Download Electron transport via metalloporphyrins C.

1381
Electron transport via metalloporphyrins
EUGENE
C.
JOHNSON, TONY NIEM,l AND DAVID DOLPHIN 2
The Department ,!(Chelllistry. The /Jnil'ersitv oIBrilish Columbia. Valleoul'(!/", B.C.. Canada V6T I WS
Received September 9, 1977
EUGENE C. JOHNSON, TONY NJEM, and DAVID DOLPHIN. Can. J. Chern. 56.1381 (1978).
The controlled potential electrolysis of Ni(lI) lIleso-tetraphenylporphyrin (Ni(II)TPP) gives
at room temperature the corresponding metalloporphyrin it-cation radical [Ni(II)TPP]+·.
Upon freezing a solution of the it-cation radical to 77 K an internal electron transfer occurs to
give [Ni(III)TPP] +. A discussion of the routes of electron transport in heme proteins is given,
and the roles of metalloporphyrin it-cation radicals in electron transport is evaluated.
EUGENE C. JOHNSON, TONY NIEM et DAVID DOLPHIN. Can. J. Chern. 56. 1381 (1978).
L'electrolyse a potentiel contra Ie de la me.l'O-tetraphenylporphyrine de Ni(II) (Ni(lI)TPP)
conduit, a temperature ambiante, au radical IT cation metalloporphyrine correspondant [Ni(Il)TPP)+·. Lorsque I'on congele une solution du radical IT cation it 77 K, un transfert interne
d'electron se produit pour conduire au [Ni(lll)TPP] +. On discute des diverses routes de transport d'electron dans des proteines contenant de l'heme et on cvalue Ie role des radicaux it cation
metalloporphyrine dans Ie transport des electrons.
[Traduit par Ie journal]
Introduction
Redox reactions play critical roles in many
biochemical transformations (I), and amongst the
coenzymes and cofactors involved the iron porphyrins (hemes) play many varied and crucial functions.
Hemes not only transport and store oxygen via
hemoglobin and myoglobin, but in the form of
cytochromes P-450 activate oxygen which, as monooxygenases bring about numerous biochemical oxidations (2). Other heme enzymes such as ca talase and
the peroxidases act as two electron reductants of
hydrogen peroxide to hydroxide, and are concomitantly oxidized by two electrons to the so-called
primary complexes of the enzymes. In the case of
catalase the primary complex then acts as a two
electron oxidant of hydrogen peroxide to oxygen,
and is reduced to the resting ferrihemoprotein.
Another group of heme-containing proteins, the cytochromes, function between Fe(II) ~ Fe(III) couples,
and by linking centres of different redox potcntials
couple the free energy change of the electron transport to covalent bond formation (e.g. oxidative
phosphorylation (3)).
An insight into the modes of electron transport of
metalloporphyrins was gained when it was shown
that the initial, photochemically promoted, electron
transport in photosynthesis involves the loss of an
electron from one of the filled n:-molecular orbitals
of chlorophyll a to generate a chlorophyll (or bacteriochlorophyll) n:-cation radical (4-6). Stable TC-cation
radicals of simpler metalloporphyrins had been re'Work carried out by T.N. as a postdoctoral fellow in (he
Chemistry department, Harvard University.
2Author to whom correspondence should be addressed.
ported earlier (7), and shortly after their occurrence
in photosynthetic systems was elucidated it was
found that similar l't-cation radicals were the catalytically active forms of both the catalases and peroxidases (8, 9), Thus when either catalase or peroxidase
is oxidized by hydrogen peroxide one electron is
removed from the ferric iron and one from the
TC-cloud to give, in the primary complexes, an Fe(IY)
TC-cation radical electronic configuration, However,
the ground states of these primary complexes differ
in that in onc the remaining unpaired electron is in
an a 2u , and in the other in an a lll orbital. The corresponding 2 A 2u and 2 Al u ground states, as a result of
their different electronic ground states, exhibit different optical and enzymic properties (8).
A change in the ground state from 2 Al u to 2 A 2u
can bc demonstrated with the n:-cation radical of
cobaltic octacthylporphyrin (7). Thus in the presence
of bromide ion the cobalt coordinates two ligands to
give [Co(III)OEP]2+' 2Br- which exhibits a 2 A lu
ground state. When the bromide ions are removed
and replaced by perchlorate the ground state changes
to that of 2 A 2u' [t is apparent that small changes in
the environment about the metal (and at the porphyrin periphery) can bring about a change in the ground
state, and this is consistent with calculations which
suggest that the two ground states differ in energy by
about 1500-2000 cm - 1 or 4.3-5.7 kcal mol- 1 (10).
