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Ligand photolysis and recombination of Fe(II) protoporphyrin IX complexes in dimethyl sulfoxide
Inorganica Chimica Acta 234 (1995) 101-107
ELSEVIER
Ligand photolysis and recombination of Fe(II) protoporphyrin IX complexes in dimethyl sulfoxide Randy W. Larsen a,., Eric W. Findsen b, Ruth E. Nalliah b "Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822, USA b Department of Chemistry, University of Toledo, Toledo, OH 43606, USA
Received 26 September 1994; revised 2 February 1995
Abstract
Steady-state and transient absorption spectroscopies have been employed to investigate ligand photolysis and recombination associated with (dimethyl sulfoxide)2Fe(II) protoporphyrin IX ((DMSO)2)Fe(II)PPIX), (imidazole)2Fe(II) protoporphyrin IX ((Imid)2Fe(II)PPIX), and (2-methylimidazole)(dimethyl sulfoxide)Fe(II) protoporphyrin IX ((2-Melm)(DMSO)Fe(II)PPIX) complexes in neat dimethyl sulfoxide (DMSO). Steady-state optical absorption spectra of these complexes are characteristic of six-coordinate low-spin heme iron. Photo-excitation of the (DMSO)2Fe(II)PPIX complexes results in the formation of a transient species consistent with a five-coordinate high-spin heme iron. The transient difference spectrum displays an absorption minimum at about 423 nm and an absorbance maximum at about 435 nm. This species decays with a first-order rate constant of (2.13+0.04)×106 s -1. In contrast, photolysis of the (2-MeIm)(DMSO)Fe(II)PPIX complex results in a species with a transient difference spectrum broadened and red-shifted A,4min~423 rim, A,4m~,~439 rim) relative to that of the (DMSO)2Fe(II)PPIX complex. This transient species decays with biphasic kinetics. The fast phase of the decay was found to be dependent on the concentration of 2-MeIm exhibiting a second-order rate constant of (7.9+0.5)×106 M -1 s -~. The slow kinetic phase displays first-order kinetics with a rate constant of (4.6+0.6)× 104 s -~. Photolysis of the (Imid)2Fe(II)PPIX yielded kinetics that were faster than the detection limit of our instrument (k> 1.0×10 s s-l). Keywords: Heme; DMSO; Ligand photolysis; Iron porphyrin complexes
1. I n t r o d u c t i o n
H e m e proteins and enzymes represent a class of biological macromolecules capable of catalyzing a wide range of chemical reactions. The active site of most heme proteins is iron protoporphyrin IX (heme) that is coordinated to one or two protein-donated axial ligands. T o a large extent the nature of this axial coordination influences the reactivity of the heme iron. H e m e proteins responsible for reversible electron transfer (e.g., b- and c-type cytochromes) have heme iron that is six-coordinate and low-spin with axial ligands derived from histidine, lysine and/or methionine, depending upon the type of protein [1-5]. In contrast, proteins and enzymes responsible for oxygen transport and activation (e.g. hemoglobin, myoglobin, cytochrome P45o, catalases and peroxidases) have h e m e iron that is either five- or six-coordinate and high-spin, prior to exogenous ligand binding [2-5]. A majority of these * Corresponding author. 0020-1693/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0020-1693(95)04488-U
proteins contains heme groups that are coordinated to a protein-donated histidine residue. Two exceptions to this are the cytochrome P45o and catalase-type enzymes. The cytochrome P450 class of enzyme contains a heme active site that is coordinated to a thiolate ligand derived from a cysteine residue, while catalases typically contain a tyrosine phenolate as the fifth heme ligand [6,7]. The critical importance ~ f heme axial ligation in modulating the biochemistry of heme proteins, as well as the potential for iron p o ~ h y r i n s in biomimetic chemistry, has stimulated efforts to understand mechanisms of ligand binding and activation in a wide range of iron porphyrin model systems [6,8]. Most of these systems are designed to mimic the reversible oxygen binding of hemoglobin and the oxygen activation chemistry of peroxidases and cytochrome P45o. These models are typically constructed to contain various imidazole or thiolate derivatives as ligands to the heme iron. H e m e model systems containing organic oxygen-donating axial ligation as models for catalase and activated oxygen intermediates of cytochrome P45o have received
102
R.W. Larsen et al. / lnorganica Chimica Acta 234 (1995) 101-107
much less attention and the ligand-binding dynamics of such ligands is not well understood [9-11]. Dimethyl sulfoxide (DMSO) is known to form complexes with iron porphyrins via coordination of the sulfoxide oxygen atom to the heme iron [12]. A variety of optical and magnetic spectroscopies and electrochemical data have shown that the (dimethyl sulfoxide)2Fe(III) protoporphyrin IX ((DMSO)2Fe(III)PPIX) complex contains an iron that is six-coordinate and high-spin with both ligands being retained upon conversion of Fe(III)PPIX to Fe(II)PPIX [9-14]. Recently, Nalliah and Findsen [15] demonstrated that the (DMSO)2Fe(II)PPIX complex is six-coordinate and low-spin with absorption properties similar to ferrous low-spin heme complexes with nitrogen-based axial ligands. These studies further show that the (DMSO)2Fe(II)PPIX complex can be photo-dissociated to form a five-coordinate high-spin (DMSO)Fe(II)PPIX species. Examination of the photolytic behavior of the (DMSO)2Fe(II)PPIX complex under varying conditions provides a unique opportunity to study structural and electron factors associated with the binding of oxygenbased organic ligands to iron porphyrins. With this in mind, the current study utilizes steadystate and transient absorption spectroscopies to examine photolysis and ligand recombination associated with (DMSO)2Fe(II)PPIX, (imidazole)2Fe(II) protoporphyrin IX ((Imid)EFe(II)PPIX) and (2-methylimidazole)(dimethyl sulfoxide)Fe(II) protoporphyrin IX ((2MeIm)(DMSO)Fe(II)PPIX)) complexes. Steady-state optical absorption spectra of these complexes are characteristic of six-coordinate low-spin heme iron. Photolysis of the (DMSO)EFe(II)PPIX complex results in the formation of a transient species with absorption maxima centered at about 435 nm, consistent with a five-coordinate high-spin heme iron. The decay of this species exhibits first-order kinetics. In contrast, photolysis of the (2-Melm)(DMSO)Fe(II)PPIX complex results in a species with a transient difference spectrum broadened and red-shifted (AAron. ~ 423 nm, AAm~ ~ 439 nm) relative to that of the (DMSO)2Fe(II)PPIX complex. This transient species decays with biphasic kinetics that are dependent upon concentration of 2-Melm. Photolysis of the (Imid)2Fe(II)PPIX yielded kinetics that were faster than the detection limit of our instrument.
