, A.; MvKenney, D. J.; Mutis, B.
Photochem. Photobiol. 1994, 59, 463-467.
14) Namiki, S.; Arai, T.; Fujimori, K. J. Am. Chem. Soc. 1997, 119,
840-3841.
15) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339-
06.
(
(
(
(
1
1
(
(
39) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Inorg. Chem. 1988, 27, 1489-
3
4
492.
(
(40) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1993, 115, 9568-9575.
(41) Laverman, L. E.; Hoshino, M.; Ford, P., C. J. Am. Chem. Soc. 1997,
(
16) Hoshino, M.; Kogure, M. J. Phys. Chem. 1989, 93, 5478-5484.
(17) Hoshino, M.; Arai, S.; Yamaji, M.; Hama, Y. J. Phys. Chem. 1986,
9
0, 2109.
119, 12663-12664.