We have been explicit in describing the electronic
configuration of an oxidized metalloporphyrin and
one might ask if it is possible to describe these configurations as resulting entirely from a metal or a
ligand oxidation. Thus a cobaltic n:-cation radical
which has two electrons less than the Co(I1) species
1382
CAN
J. CIIEM. VOL. 56. 1978
could formally be described as both a derivative of
cobalt(IV) or a n-dication of cobalt(Il). One might
be inclined to conclude that" each of these descriptions are extreme resonance forms of the same
molecule and that the molecule should be described
as some contribution of all of them. This is not,
however, th::: case since the epr spectrum of [Co(lll)OEP]2+' shows that the unpaired electron spends
less than 5% of its time on the metal (9). Since the
unpaired electron is in a ligand based orbital it can
be described as a n-cation radical of cobaJt(III).
Similarly the oxidation of ferric porphyrins removes
electrons from the metal, not the ligand, since the
optical spectra (which are dominated by rr-rr* transitions of the ligand) remain essentially unchanged
upon oxidation whereas a dramatic change in the
optical spectra would be observed if the ligand were
oxidized (11).
The ground state electronic contiguration of a
metalloporphyrin can, from optical and epr measurements, be accurately determined; and the el ectronic
configuration adopted will depend upon a variety of
factors including the metal, the porphyrin and its
peripheral substituents, and the axial ligands. Even
in the 'simplest' metalloporphyrins small changes
will influence the ground state. Thus the electrochemical one-electron oxidation of ruthenium(II)
complexes of meso-tetraphenylporphyrin (TPP) gave
a n-cation of [Ru(II)TPP](CO)(py) (viz. [Ru(II)TPP] + . (CO)(py)) but caused oxidation of the metal
with [Ru(II)TPP](pY)2 to give [Ru(III)TPP] +(PY)2
(12). Hence the su bstitution of the weakly backbonding ligand pyridine for the more strongly backbonding
CO destabilized Ru(IJ) vs. Ru(III) and changed the
electronic configuration of the one-electron oxidized
species.
Even though the electronic configuration of a
metalloporphyrin can now be readily determined,
the mechanism, or route, of electron transfer by
which such systems undergo redox chemistry is still
a subject of much speCUlation with respect to even
simple metalloporphyrins, which may be used as
models of heme proteins; while the mechanisms by
which heme proteins themselves transport electrons
still remain, in general, largely unexplained (1).
As with other coordination complexes metalloporphyrins have been found to effect electron transfer
via two well-characterized mechanisms; inner sphere
(13), where electron transfer occurs through a bridging species between oxidant and reductant, and outer
sphere, where a bridging species is absent and electron transfer must occur through the extended
rr-systems of the oxidant and reductant (14). While
inner sphere mechanisms may operate with some
model meta\loporphyrins it is apparent, on spatial
considerations alone, that for most heme proteins in
general an inner sphere mechanism cannot obtain.
Wolberg and Manassen (15) have reported that the
electrochemical oxidation ofNi(II)TPP in benzonitrile
initially gives a Ni(III)TPP+ species which decayed to
a Ni(II)TPP+' species. The presence of the two complexes was suggested by epr. In contrast we have
recently reported that a similar oxidation in CH 2 Ci?
gives the [Ni(II)TPP]+ . species, which is in thermo~
dynamic equilibrium with a [Ni(III)TPP]+ species at
77 K, as shown by epr and electronic absorption
spectra (16). We describe here a detailed .study of
these systems and suggest evidence of possible modes
for outer sphere mechanisms of electron transfer in
metalloenzymes.
Experimental
Materials
Dichloromethane (Fischer, reagent) was ref1uxed over
calcium hydride and distilled and storcd over 4 A molecular
sieves. Tetra-n-butylammonium hexaAuorophosphate (TBAH)
was prepared by treating an acetone solution of tetra-n-butylammonium iodide and ammonium hexafluorophosphate with
water. The precipitated product was twice recrystallized from
ethanol-water and dried in vacuum for 10 h at 100'C. Tetra-Ilbutylammonium perchlorate (TBAP) (Eastman) was twice recrystallized from ethanol-water and dried under vacuum for
10 h at 70°C. Tetraphenylporphyrinato nickel(II) was prepared
from chlorin-free TPP since Ni(II)TPP is sparingly soluble
and difficult to separate from chlorin impurity. 61Ni(II)TPP
was prepared similarly except that 6l Ni (997,; enriched,
Oakridge, TN) was initially dissolved in concentrated Hel,
evaporated to dryness, and the resultant 6lNiCl 2 used in place
of Ni(OAc)2.