2. Materials and methods
Hematin (Sigma), imidazole (Aldrich), 2-methylimidazole (Aldrich), sodium dithionite (Aldrich) and DMSO (Fisher) were used without further purification. Samples for steady-state and transient absorption studies were prepared by diluting hematin from a 3 mM stock solution into a 1 cm quartz optical cuvette containing
neat DMSO to give a final concentration of 5 ~M (determined using e402nm=174 mM -1 cm -1) [16]. For some samples either imidazole (Imid) or 2-methylimidazole (2-Melm) was added from a 1 M stock solution of the ligand in neat DMSO. Each sample was then sealed using a septum cap and purged with Ar for 30 min. Solid sodium dithionite was then added and the samples were purged with Ar for an additional 20 min. Optical absorption spectra of the ferrous heme complexes were obtained after a further 1 h incubation to insure complete reduction of the iron. The same samples were used for both the steady-state and transient absorption measurements. Nanosecond transient absorption data were obtained by photo-exciting a given sample with a 532 nm pulse (10 ns, 3.0 mJ/pulse) from a frequency doubled Nd:YAG laser (Continium, Surelite II). The change in absorption at various wavelengths was monitored by focusing the arc of a 75 W Xe arc lamp (Oreil) through the sample and overlapping with the volume illuminated by the laser pulse. The light passing through the sample was imaged onto the entrance slit of a SPEX 1680 0.25 M double monochromator. The resulting signal was detected using a thermoelectrically cooled photomultiplier tube (Hamamatsu R928 with a Products for Research cooling unit) coupled to a preamplifier of our own design. The resulting signal was digitized using a Tektronix RTD710A 200 MHz transient digitizer that was triggered with a photodiode exposed to the laser pulse. The data were transferred to an IBM-based 486DX personal computer for data manipulation. The data were fit using nonlinear least-squares methods. Steadystate absorption spectra were obtained using a Milton Roy Spectronic 3000 diode array spectrophotometer. Dipole moments for DMSO and 2-Melm were calculated using the AM1 quantum mechanical package with HyperChem T M software. The calculations were performed for structures that had been energy minimized using an MM2 force field with default parameters.
3. Results and discussion
3.1. Steady-state absorption spectra Fig. 1 displays the steady-state optical absorption spectra Of Fe(II)PPIX in DMSO both in the presence and absence of imidazole-based ligands. The spectrum of Fe(II)PPIX in DMSO in the absence of added ligands displays a Soret maximum at 424 nm, a-band at 556 nm and /3-band at 527 nm. Upon addition of 100 mM 2-Melm no significant changes are observed in the positions of the absorption bands. The ratio of the a- and/3-band absorbances, however, is increased upon addition of 2-Melm (I,u# = 2.4 for Fe(II)PPIX in DMSO containing 100 mM 2-Melm relative to 1.17
R.W. Larsen et al. / Inorganica Chimica Acta 234 (1995) 101-107
424 I
I
\
c
<
\
\ '
350
'
'
'
'
'
'
'
'
I
'
450 Wavelength
'
'
'
500
550 Wavelength
600
Fig. 1. Optical absorption spectra in the Soret (A) and visible (B) regions of (a) Fe(II)PPIX in D M S O containing 100 m M 2-MeIm, (b) Fe(II)PPIX in DMSO, and (c) Fe(II)PPIX in D M S O containing 100 m M Imid. Sample concentration is 5 /~M Fe(II)PPIX. Spectra were obtained in a 1 cm quartz optical cuvette.
for Fe(II)PPIX in DMSO), indicating complexation of the 2-Melm to the heme iron. In contrast, addition of 100 mM Imid to Fe(II)PPIX in DMSO results in red shifts and changes in relative intensities of all absorption bands. Specifically, the absorption spectrum displays bands at 426 nm (Soret), 529 nm (/3-band) and 559 nm (a-band). Examination of the absorption maxima of various ferrous heme model complexes (see Table 1) indicates that Fe(II)PPIX in DMSO forms a six-coordinate lowspin complex, similar to ferrous heme complexes con-
103
taining nitrogen-based ligands. The formation of a lowspin (DMSO)2Fe(II)PPIX has recently been confirmed by resonance Raman spectroscopy [15]. The formation of a low-spin (DMSO)2Fe(II)PPIX complex is in contrast to coordination of other oxygen-donating ligands such as phenoxides to ferrous iron porphyrins. Previous studies have shown that Fe(II)PPIX dimethyl ester and Fe(II)PPIX di-tert-butyl ester complexed with 3,4-dimethyl sodium phenoxide solubilized in either toluene or DMSO exhibit optical properties resembling those of ferrous high-spin iron complexes [9]. The absorption spectrum of Fe(II)PPIX in DMSO containing 100 mM imidazole is nearly identical to the (Imid)2Fe(II)PPIX dimethyl ester complex in aqueous solutions containing sodium dodecyl sulfate [17]. This indicates that the low-spin (DMSO)2Fe(II)PPIX complex is converted to a corresponding low-spin (Imid)2Fe(II)PPIX species upon addition of Imid and further suggests that the Fe(II)PPIX has a much higher ligand affinity for Imid than for DMSO. In contrast, the absorption spectrum of Fe(II)PPIX in DMSO containing 2-MeIm is similar to that of the (DMSO)2Fe(II)PPIX complex. Changes in the od/3ratio, however, indicate that 2-Melm displaces a coordinated DMSO to form a mixed-ligand (2-Melm)(DMSO)Fe(II)PPIX complex. Similarities in the absorption spectra of the (2-Melm)(DMSO)Fe(II)PPIX complex to other ferrous low-spin hemes suggest that this complex is also low-spin. This is consistent with the transient absorption data discussed later. It is interesting to note that 2-Melm bound Fe(II) porphyrins are typically fivecoordinate and high-spin in aqueous solution and in non-coordinating solvents [17]. This presumably arises
Table 1 Steady-state absorption maxima for various five- and six-coordinate Fe(II)PPIX complexes Complex
(DMSO)2Fe(II)PPIX (Imid)2Fe(II)PPIX" (2-Melm)(DMSO)Fe(II)PPIX (2-Melm)Fe(II)PPIX b (Imid)2Fe(II)PPIX c (Pyridine)2Fe (II)PPIX d (OC6H3-3,4Me2)2Fe(II)PPDME c Fe(II)PPDME(DMSO) f H93Y bib s
a~
(nm)
Soret
g-Band
a-Band
424 426 424 431 425 420 434 424 427
527 529 527 -530 527 562 525 -
556 559 556 559 560 557 593 557 560
Fe spin state
Ref.
low low low high low low high _h high
this work, [15] this work this work
• In neat DMSO. b NO fl-band given. c In 1% (wt.Nol.) sodium dodecyl sulfate/0.01 M N a O H solution. a In pyridine/water (35% wt./vol.). c O C ~ 3 . 3 , 4 M e 2 = 3 , 4 . d i m e t h y I phenoxide; P P D M E = p r o t o p o r p h y r i n IX dimethyl ester. f R e p o r t e d as 'solvated' in DMSO. s H93Y M b = m u t a n t myglobin with the proximal histidine (His 93) replaced with a tyrosine as a catalase model. h NO spin state reported.
[17l [171 [17] [9] [91 [251
104
R.W. Larsen et al. / Inorganica Chimica Acta 234 (1995) 101-107
from steric hindrance of the 2-MeIm group that restricts the movement of the iron atom into the plane of the porphyrin ring upon conversion to the low-spin configuration. Thus, it is likely that coordination of DMSO to the sixth position causes the iron to move closer to the plane of the porphyrin, but steric hindrance of the 2-Melm prevents the planar configuration typical of low-spin configurations.
(2-Melm)Fe(II)PPIX complex in non-coordinating solvents (absorption maximum at about 430 nm) [17]. The corresponding single wavelength transient absorption data obtained at 435 nm (Fig. 3(A), trace a) demonstrates that recombination of a DMSO molecule to the vacant coordination site of the heme subsequent to photolysis occurs mono-phasically according to the following scheme:
3.2. Nanosecond transient absorption studies
(DMSO)Fe(II)PPIX*+DMSO.
kz
" k-I
Previous resonance Raman studies have demonstrated that photolysis of a solution containing Fe(II)PPIX in DMSO results in the formation of a transient fivecoordinate high-spin species within 10 ns [15]. The time-resolved absorption difference spectrum of this intermediate at various times subsequent to photolysis is displayed in Fig. 2(A). The intermediate Fe(II)PPIX complex displays an absorption maximum at about 435 nm and a minimum at about 423 nm. The bleach observed at about 423 nm can be attributed to depletion of the six-coordinate low-spin ground-state complex. The increase in absorbance at about 435 nm is due to the formation of a transient five-coordinate high-spin complex ((DMSO)Fe(II)PPIX*). This conclusion is based upon similarities of the absorption maxima observed in the difference spectrum of the (DMSO)Fe(II)PPIX* photolytic transient to the absorption maxima of the five-coordinate and high-spin
(DMSO)2 Fe(II)PPIX Since the concentration of the ligand is in excess relative to the concentration of Fe(II)PPIX, the reaction follows pseudo-first-order kinetics with the rate, kobs((DMSO)Fe(II)PPIX). A single exponential fit to the transient absorption data gives a rate constant kobs= (2.13 + 0.04)X 106 S - 1 . Interestingly, the rate constant of ligand recombination to (DMSO)Fe(II)PPIX* is slow relative to recombination rates of other heme complexes containing or-donating ligands. For example, observed rate constants for ligand recombination of various isocyanides and 1-methylimidazole to protoporphyrin IX dimethyl ester are on the order of 109 s-1 [18-20]. In addition, ligand recombination of Imid to the photo-dissociated (Imid)2Fe(II)PPIX in DMSO (data not shown) occurs on a time scale approaching the resolution of the current instrument (above 10s
(8)
(A)
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424
434 Wavelength (nm)
444
415
425 43S Wavelength (nm)
445
Fig. 2. Transient difference spectra associated with the photolytic transient of Fe(II)PPIX in D M S O (A) and Fe(II)PPIX in D M S O containing 100 m M 2-MeIm (B). Transient difference spectra were reconstructed from single wavelength transient absorption data. Time scales are as follows. (A): E], 50 ns; A, 250 ns; O, 500 ns; *, 1 /zs. (B): El, 50 ns; A, 250 ns; * with dashed line, 500 ns; O, 1 /as; * with solid line, 10 /.~s. Difference spectra are post-flash minus pre-flash species. Sample conditions are as described in Fig. 1.
105
R. W. Larsen et aL / lnorganica Chimica Acta 234 (1995) 101-107
i
5.00E+006
(A) I
T
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.... L _ _
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.
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2.50E+006 o
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gi'ob~
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Time (ms)
......... i i ' c 02 0 E + 0 0 0
i .........
2E-002
Time (ms)
i .....
4E-002
Fig. 3. (A) Single wavelength transient absorption traces of (a) Fe(II)PPIX in DMSO, (b) Fe(II)PPIX in DMSO containing 50 mM 2-Melm, and (c) Fe(II)PPIX in DMSO containing 100 mM 2-MeIm on a 10 p,s time scale. (B) Single wavelength transient absorption traces of (a) Fe(II)PPIX in DMSO containing 25 mM 2-Melm, (b) Fe(II)PPIX in DMSO containing 50 mM 2-Melm, and (c) Fe(II)PPIX in DMSO containing 100 mM 2-MeIm on a 50 p,s time scale. Sample conditions are as in Fig. 1.
s-~). Lavalette et al. [18] have argued that a large dipole moment associated with the rebinding ligand can reduce the overall rate of recombination due to increased electrostatic repulsion between the ligand and the d~2 orbital of the five-coordinate heme. DMSO has a much larger dipole moment than nitrogen-based ligands, such as Imid (/~ = 5.3 D for DMSO versus 4.3 D for Imid) which is consistent with the slower rate of recombination. An alternative explanation involves strong dipole-dipole interactions of the photo-dissociated DMSO molecule with the bulk solvent. It has been suggested that liquid state DMSO assumes a chain-like structure that arises from the alignment of S-O dipoles [21]. Association of the photo-dissociated DMSO molecule into a chain-like aggregate introduces an additional energy barrier to recombination, thus reducing the recombination rate. Although it is not known for certain which of these effects dominates the recombination kinetics, both may have significant contributions. Photolysis of a solution of Fe(II)PPIX in DMSO containing 2-Melm produces a transient difference spectrum that is distinct from that of Fe(II)PPIX in DMSO (Fig. 2(B)). The difference spectrum displays a broad increase in absorbance centered at about 440 nm with a trough at about 424 nm. Examination of single wavelength transient absorption data obtained at 440 nm (Fig. 3, traces b and c) reveals that ligand recombination is biphasic with the rate constant of the fast phase being dependent upon the concentration of 2-Melm. A plot of kobs versus concentration of 2-Melm (Fig. 4) provides a second-order rate constant of (7.9 + 0.5) x 106 M - 1 s- ~ for the fast phase. Fig. 3(B) displays the single wavelength transient absorption data taken at 435 nm with three concentrations of 2-Melm on longer time
2.00E+006
.........
0E+000
, .........
L .........
1E-001 2E-001 Cone. 2-Melm (M)
3E-001
Fig. 4. Plot of the observed rate for the fast phase decay subsequent to photolysis of Fe(II)PPIX in DMSO as a function of 2-MeIm concentration. The concentration of Fe(II)PPIX is fixed at 15 /zM. Other conditions are as in Fig. 1.
scales, The data demonstrate that, although the rate constant of the slow phase is found to be independent of 2-MeIm concentration, the magnitude of the slow phase increases as the concentration of 2-MeIm increases. The transient absorption data obtained for photolysis of the Fe(II)PPIX complex in DMSO containing 2Melm are consistent with a model for ligand recombination described as follows: kt
DMSO+(2-Melm)Fe(II)PPIX.
" k-1 k2
(DMSO)(2-Melm)Fe(II)PPIX.