M easlIrements
Ultraviolet-visible absorption spectra were recorded on a
Cary 17 recording spectrophotometer. Room temperature
spectra were recoded in CH 2 C1 2 using 1.0 cm cells. Spectra at
77 K were me<lsured using a variation of the opal glass method
(17). A Pyrex cell was made by flattening a tube (0.5 cm OD)
until the path length was ~0.1 mm. A quantity of a methylene
dichloride solution was transferred to the cell by syringe and
the sample frozen, evacuated, and sealed. After warming the
cell to room temperature a strip of thin Teflon tape was
stretched over the flat face and the cell was then placed in a
windowed quartz Dewar filled with liquid nitrogen and placed
in the sample beam of the spectrometer. A piece of opal glass
was placed in the reference beam and optical spectra were then
recorded as usual. The epr measurements, at both room
temperature and 77 K, were made on a Varian E-3 EPR
spectrometer in degassed methylene dichloride solutions in
evacuated quartz epr tubes (4 mm 00).
Electrochemical measurements were made at a Pt electrode
in CHzCl z using 0.1 lv[ TBAH or TBAP as supporting electrolyte. A Ag/AgCI couple was used as reference. Standard 3
electrode operational amplifier circuitry was utilized as described previously (6). Due to the sparing solubility of Ni(II)TPP, special care was taken to ensure that no solid material
was present during electrochemical or epr measurements.
Solutions were either filtered prior to use or, for coulometric
measurements, accurately weighted samples of ~ 3 mg were
stirred for 30 min in 30 ml of degassed CH zCI 2 prior to use to
1383
JOHNSON ET AL.
ensure complete solution. Such solutions were then used
without further treatment.
Results
solutions of the second electrolysis product gave, by
coulometry, n = 0.92 ± 0.05, but the appearance of
additional cycles indicated decomposition.
Spectra
Electrochemistry
Figure 2 follows the changes in the absorption
The shape of the cyclic voltammograms varied
significantly according to the supporting electrolyte spectra of Ni(II)TPP during electrolysis at 1.24 V to
used. Figure I shows cyclic voltammograms of give the green one-electron oxidation product. The
Ni(II)TPP in CH 2 Cl 2 using TBAH and TBAP. In the appearance of isosbestic points in the spectra indica tes
presence of TBAP the cyclic voltammogram, as pre- a clean conversion to the oxidized form. A compariviously reported (16), is consistent with two overlap- son of the optical spectrum of the green oxidation
ping electrochemically reversible couples centered at product to that of the well characterized IT-cation
1.17 and 1.29 V, while the data for the TBAH system radical of Mg(II)OEP (Fig. 3) shows that the product
display two well-defined electrochemically reversible is the 1t-cation radical [Ni(lI)TPPt· and that further
couples centred at 1.07 and 1.39 V. Exhaustive elec- electrolysis generates the corresponding rr-dication
trolysis at 1.24 V in either system produced a green [Ni(II)TPP]2+ (Fig. 3) which has an optical spectrum
solution which gave a cyclic voltammogram identical similar to other metalloporphyrin rr-dications. This
to the starting material. Controlled potential coul- dication is a powerful electrophile reacting with a
ometry (at 1.24 V) gave n-values of 1.01 ± 0.05, variety of nucleophiles such as methanol, or water,
consistent with a one-electron oxidation of the to give isoporphyrins (18).
Figure 4 shows the optical spectrum of [Ni(II)Ni(II)TPP. The green solution was stable for several
days when protected from the light and the atmo- TPP] + ·PF 6 - at room temperature and after freezing
sphere, and upon electrolysis at 0.80 V the starting to 77 K. The dramatic spectral changes are accommaterial was quantitatively regenerated (n = I), such panied by a change from a green solution to an
that the cycle was completely reversible. Further electrolysis of the green oxidation product at potentials
anodic to the second couple gave a brown solution
which decomposed over a period of a few hours.
15
Immediate cathodic electrolysis of freshly prepared
'"
Q
~
'"
a
'"
f'2?5.1SO
(I
~
20
10
. FIG. 2. Optical absorption spectra of Ni(II)TPP (A), during
Its electrochemical oxidation, in CH,CJ 2 , to the x-cation
radical [Ni(II)TPP] + . (E).
b
J20
10
A
Q
~
'"\
05
1.4 1.2 1.0
FIG.