" k-2
2-Melm + (DMSO)Fe(II)PPIX The rate of recombination of DMSO to (2Melm)Fe(II)PPIX is pseudo-first order due to the large excess of DMSO with kobs= (4.6+0.6)X 104 s -1. The recombination rate of 2-Melm to (DMSO)Fe(II)PPIX, on the other hand, is dependent upon the concentration of 2-Melm with a bimolecular rate constant k_z = (7.9 + 0.5)X 106 M-1 s-1. The initial event subsequent to photolysis is the transient generation of both (DMSO)Fe(II)PPIX* and (2-Melm)Fe(II)PPIX* complexes. This could be the direct result of photo-dissociation of either 2-Melm or DMSO ligand, respectively, from the ground-state (2-Melm)(DMSO)Fe(II)PPIX complex. Alternatively, a four-coordinate Fe(II)PPIX complex may be generated within the first hundreds of femtoseconds subsequent to photolysis, followed by recombination of either a DMSO or a 2Melm molecule to the fifth coordination site [22-24]. This recombination is also likely to occur on a subnanosecond time scale. In either case, at the beginning of the current observation window (about 50 ns subsequent to photolysis) two species are present:
106
1~ W. Larsen et al. / lnorganica Chimica Acta 234 (1995) 101-107
(DMSO)Fe(II)PPIX* and (2-Melm)Fe(II)PPIX*. This assignment is consistent with the transient difference spectrum of the photolyzed (2-Melm)(DMSO)Fe(II)PPIX which displays a broad absorption increase that is red-shifted relative to the (DMSO)2Fe(II)PPIX photolytic transient. The broad nature of this band is most likely due to the presence of a (DMSO)Fe(II)PPIX* complex (absorption maximum at about 435 nm) and a (2-Melm)Fe(II)PPIX* complex with an absorbance maximum near 440 nm. We attribute the biphasic recombination observed on the microsecond time scale to ligand recombination to the two intermediates present. In the case of the (2-Melm)Fe(II)PPIX transient, rebinding of a second 2-Melm molecule to form a (2-Melm)2Fe(II)PPIX complex is highly unfavorable due to steric interactions between the imidazole methyl grouP and the porphyrin. Thus, the only avenue for recombination is the binding of a DMSO molecule to the sixth coordination site. Since the rate of the slow phase of recombination is independent of 2-Melm concentration and obeys firstorder kinetics, it can be assigned as the recombination of DMSO to the (2-Melm)Fe(II)PPIX* complex. This assignment is also consistent with the dependence of the magnitude of the slow phase on the concentration of 2-Melm since the overall population of transiently formed (2-Melm)Fe(II)PPIX should increase as the concentration of 2-Melm is increased. On the other hand, the (DMSO)Fe(II)PPIX* complex has two avenues for recombination. One possibility is the addition of a second DMSO molecule to form a (DMSO)2Fe(II)PPIX complex which can then exchange one DMSO for a 2-Melm molecule to reform the (2Melm)(DMSO)Fe(II)PPIX complex. This pathway, however, would not be expected to show any dependence upon the concentration of 2-Melm. In addition, the observed rate of recombination of DMSO to the (DMSO)Fe(II)PPIX* complex in the absence of added ligands is roughly a factor of four smaller than that of the observed fast phase decay. Taken together these data suggest that the fast phase recombination arises from 2-Melm rebinding to a (DMSO)Fe(II)PPIX* complex. The slow rebinding of DMSO to the (2Melm)Fe(II)PPIX* relative to the (DMSO)Fe(II)PPIX* complex arises from both electronic and steric constraints of the ligand. Ferrous heme complexes with coordinated 2-Melm are typically five-coordinate and high-spin in non-coordinating or weakly coordinating solvents, resulting in a porphyrin structure with the iron out-of-plane and a domed-ring conformation. By binding a sixth ligand the iron is forced to move further into the porphyrin plane. This motion would have a large energy barrier due to steric hindrance of the methyl group associated with the imidazole group. This steric constraint coupled with increased electrostatic
repulsion between the rebinding DMSO molecule and the iron d~, orbital results in a reduced rate of DMSO recombination. In contrast, recombination of 2-Melm to the (DMSO)Fe(II)PPIX appears to be less restricted than DMSO rebinding to the (DMSO)Fe(II)PPIX. This result is somewhat surprising since recombination of the 2-Melm group should be associated with steric hindrance of the methyl group. The fact that the observed rate is larger than the corresponding DMSO rebinding to the (2-Melm)Fe(II)PPIX complex indicates that either the structure of the (DMSO)Fe(II)PPIX* complex is such that the iron atom lies closer to the plane of the porphyrin ring than the corresponding (2Melm)Fe(II)PPIX* complex, or that the reduced dipole moment of the rebinding imidazole group plays a more significant role in modulating ligand rebinding.
4. Conclusions
The data presented here demonstrate several unusual aspects of the ligand-binding dynamics of DMSO to Fe(II)PPIX complexes. It is apparent that the effect of the large dipole moment of the sulfoxide group can significantly influence the rate of ligand recombination. In addition, data presented for the mixed-ligand (2Melm)(DMSO)Fe(II)PPIX complex demonstrate a balance between dipolar forces and steric constraints in modulating ligand recombination to five-coordinate Fe(II)PPIX complexes. Thus, this study provides new insight into the dynamics of polar oxygen-donating ligands to iron porphyrins and lays a foundation for future structural studies in this unusual system.
Acknowledgements
R.W.L. gratefully acknowledges Don Cole for assistance in the development of the nanosecond transient absorption spectrometer used in this study and Roger Cramer for helpful discussions during the course of this work. He also thanks the contributors to the Petroleum Research Fund administered by the American Chemical Society for support of this work.
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R.IV. Larsen et al. / Inorganica Chimica Acta 234 (1995) 101-107
[5] J.L. Hoard, in B. Chance, R.W. Estabroock and T. Yonetani (eds.), Heroes and Heine Proteins, Academic Press, New York, 1966. [6] P. Ortiz de Montellano, Cytochrome P-450: Structure, Mechanism, and Biochemistry, Plenum, New York, 1986. [7] G.R. Schonbaum and B. Chance, in P. Boyer (ed.), The Enzymes, Academic Press, New York, 3rd edn., 1976. [8] For reviews, see: (a) T.G. Traylor, PureAppl. Chem., 63 (1991) 265; (b) G.B. Jameson and J.A. Ibers, Comments Inorg. Chem., 2 (1983) 97; (c) B. Morgan and D. Dolphin, Struct. Bonding (Berlin), 64 (1987) 115. [9] E.W. Ainscough, A.W. Addison, D. Dolphin and B.R. James, J. Am. Chem. Soc., 100 (1978) 7585. [10] T. Otsuka, T. Ohya and M. Sato, Inorg. Chem., 24 (1985) 776. [11] R. Belal, M. Momenteau and B. Meunier, New J. Chem., 13 (1989) 853. [12] T. Mashiko, M.E. Kastner, K. Spartalian, W.R. Scheidt and C.A. Reed, Z A r t Chem. Soc., 100 (1978) 6354-6362. [13] L.A. Constant and D.G. Davis, Anal. Chem., 47 (1975) 2253. [14] L.A. Bottemly and K.M. Kadish, Inorg. Chem., 20 (1981) 1348.