Volts vs Ag/AgCI
1. Cyclic voltammogram of Ni(IJ)TPP in methylene
dichloride using a Ag/AgCI couple as reference; (a) tetra-nbutylammonium perchlorate and (b) tetra-n-butylammonium
hexafluorophosphate as supporting electrolytes at 0.1 M.
\
\
j~
FIG. 3. Optical absorption spectra, in CH 2 Cl 2 , of the
x-cation radical [Ni(II)TPP] +. (A), the rc-dication [Ni(II)TPP]H (E). C is the optical spectrum during oxidation of
A to B showing isosbestic points. D is the rc-cation radical
[Mg(II)OEP] +.
1384
CAN. 1. CHEM. VOL. 56. 1978
no color change is observed. Similarly, these same
solutions show no change in their epr spectra when
cooled to 77 K, and only the isotropic signal at g =
2.0041 is present.
FIG. 4. Optical absorption spectra, in CH 2 CI 2 , of [Ni(II)TPP] +. PF6 - at room temperature (A), the same sample at 77
K showing spectrum of [Ni(III)TPPJ+PF6 - (B), and Ni(II)TPP at 77 K (e).
orange-red frozen solid which has a strong band at
526 nm. The spectra of both Ni(II)TPP and [Ni(II)TPPt· when measured in CH 2 Cl 2 in the presence
of both TBAH and TBAP at 77 K show a band
centered at 654 nm (in the absence of TBAP or
TBAH Ni(II)TPP does not exhibit this absorption).
In addition Ni(II)TPP+' at room temperature exhibits a broad absorption at 668 which moves to 656
at 77 K.
The optical spectra of methylene dichloride solutions of the n-cation radical [Ni(II)TPP] +. [PF 6 or
C10 4
showed no major changes upon cooling to
77 K when small amounts of water were present, nor
were any changes observed with the bromide salt
[Ni(II)TPPt· Br-.
r
Electron Paramagnetic Resonance
The epr of the one-electron oxidation product were
recorded at room temperature and at 77 K. At room
temperature the green solution in the presence of
PF 6 -, C10 4 -, or Br- exhibited a single signal, g =
2.0041 with a peak-to-peak separation of 47.2 G,
indicative of a n-cation radical. At 77 K the orangered solid showed signals at gJ. = 2.286 and gil =
2.086, consistent with a low spin d 7 (Ni(III)) electronic configuration and in good agreement with
spectra reported by Busch and co-workers (19) and
by Wolberg and Manassen (15) for Ni(III) complexes. The epr signal of the Ni(III) species slowly
decayed with time, being replaced after ~ 24 h by a
weak isotropic signal which also decayed with time.
We have already noted above that upon cooling
solutions of the PF 6 - or C10 4 - salts, in the presence
of small amounts of water, or the pure bromide salt
Discussion
From the physical studies described above we can
conclude that when Ni(II)TPP is electrolyzed in
methylene dichloride (with either TBAH, or TBAP
as supporting electrolyte) at 1.24 V it undergoes a
clean, reversible, one-electron oxidation to give the
corresponding n-cation radical [Ni(II)TPPt·. At
room temperature this species shows all of the properties of other similar, and well characterized, metalloporphyrin n-cation radicals with a 2 A2l1 ground
state (5). The stability and ease of preparation of
this n-cation radical is indicated by the clean electrochemistry and solution stability over long time
periods. In addition this n-cation radical behaves
like other metalloporphyrin n-cation radicals in that
it can quantitatively be reduced back to Ni(II)TPP,
and can be made to undergo a further one-electron
oxidation to the n-dication [Ni(II)TPP]2+.
In all of these respects [Ni(II)TPPt· is no different from other porphyrin n-cation radicals, which
have been reported on in great detail, except for its
quite exceptional thermal behavior. All of the above
data are consistent with an internal electron transfer
occurring as the temperature is lowered, with the hole
jumping from the periphery to the metal. These redox
reactions can be summarized as follows:
-e
-e
Ni(II)TPP ~ [Ni(II)TPP] +. ~[Ni([I)TPPF +
+e+e-
HLl
[Ni(III)TPP] +
The strongest electronic transitions in metalloporphyrins (and their oxidized and reduced derivatives)
are n -> n* in nature with molecular extinction coefficients offrom 10000 to 1 000000 (20, 21). In such
systems the weak d-d transitions of the metal are
hidden. Except when charge transfer between metal
and ligand occurs, the oxidation state of a specific
metal makes only minor perturbation on the optical
transitions of metalloporphyrins. The spectra of
Ni(II)TPP and that of the orange frozen samples of
the n-cation radical [Ni(II)TPPt· are quite similar
(Fig. 4) which show that all of the bonding 1tmolecular orbitals are filled. Since the n-cation radical has lost an electron from its highest filled bonding
n-MO, then upon cooling an electron must move
from the metal to fill the n-system and generate
Ni(III). The formation of a nickel(III) complex at low
temperatures is confirmed by epr spectroscopy which
shows two signals having g values comparable to
1385
JOHNSON ET AL.
those of previously reported Ni(III) complexes (I 7,
22). When the same series of epr experiments were
carried out using 61Ni (J = 3/2) no change in the epr
spectra of either the cation radical or the Ni(lII)
species was observed. However, 61 Ni has a small
magnetic moment ( - 0.75) resulting in a small hyperfine interaction which is lost under the relatively
broad signal.
W olberg and Manassen (15) have reported that
the initial one-electron oxidation product of Ni(II)TPP in benzonitrile/TBAP is the metal oxidized
[Ni(III)TPPt, and that upon standing at room temperature solutions of [Ni(III)TPPt began to show a
weak epr signal which was assigned to [Ni(II)TPP] +'.
The epr signal which they assigned to [Ni(III)TPPt
was only observed at 77 K, and it is possible that
these authors were observing a similar phenomenon to
that reported here. However, the cyclic voltammogram reported for Ni(II)TPP in benzonitrilejTBAP
and that reported here using methylene dichloride/
TBAP exhibit features which are atypical of most
metalloporphyrins (22-25). In the former case the
cyclic voltammogram showed two overlapping successive one-electron oxidations which were separated
by not more than 100 mY. In the latter case we can
resolve the two peaks at 1.17 and 1.29 V which differ
by 120 mY. In the numerous cases where it has been
shown that the two oxidations occur at the porphyrin
periphery (to give rr-cation and rr-dication) the difference between the two potentials is 290 ± 50 m V (23),
and deviations from this value suggest that the metal,
rather than the ring, is the redox reactive centre.
When Ni(II)TPP is oxidized in CH 2 CI 2 /TBAH the
two oxidation waves are separated by 320 mV consistant with two ring oxidations, suggesting that the
oxidations described above, which were carried out
in the presence of perchlorate, involve an initial oxidation of the metal. In benzonitrile the (Ni(I1I)TPP]+
is sufficiently stable to be observed at room temperature before it converts over to the [Ni(II)TPP]+'
whereas we find that at the end of a bulk electrolysis
in methylene dichloride any [Ni(III)TPP]+ ,if formed,
has already converted to (Ni(II)TPP] +'.
Similar internal electron transfers have been reported previously (19,26), and Barefield and Mocella
(27) showed that a nickel Curtis-type macrocycle
exhibited a similar temperature-dependent internal
electron transfer, i.e. 1 ¢ 2. Although the total charge
on these complexes is 3 + instead of the I + for the
systems reported here, the same general physical
behavior was noted. Barefield attributed this phenomenon to the phase change in going from solution
to the solid state, and suggested that axial ligation
might play an important role in the process. A
similar situation obtains here, for when a frozen
sample is in thermal equilibrium with its solution the
solid is orange while the solution is green, and only
when the green solution freezes does it change color.
It is not yet clear what happens, as the phase changes,
to bring about the internal electron transfer and
stabilize the hole on the metal rather than the periphery. One might anticipate that the higher oxidation
state of the metal would be stabilized by the axial
coordination of a donor ligand. The situation is not
as clear cut as this, however, since in the presence Qf
bromide or water only the n-cation radical is observed even at 77 K. Moreover, the fact that we have
found no other prophyrin rr-cation which exhibits
this internal electron transfer (including the n-cation
radical of Ni(II) octaethylporphyrin) suggests that
the phenomenon must depend upon a number of
closely balanced factors such that small perturbations
can stabilize one or another of the two species undergoing the internal electron transfer. Nonetheless, the
degree of axial ligation does playa role here since in
addition to the intense band at 525 nm the nickel(IIl)
complex shows a broad weak absorption around 650
nm. While the origin of this charge transfer band is
unclear, a similar band also occurs in the Ni(lI) complex, but only in the presence of ligands which would
axially coordinate, and the appearance of a similar
band in the spectrum of the Ni(III) com plex suggests
that at 77 Keven PF 6 - and CI0 4 - are coordinating
to the metal.