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[15] R.E. Nalliah and E.W. Findsen, J. Raman Spectrosc., 24 (1993) 867. [16] J.W. Owens, J. Robinson and C.J. O'Connor, Inorg. Chem. Acta, 206 (1993) 141. [17] A. Desbois, Y. Henry and M. Lutz, Biochim. Biophys. Acta~ 785 (1984) 148. [18] D. Lavalette, C. Tetreau and M. Momenteau, Z Am. Chem. Soc., 101 (1979) 5395. [19] D.K. White, J.B. Cannon and T.G. Traylor, ./. Am. Chem. Soc., 101 (1979) 2443. [20] M. Momenteau and D. Lavalctte, J. Am. Chem. Soc., 100 (1978) 4322. [21] S.W. Jacob, E.E. Rosenbaum and D.C. Wood, Dimethyl Sulphoxide, Marcel Dekker, New York, 1971. [22] Q.H. Gibson, in C. Ho (ed.), Hemoglobin and Oxygen Binding, Elsevier, Amsterdam, 1982. [23] D.W. Dixon, C. Kirmaier and D. Holten, J. Am. Chem. Soc., 107 (1985) 808. [24] T.G. Traylor, D. Magde, D. Taube and K. Jongeward, J. Am. Chert Soc., 109 (1987) 5864. [25] S. Adachi, S. Nagano, Y. Watanabe, K. Ishimori and I. Morishima, Biochem. Biophys. Res. Commun., 180 (1991) 138.
ELSEVIER
Ligand photolysis and recombination of Fe(II) protoporphyrin IX complexes in dimethyl sulfoxide Randy W. Larsen a,., Eric W. Findsen b, Ruth E. Nalliah b "Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822, USA b Department of Chemistry, University of Toledo, Toledo, OH 43606, USA
Received 26 September 1994; revised 2 February 1995
Abstract
Steady-state and transient absorption spectroscopies have been employed to investigate ligand photolysis and recombination associated with (dimethyl sulfoxide)2Fe(II) protoporphyrin IX ((DMSO)2)Fe(II)PPIX), (imidazole)2Fe(II) protoporphyrin IX ((Imid)2Fe(II)PPIX), and (2-methylimidazole)(dimethyl sulfoxide)Fe(II) protoporphyrin IX ((2-Melm)(DMSO)Fe(II)PPIX) complexes in neat dimethyl sulfoxide (DMSO). Steady-state optical absorption spectra of these complexes are characteristic of six-coordinate low-spin heme iron. Photo-excitation of the (DMSO)2Fe(II)PPIX complexes results in the formation of a transient species consistent with a five-coordinate high-spin heme iron. The transient difference spectrum displays an absorption minimum at about 423 nm and an absorbance maximum at about 435 nm. This species decays with a first-order rate constant of (2.13+0.04)×106 s -1. In contrast, photolysis of the (2-MeIm)(DMSO)Fe(II)PPIX complex results in a species with a transient difference spectrum broadened and red-shifted A,4min~423 rim, A,4m~,~439 rim) relative to that of the (DMSO)2Fe(II)PPIX complex. This transient species decays with biphasic kinetics. The fast phase of the decay was found to be dependent on the concentration of 2-MeIm exhibiting a second-order rate constant of (7.9+0.5)×106 M -1 s -~. The slow kinetic phase displays first-order kinetics with a rate constant of (4.6+0.6)× 104 s -~. Photolysis of the (Imid)2Fe(II)PPIX yielded kinetics that were faster than the detection limit of our instrument (k> 1.0×10 s s-l). Keywords: Heme; DMSO; Ligand photolysis; Iron porphyrin complexes
1. I n t r o d u c t i o n
H e m e proteins and enzymes represent a class of biological macromolecules capable of catalyzing a wide range of chemical reactions. The active site of most heme proteins is iron protoporphyrin IX (heme) that is coordinated to one or two protein-donated axial ligands. T o a large extent the nature of this axial coordination influences the reactivity of the heme iron. H e m e proteins responsible for reversible electron transfer (e.g., b- and c-type cytochromes) have heme iron that is six-coordinate and low-spin with axial ligands derived from histidine, lysine and/or methionine, depending upon the type of protein [1-5]. In contrast, proteins and enzymes responsible for oxygen transport and activation (e.g. hemoglobin, myoglobin, cytochrome P45o, catalases and peroxidases) have h e m e iron that is either five- or six-coordinate and high-spin, prior to exogenous ligand binding [2-5]. A majority of these * Corresponding author. 0020-1693/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0020-1693(95)04488-U
proteins contains heme groups that are coordinated to a protein-donated histidine residue. Two exceptions to this are the cytochrome P45o and catalase-type enzymes. The cytochrome P450 class of enzyme contains a heme active site that is coordinated to a thiolate ligand derived from a cysteine residue, while catalases typically contain a tyrosine phenolate as the fifth heme ligand [6,7]. The critical importance ~ f heme axial ligation in modulating the biochemistry of heme proteins, as well as the potential for iron p o ~ h y r i n s in biomimetic chemistry, has stimulated efforts to understand mechanisms of ligand binding and activation in a wide range of iron porphyrin model systems [6,8]. Most of these systems are designed to mimic the reversible oxygen binding of hemoglobin and the oxygen activation chemistry of peroxidases and cytochrome P45o. These models are typically constructed to contain various imidazole or thiolate derivatives as ligands to the heme iron. H e m e model systems containing organic oxygen-donating axial ligation as models for catalase and activated oxygen intermediates of cytochrome P45o have received
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R.W. Larsen et al. / lnorganica Chimica Acta 234 (1995) 101-107
much less attention and the ligand-binding dynamics of such ligands is not well understood [9-11]. Dimethyl sulfoxide (DMSO) is known to form complexes with iron porphyrins via coordination of the sulfoxide oxygen atom to the heme iron [12]. A variety of optical and magnetic spectroscopies and electrochemical data have shown that the (dimethyl sulfoxide)2Fe(III) protoporphyrin IX ((DMSO)2Fe(III)PPIX) complex contains an iron that is six-coordinate and high-spin with both ligands being retained upon conversion of Fe(III)PPIX to Fe(II)PPIX [9-14]. Recently, Nalliah and Findsen [15] demonstrated that the (DMSO)2Fe(II)PPIX complex is six-coordinate and low-spin with absorption properties similar to ferrous low-spin heme complexes with nitrogen-based axial ligands. These studies further show that the (DMSO)2Fe(II)PPIX complex can be photo-dissociated to form a five-coordinate high-spin (DMSO)Fe(II)PPIX species. Examination of the photolytic behavior of the (DMSO)2Fe(II)PPIX complex under varying conditions provides a unique opportunity to study structural and electron factors associated with the binding of oxygenbased organic ligands to iron porphyrins. With this in mind, the current study utilizes steadystate and transient absorption spectroscopies to examine photolysis and ligand recombination associated with (DMSO)2Fe(II)PPIX, (imidazole)2Fe(II) protoporphyrin IX ((Imid)EFe(II)PPIX) and (2-methylimidazole)(dimethyl sulfoxide)Fe(II) protoporphyrin IX ((2MeIm)(DMSO)Fe(II)PPIX)) complexes. Steady-state optical absorption spectra of these complexes are characteristic of six-coordinate low-spin heme iron. Photolysis of the (DMSO)EFe(II)PPIX complex results in the formation of a transient species with absorption maxima centered at about 435 nm, consistent with a five-coordinate high-spin heme iron. The decay of this species exhibits first-order kinetics. In contrast, photolysis of the (2-Melm)(DMSO)Fe(II)PPIX complex results in a species with a transient difference spectrum broadened and red-shifted (AAron. ~ 423 nm, AAm~ ~ 439 nm) relative to that of the (DMSO)2Fe(II)PPIX complex. This transient species decays with biphasic kinetics that are dependent upon concentration of 2-Melm. Photolysis of the (Imid)2Fe(II)PPIX yielded kinetics that were faster than the detection limit of our instrument.