Other metaIIoporphyrins show similar internal
electron transfers. Ferguson et al. (28) while studying
octaethylporphyrinato Pb(II) (Pb(II)OEP) found two
reversible oxidations on the cyclic voltammetric time
scale (s) which they attributed to successive, and
reversible, one-electron oxidations of the ring. The
resulting rr-dication decays, in rigorously purified solvents, with a half-life of ~ 5 min to a Pb(IV) com plex
(6), i.e.
-e
-c
Pb(II)OEP~ [Pb(II)OEP] + • .;=..-'[Pb(II)OEP]2+
t
[Pb(IV)OEP]2+
Having shown that a facile and reversible internal
electron transfer can occur between a metal and the
periphery of a coordinating porphyrin one can ask
could such an internal transfer play any role in biochemical systems? Even though many cytochromes
function via an Fe(II)-Fe(III) couple, and while it is
1386
CAN. J. CHEM. VOL. 56, 197X
of cytochrome c to form a channel from the surface
of the protein to the heme. Similar remote pathways
with an electron tunneling through several Angstroms
Reference of a hydrophobic protein channel have been proReagent
posed for some non-heme iron proteins, the ferre1,3 X 1011
29
enQ
doxins (46). Thorough articulations of these argu8
Fe(dipy),(CN), +
1. 9 X 10
35
ments can be found elsewhere (47), but it should be
8,0 X 10 6
35
Fe(CN)6 3 37
6,25 X 10 6
Azurin
noted such a pathway does not appear to be opera32
3,8 X 104
Ru(NH 3)62+
tive in cytochrome c.
2,6 X 10451
Fe(CN)6 4 (ii) A direct electron transfer through space-In
2
X 10452
Fe(II) cyt c
addition to the classical inner- and outer-sphere
1.2 X 10433
Cr(Il)
32
Co(phenh'+
1. 5 x 10 3
modes of electron transfer the possibilities that direct
electron transfer via quantum mechanical electron
tunneling, or transfer to and transport through a solabundantly clear that the final oxidation states are vent (as a hydrated electron (e aq - ) in biochemical
indeed ferric and ferrous, how is ferrous cytochrome systems) must be considered. Even in redox reactions
with simple inorganic ions the intermediacy, or lack
oxidized to its ferric state?
Much kinetic data have been gathered on the redox thereof, of eaq - has yet to be satisfactorily demonreactions of cytochromes, particularly the cyto- strated (48). The same situation applies to the more
chromes c, using a variety of redox reagents ranging complex biochemical systems even though the reducfrom hydrated electrons (e aq - ) (29-31) through si m- tion of ferricytochrome c by externally generated
pie inner- and outer-sphere inorganic reagents (32- eaq - has been reported (49).
36) to those macromolecules which transfer electrons
Quantum mechanical electron tunneling is, howto and from cytochrome c in nature (37-41), Table I ever, well documented, and tunneling through dislists some of these redox reagents and observed rates tances of tens of Angstroms of inert solvents has
of electron transfer, and while the rates vary they are been observed with non-biochemical systems (49).
generally in the range of 10 3 -10 6 M - 1 S - 1. Cyto- While application of the simple one-dimensional
chrome c contains an axially coordinated methionine theory (49) to complex macromolecules is unwar(42-43). Sutin has measured the rate of breaking the
ranted, it does suggest that such mechanisms are
Fe-S bond, and of opening a heme crevice, which
possible in nature even though none has yet been
should be rate limiting for an inner sphere pathway documented.
(44). The measured value of 60 s -1 would appear to
(iii) Oxidation at the porphyrin periphery-A numrule out an inner sphere process as a major route of ber of both kinetic and product ISO lation studies sugelectron transfer even with small inorganic redox
gest that the oxidation of a coordinated metal can
reagents.
proceed through an outer-sphere mechanism, and
X-ray crystallography has shown that both the studies with ferrocytochrome c and complexes of
reduced and oxidized forms of cytochrome c have tris(i,lO-phenantholine)cobalt(IIl) ions show that an
essentially the same structure (43). One striking feaouter sphere mechanism is operative and these studies
ture of the crystal structure is the partially exposed
have led Gray and co-workers (14) to suggest that
edge of the heme porphyrin ring (a similar structural
the point of electron transfer is the partially exposed
feature is observed with cytochrome b s (45), and we
heme edge. But having concluded that electron transanticipate that other cytochromes in the respiratory fer occurs via the porphyrin TC-cloud how, in fact, is
chain will show similar exposed hemes). The tertiary the ferrocytochrome c oxidized to the ferric state? We
structure of the protein will, of course, impose severe suggest that a process analogous to the oxidation of
steric restraints upon the accessibility of the heme the Ni(II)TPP to the Ni(IIl)TPP+ might be operacoordinated iron to even the smallest redox reagent, tive, that is via the intermediacy of a porphyrin
such that it is inconceivable that the iron atoms of TC-cation radical, such that the oxidation of ferro- to
two adjacent cytochromes could come close enough ferricytochrome c could be descri bed as:
to transfer an electron via an inner sphere mechanism.
-eHow then do cytochromes transfer electrons amongst
Fe(II) cyt c ~[Fe(II) cyt c]+';;=:=::':[Fe(lII) cyt c]+
themselves? Kinetic studies have suggested three
+e
main routes.
III
II
(i) A remote transfer through aromatic residues in
Since ferric and ferrous iron have different covalent
the protein-which can be seen in the crystal structure
TABLE
I, Electron transfer rates for oxidation/reduction of
cytochrome c
JOHNSON ET AL.
radii there must be conformational changes around
the iron and its ligands between ferro- and ferricytochrome c (I and III) and between the postulated
ferrocytochrome en-cation radical (II) and ferricytochrome c (III). Thus II and III would not be extreme
resonance forms of the same species and an activation energy will be required to convert II to III. It has
been suggested that the slow interconversion of
[Pb(II)OEP]2+ to [Pb(IV)OEPf+, discussed above,
may be due to a Frank-Condon forbidden transition
since the Pb(II) is out of plane and the Pb(IV) in
plane (50); and that since there is no overlap between
the empty a 1u porphyrin orbital and the a2u(6p.) lead
orbital the probability for charge transfer is small.
In the case of the nickel complexes described here the
system is more finely tuned and the electronic configurations are adjusted to a thermally controlled
level. A similar situation might be anticipated in the
cytochromes, and the rapid rate of electron transfer
for self exchange in cytochromes c suggest that an
intermediate such as II will be short lived « 10- 5
s), and as yet none have been detected.
Acknowledgements
This work is a contribution from the Bioinorganic
Chemistry Group and was supported by operating
and negotiated development grants from the National
Research Council of Canada and the United States
National Institutes of Health (AM 17989).
I. G. R. MOORE and R. 1. P. WILLIAMS. Coord. Chern. Rev.
18. 125 (1976).
2. R. LEMBERG and 1. BARRETT. Cytochromes. Academic
Press, New York, NY. 1973.
3. 1. H. WANG. On the coupl ing of oxidation to phosphorylation in biological systems. III Biological aspects ofinorganic
chemistry. Edited by A. W. Addison, W. R. Cullen, D.
4.
5.
6.
7.
8.
Dolphin, and B. R. lames. Wiley Interscience, New York,
NY. 1977. p. l.
D. H. KOHL. In Biological applications of electron spin
resonance. Edited by H. M. Swartz, 1. R. Bolton, and D. C.
Borg. Wiley Interscience, New York, NY. 1972. p. 213.
1. D. McELROY, G. FEKER, and D. MAUZERALL. Biochem.
Biophys. Acta, 267, 363 (1972).
D. C. BORG, 1. FAJER, R. H. FELTON, and D. DOLPHIN.
Proc. Natl. Acad. Sci. U.S.A. 67,813 (1970).
1. FAJER, D. C. BORG, A. FORMAN, D. DOLPHIN, and R. H.
FELTON. J. Am. Chern. Soc. 92, 3451 (1970).
D. DOLPHIN and R. H. FELTON. Acc. Chern. Res. 7, 26
(1974).
9. D. DOLPHIN, A. FORMAN, D. C. BORG, 1. FAJER, and R. H.
FELTON. Proc. Natl. Acad. Sci. U.S.A. 68, 614(1971).
10. 1. FAJER, D. C. BORG, A. FORMAN, R. H. FELTON, L.
VEGH, and D. DOLPHIN. Ann. N.Y. Acad. Sci. 206, 349
(1973).
II. R. H. FELTON, G. S. OWEN, D. DOLPHIN, and 1. FAJER. J.
Am. Chern. Soc. 93, 6332 (1971).
12. G. M. BROWN, F. R. HOPF, 1. A. FERGUSON, T. 1. MEYER,
and D. G. WHITTEN. 1. Am. Chern. Soc. 95, 5939 (1973).