2. Materials and methods
Hematin (Sigma), imidazole (Aldrich), 2-methylimidazole (Aldrich), sodium dithionite (Aldrich) and DMSO (Fisher) were used without further purification. Samples for steady-state and transient absorption studies were prepared by diluting hematin from a 3 mM stock solution into a 1 cm quartz optical cuvette containing
neat DMSO to give a final concentration of 5 ~M (determined using e402nm=174 mM -1 cm -1) [16]. For some samples either imidazole (Imid) or 2-methylimidazole (2-Melm) was added from a 1 M stock solution of the ligand in neat DMSO. Each sample was then sealed using a septum cap and purged with Ar for 30 min. Solid sodium dithionite was then added and the samples were purged with Ar for an additional 20 min. Optical absorption spectra of the ferrous heme complexes were obtained after a further 1 h incubation to insure complete reduction of the iron. The same samples were used for both the steady-state and transient absorption measurements. Nanosecond transient absorption data were obtained by photo-exciting a given sample with a 532 nm pulse (10 ns, 3.0 mJ/pulse) from a frequency doubled Nd:YAG laser (Continium, Surelite II). The change in absorption at various wavelengths was monitored by focusing the arc of a 75 W Xe arc lamp (Oreil) through the sample and overlapping with the volume illuminated by the laser pulse. The light passing through the sample was imaged onto the entrance slit of a SPEX 1680 0.25 M double monochromator. The resulting signal was detected using a thermoelectrically cooled photomultiplier tube (Hamamatsu R928 with a Products for Research cooling unit) coupled to a preamplifier of our own design. The resulting signal was digitized using a Tektronix RTD710A 200 MHz transient digitizer that was triggered with a photodiode exposed to the laser pulse. The data were transferred to an IBM-based 486DX personal computer for data manipulation. The data were fit using nonlinear least-squares methods. Steadystate absorption spectra were obtained using a Milton Roy Spectronic 3000 diode array spectrophotometer. Dipole moments for DMSO and 2-Melm were calculated using the AM1 quantum mechanical package with HyperChem T M software. The calculations were performed for structures that had been energy minimized using an MM2 force field with default parameters.
3. Results and discussion
3.1. Steady-state absorption spectra Fig. 1 displays the steady-state optical absorption spectra Of Fe(II)PPIX in DMSO both in the presence and absence of imidazole-based ligands. The spectrum of Fe(II)PPIX in DMSO in the absence of added ligands displays a Soret maximum at 424 nm, a-band at 556 nm and /3-band at 527 nm. Upon addition of 100 mM 2-Melm no significant changes are observed in the positions of the absorption bands. The ratio of the a- and/3-band absorbances, however, is increased upon addition of 2-Melm (I,u# = 2.4 for Fe(II)PPIX in DMSO containing 100 mM 2-Melm relative to 1.17
R.W. Larsen et al. / Inorganica Chimica Acta 234 (1995) 101-107
424 I
I
\
c
<
\
\ '
350
'
'
'
'
'
'
'
'
I
'
450 Wavelength
'
'
'
500
550 Wavelength
600
Fig. 1. Optical absorption spectra in the Soret (A) and visible (B) regions of (a) Fe(II)PPIX in D M S O containing 100 m M 2-MeIm, (b) Fe(II)PPIX in DMSO, and (c) Fe(II)PPIX in D M S O containing 100 m M Imid. Sample concentration is 5 /~M Fe(II)PPIX. Spectra were obtained in a 1 cm quartz optical cuvette.
for Fe(II)PPIX in DMSO), indicating complexation of the 2-Melm to the heme iron. In contrast, addition of 100 mM Imid to Fe(II)PPIX in DMSO results in red shifts and changes in relative intensities of all absorption bands. Specifically, the absorption spectrum displays bands at 426 nm (Soret), 529 nm (/3-band) and 559 nm (a-band). Examination of the absorption maxima of various ferrous heme model complexes (see Table 1) indicates that Fe(II)PPIX in DMSO forms a six-coordinate lowspin complex, similar to ferrous heme complexes con-
103
taining nitrogen-based ligands. The formation of a lowspin (DMSO)2Fe(II)PPIX has recently been confirmed by resonance Raman spectroscopy [15]. The formation of a low-spin (DMSO)2Fe(II)PPIX complex is in contrast to coordination of other oxygen-donating ligands such as phenoxides to ferrous iron porphyrins. Previous studies have shown that Fe(II)PPIX dimethyl ester and Fe(II)PPIX di-tert-butyl ester complexed with 3,4-dimethyl sodium phenoxide solubilized in either toluene or DMSO exhibit optical properties resembling those of ferrous high-spin iron complexes [9]. The absorption spectrum of Fe(II)PPIX in DMSO containing 100 mM imidazole is nearly identical to the (Imid)2Fe(II)PPIX dimethyl ester complex in aqueous solutions containing sodium dodecyl sulfate [17]. This indicates that the low-spin (DMSO)2Fe(II)PPIX complex is converted to a corresponding low-spin (Imid)2Fe(II)PPIX species upon addition of Imid and further suggests that the Fe(II)PPIX has a much higher ligand affinity for Imid than for DMSO. In contrast, the absorption spectrum of Fe(II)PPIX in DMSO containing 2-MeIm is similar to that of the (DMSO)2Fe(II)PPIX complex. Changes in the od/3ratio, however, indicate that 2-Melm displaces a coordinated DMSO to form a mixed-ligand (2-Melm)(DMSO)Fe(II)PPIX complex. Similarities in the absorption spectra of the (2-Melm)(DMSO)Fe(II)PPIX complex to other ferrous low-spin hemes suggest that this complex is also low-spin. This is consistent with the transient absorption data discussed later. It is interesting to note that 2-Melm bound Fe(II) porphyrins are typically fivecoordinate and high-spin in aqueous solution and in non-coordinating solvents [17]. This presumably arises
Table 1 Steady-state absorption maxima for various five- and six-coordinate Fe(II)PPIX complexes Complex
(DMSO)2Fe(II)PPIX (Imid)2Fe(II)PPIX" (2-Melm)(DMSO)Fe(II)PPIX (2-Melm)Fe(II)PPIX b (Imid)2Fe(II)PPIX c (Pyridine)2Fe (II)PPIX d (OC6H3-3,4Me2)2Fe(II)PPDME c Fe(II)PPDME(DMSO) f H93Y bib s
a~
(nm)
Soret
g-Band
a-Band
424 426 424 431 425 420 434 424 427
527 529 527 -530 527 562 525 -
556 559 556 559 560 557 593 557 560
Fe spin state
Ref.
low low low high low low high _h high
this work, [15] this work this work
• In neat DMSO. b NO fl-band given. c In 1% (wt.Nol.) sodium dodecyl sulfate/0.01 M N a O H solution. a In pyridine/water (35% wt./vol.). c O C ~ 3 . 3 , 4 M e 2 = 3 , 4 . d i m e t h y I phenoxide; P P D M E = p r o t o p o r p h y r i n IX dimethyl ester. f R e p o r t e d as 'solvated' in DMSO. s H93Y M b = m u t a n t myglobin with the proximal histidine (His 93) replaced with a tyrosine as a catalase model. h NO spin state reported.
[17l [171 [17] [9] [91 [251
104
R.W. Larsen et al. / Inorganica Chimica Acta 234 (1995) 101-107
from steric hindrance of the 2-MeIm group that restricts the movement of the iron atom into the plane of the porphyrin ring upon conversion to the low-spin configuration. Thus, it is likely that coordination of DMSO to the sixth position causes the iron to move closer to the plane of the porphyrin, but steric hindrance of the 2-Melm prevents the planar configuration typical of low-spin configurations.
(2-Melm)Fe(II)PPIX complex in non-coordinating solvents (absorption maximum at about 430 nm) [17]. The corresponding single wavelength transient absorption data obtained at 435 nm (Fig. 3(A), trace a) demonstrates that recombination of a DMSO molecule to the vacant coordination site of the heme subsequent to photolysis occurs mono-phasically according to the following scheme:
3.2. Nanosecond transient absorption studies
(DMSO)Fe(II)PPIX*+DMSO.
kz
" k-I
Previous resonance Raman studies have demonstrated that photolysis of a solution containing Fe(II)PPIX in DMSO results in the formation of a transient fivecoordinate high-spin species within 10 ns [15]. The time-resolved absorption difference spectrum of this intermediate at various times subsequent to photolysis is displayed in Fig. 2(A). The intermediate Fe(II)PPIX complex displays an absorption maximum at about 435 nm and a minimum at about 423 nm. The bleach observed at about 423 nm can be attributed to depletion of the six-coordinate low-spin ground-state complex. The increase in absorbance at about 435 nm is due to the formation of a transient five-coordinate high-spin complex ((DMSO)Fe(II)PPIX*). This conclusion is based upon similarities of the absorption maxima observed in the difference spectrum of the (DMSO)Fe(II)PPIX* photolytic transient to the absorption maxima of the five-coordinate and high-spin
(DMSO)2 Fe(II)PPIX Since the concentration of the ligand is in excess relative to the concentration of Fe(II)PPIX, the reaction follows pseudo-first-order kinetics with the rate, kobs((DMSO)Fe(II)PPIX). A single exponential fit to the transient absorption data gives a rate constant kobs= (2.13 + 0.04)X 106 S - 1 . Interestingly, the rate constant of ligand recombination to (DMSO)Fe(II)PPIX* is slow relative to recombination rates of other heme complexes containing or-donating ligands. For example, observed rate constants for ligand recombination of various isocyanides and 1-methylimidazole to protoporphyrin IX dimethyl ester are on the order of 109 s-1 [18-20]. In addition, ligand recombination of Imid to the photo-dissociated (Imid)2Fe(II)PPIX in DMSO (data not shown) occurs on a time scale approaching the resolution of the current instrument (above 10s
(8)
(A)
T
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424
434 Wavelength (nm)
444
415
425 43S Wavelength (nm)
445
Fig. 2. Transient difference spectra associated with the photolytic transient of Fe(II)PPIX in D M S O (A) and Fe(II)PPIX in D M S O containing 100 m M 2-MeIm (B). Transient difference spectra were reconstructed from single wavelength transient absorption data. Time scales are as follows. (A): E], 50 ns; A, 250 ns; O, 500 ns; *, 1 /zs. (B): El, 50 ns; A, 250 ns; * with dashed line, 500 ns; O, 1 /as; * with solid line, 10 /.~s. Difference spectra are post-flash minus pre-flash species. Sample conditions are as described in Fig. 1.
105
R. W. Larsen et aL / lnorganica Chimica Acta 234 (1995) 101-107
i
5.00E+006
(A) I
T
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.... L _ _
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. . . . .
.
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oE.o'od . . . . .
gi'ob~
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Time (ms)
......... i i ' c 02 0 E + 0 0 0
i .........
2E-002
Time (ms)
i .....
4E-002
Fig. 3. (A) Single wavelength transient absorption traces of (a) Fe(II)PPIX in DMSO, (b) Fe(II)PPIX in DMSO containing 50 mM 2-Melm, and (c) Fe(II)PPIX in DMSO containing 100 mM 2-MeIm on a 10 p,s time scale. (B) Single wavelength transient absorption traces of (a) Fe(II)PPIX in DMSO containing 25 mM 2-Melm, (b) Fe(II)PPIX in DMSO containing 50 mM 2-Melm, and (c) Fe(II)PPIX in DMSO containing 100 mM 2-MeIm on a 50 p,s time scale. Sample conditions are as in Fig. 1.
s-~). Lavalette et al. [18] have argued that a large dipole moment associated with the rebinding ligand can reduce the overall rate of recombination due to increased electrostatic repulsion between the ligand and the d~2 orbital of the five-coordinate heme. DMSO has a much larger dipole moment than nitrogen-based ligands, such as Imid (/~ = 5.3 D for DMSO versus 4.3 D for Imid) which is consistent with the slower rate of recombination. An alternative explanation involves strong dipole-dipole interactions of the photo-dissociated DMSO molecule with the bulk solvent. It has been suggested that liquid state DMSO assumes a chain-like structure that arises from the alignment of S-O dipoles [21]. Association of the photo-dissociated DMSO molecule into a chain-like aggregate introduces an additional energy barrier to recombination, thus reducing the recombination rate. Although it is not known for certain which of these effects dominates the recombination kinetics, both may have significant contributions. Photolysis of a solution of Fe(II)PPIX in DMSO containing 2-Melm produces a transient difference spectrum that is distinct from that of Fe(II)PPIX in DMSO (Fig. 2(B)). The difference spectrum displays a broad increase in absorbance centered at about 440 nm with a trough at about 424 nm. Examination of single wavelength transient absorption data obtained at 440 nm (Fig. 3, traces b and c) reveals that ligand recombination is biphasic with the rate constant of the fast phase being dependent upon the concentration of 2-Melm. A plot of kobs versus concentration of 2-Melm (Fig. 4) provides a second-order rate constant of (7.9 + 0.5) x 106 M - 1 s- ~ for the fast phase. Fig. 3(B) displays the single wavelength transient absorption data taken at 435 nm with three concentrations of 2-Melm on longer time
2.00E+006
.........
0E+000
, .........
L .........
1E-001 2E-001 Cone. 2-Melm (M)
3E-001
Fig. 4. Plot of the observed rate for the fast phase decay subsequent to photolysis of Fe(II)PPIX in DMSO as a function of 2-MeIm concentration. The concentration of Fe(II)PPIX is fixed at 15 /zM. Other conditions are as in Fig. 1.
scales, The data demonstrate that, although the rate constant of the slow phase is found to be independent of 2-MeIm concentration, the magnitude of the slow phase increases as the concentration of 2-MeIm increases. The transient absorption data obtained for photolysis of the Fe(II)PPIX complex in DMSO containing 2Melm are consistent with a model for ligand recombination described as follows: kt
DMSO+(2-Melm)Fe(II)PPIX.
" k-1 k2
(DMSO)(2-Melm)Fe(II)PPIX.
" k-2
2-Melm + (DMSO)Fe(II)PPIX The rate of recombination of DMSO to (2Melm)Fe(II)PPIX is pseudo-first order due to the large excess of DMSO with kobs= (4.6+0.6)X 104 s -1. The recombination rate of 2-Melm to (DMSO)Fe(II)PPIX, on the other hand, is dependent upon the concentration of 2-Melm with a bimolecular rate constant k_z = (7.9 + 0.5)X 106 M-1 s-1. The initial event subsequent to photolysis is the transient generation of both (DMSO)Fe(II)PPIX* and (2-Melm)Fe(II)PPIX* complexes. This could be the direct result of photo-dissociation of either 2-Melm or DMSO ligand, respectively, from the ground-state (2-Melm)(DMSO)Fe(II)PPIX complex. Alternatively, a four-coordinate Fe(II)PPIX complex may be generated within the first hundreds of femtoseconds subsequent to photolysis, followed by recombination of either a DMSO or a 2Melm molecule to the fifth coordination site [22-24]. This recombination is also likely to occur on a subnanosecond time scale. In either case, at the beginning of the current observation window (about 50 ns subsequent to photolysis) two species are present:
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1~ W. Larsen et al. / lnorganica Chimica Acta 234 (1995) 101-107
(DMSO)Fe(II)PPIX* and (2-Melm)Fe(II)PPIX*. This assignment is consistent with the transient difference spectrum of the photolyzed (2-Melm)(DMSO)Fe(II)PPIX which displays a broad absorption increase that is red-shifted relative to the (DMSO)2Fe(II)PPIX photolytic transient. The broad nature of this band is most likely due to the presence of a (DMSO)Fe(II)PPIX* complex (absorption maximum at about 435 nm) and a (2-Melm)Fe(II)PPIX* complex with an absorbance maximum near 440 nm. We attribute the biphasic recombination observed on the microsecond time scale to ligand recombination to the two intermediates present. In the case of the (2-Melm)Fe(II)PPIX transient, rebinding of a second 2-Melm molecule to form a (2-Melm)2Fe(II)PPIX complex is highly unfavorable due to steric interactions between the imidazole methyl grouP and the porphyrin. Thus, the only avenue for recombination is the binding of a DMSO molecule to the sixth coordination site. Since the rate of the slow phase of recombination is independent of 2-Melm concentration and obeys firstorder kinetics, it can be assigned as the recombination of DMSO to the (2-Melm)Fe(II)PPIX* complex. This assignment is also consistent with the dependence of the magnitude of the slow phase on the concentration of 2-Melm since the overall population of transiently formed (2-Melm)Fe(II)PPIX should increase as the concentration of 2-Melm is increased. On the other hand, the (DMSO)Fe(II)PPIX* complex has two avenues for recombination. One possibility is the addition of a second DMSO molecule to form a (DMSO)2Fe(II)PPIX complex which can then exchange one DMSO for a 2-Melm molecule to reform the (2Melm)(DMSO)Fe(II)PPIX complex. This pathway, however, would not be expected to show any dependence upon the concentration of 2-Melm. In addition, the observed rate of recombination of DMSO to the (DMSO)Fe(II)PPIX* complex in the absence of added ligands is roughly a factor of four smaller than that of the observed fast phase decay. Taken together these data suggest that the fast phase recombination arises from 2-Melm rebinding to a (DMSO)Fe(II)PPIX* complex. The slow rebinding of DMSO to the (2Melm)Fe(II)PPIX* relative to the (DMSO)Fe(II)PPIX* complex arises from both electronic and steric constraints of the ligand. Ferrous heme complexes with coordinated 2-Melm are typically five-coordinate and high-spin in non-coordinating or weakly coordinating solvents, resulting in a porphyrin structure with the iron out-of-plane and a domed-ring conformation. By binding a sixth ligand the iron is forced to move further into the porphyrin plane. This motion would have a large energy barrier due to steric hindrance of the methyl group associated with the imidazole group. This steric constraint coupled with increased electrostatic
repulsion between the rebinding DMSO molecule and the iron d~, orbital results in a reduced rate of DMSO recombination. In contrast, recombination of 2-Melm to the (DMSO)Fe(II)PPIX appears to be less restricted than DMSO rebinding to the (DMSO)Fe(II)PPIX. This result is somewhat surprising since recombination of the 2-Melm group should be associated with steric hindrance of the methyl group. The fact that the observed rate is larger than the corresponding DMSO rebinding to the (2-Melm)Fe(II)PPIX complex indicates that either the structure of the (DMSO)Fe(II)PPIX* complex is such that the iron atom lies closer to the plane of the porphyrin ring than the corresponding (2Melm)Fe(II)PPIX* complex, or that the reduced dipole moment of the rebinding imidazole group plays a more significant role in modulating ligand rebinding.
4. Conclusions
The data presented here demonstrate several unusual aspects of the ligand-binding dynamics of DMSO to Fe(II)PPIX complexes. It is apparent that the effect of the large dipole moment of the sulfoxide group can significantly influence the rate of ligand recombination. In addition, data presented for the mixed-ligand (2Melm)(DMSO)Fe(II)PPIX complex demonstrate a balance between dipolar forces and steric constraints in modulating ligand recombination to five-coordinate Fe(II)PPIX complexes. Thus, this study provides new insight into the dynamics of polar oxygen-donating ligands to iron porphyrins and lays a foundation for future structural studies in this unusual system.
Acknowledgements
R.W.L. gratefully acknowledges Don Cole for assistance in the development of the nanosecond transient absorption spectrometer used in this study and Roger Cramer for helpful discussions during the course of this work. He also thanks the contributors to the Petroleum Research Fund administered by the American Chemical Society for support of this work.
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