1387
13. E. B. FLEISCHER and S. K. CHEUNG. 1. Am. Chern. Soc. 98,
8381 (1976).
14. 1. V. McARDLE, K. YOCOM, and H. B. GRAY. J. Am.
Chern. Soc. 99, 4141 (1977).
15. A. WOLBERG and 1. MANASSEN. Inorg. Chern. 9, 2365
(1970).
16. D. DOLPHIN, T. NIEM, R. H. FELTON, and S. FUJITA. 1.
Am. Chern. Soc. 97, 5288 (1975).
17. K. SHIBATA. Methods Biochem. Anal. 7,7 (1959).
18. D. DOLPHIN, R. H. FELTON, D. BORG, and J. FAJER. 1. Am.
Chern. Soc. 92, 743 (1970).
19. N. E. TOKEL, K. FARMERY, L. ANDERSON, F. V. LOVECCHIO, E. S. GORE, and D. H. BUSCH. J. Am. Chern. Soc. 96,
731 (1974).
20. M. GOUTERMAN. Optical spectra and electron structure of
porphyrins. 111 The porphyrins. Vol. III. Edited by D. Dolphin. Academic Press, New York. NY. In press. Chap!. I.
21. M. P. GOUTERMAN. J. Mol. Spectrosc. 6, 138 (1961); C.
WEISS, H. KOBAYASHI, and M. P. GOUTERMAN. J. Mol.
Spectrosc. 16,415 (1965).
22. 1 .-H. FUHRHOP and D. MAUZERALL. J. Am. Chern. Soc. 91,
4174 (1969).
.
23. 1.-H. FUHRHOP, K. M. KADISH, and D. G. DAVIS. J. Am.
Chern. Soc. 95, 5140 (1973).
24. A. WOLBERG and 1. MANASSEN. J. Am. Chern. Soc. 92,
2982 (1970).
25. D. G. DAVIS. Electrochemistry of porphyrins. III The porphyrins. Vol. V. Edited by D. Dolphin. Academic Press,
New York, NY. In press. Chap!. 4.
26. N. S. HUCK and l. S. WOOLSEY. 1. Am. Chern. Soc. 94,4107
(1972).
27. E. K. BAREFIELD and M. T. MOCELLA. 1. Am. Chern. Soc.
94,4107 (1972).
27. E. K. BAREFIELD ,md M. T. MOCELLA. 1. Am. Chern. Soc.
97,4238 (1975).
28. 1. A. FERGUSON, T. 1. MEYER, and D. G. WHrITEN. lnorg.
Chern. 11,2767 (1972).
29. 1. PECHT and M. FARAGGI. Proc. Natl. Acad. Sci. U.S.A.
69,902 (1972).
30. A. SHAFFERMAN and G. STEIN. Biochim. Biophys. Acta,
416,287 (1975).
31. N. N. LICHTlN, A. SHAFFERMAN, and G. STEIN. Biochim.
Biophys. Acta, 314, 117 (1973).
32. S. WHERLAND and H. B. GRAY. Proc. Natl. Acad. Sci.
U.S.A. 73, 2950(1976).
33. J. W. DAWSON, H. B. GRAY, R. A. HOLWERDA, and E. W.
WESTHEAD. Proc. Natl. Acad. Sci. U.S.A. 69,30 (1972).
34. 1. K. YANDELL, D. P. FAY, and N. SUTIN. J. Am. Chern.
Soc. 95, 1131 (1973).
35. 1. C. CASSATT and C. P. MARINI. Biochemistry, 13,5323
(1974).
36. C. CREUTZ and N. SUTIN. Proc. Natl. AC'ld. Sci. U.S.A.
70,1701 (1973).
37. E. ANTONINI, A. FINAZZ1-AGRO, L. AVIGLlANO, P. GUERRIERI, G. ROTlLlO, and B. MONDOVI. 1. BioI. Chern. 245,
4847 (1970).
38. S. PECHT and P. ROSEN. Biochem. Biophys. Res. Cornmun.
50,853 (1973).
39. M. BRUNORI, C. GREENWOOD, and M. T. WILSON.
Biochem. 1.137, 113(1974).
40. M. T. WILSON, C. GREENWOOD, M. BRUNORI, and E.
ANTONINI. Biochem. 1.145,449 (1975).
41. P. ROSEN and S. PECHT. Biochemistry, 15, 775 (1976).
42. R. E. DICKERSON, T. TAKANO, D. EISENBERG, O. B. KALLAl, L. SAMSON, A. COOPER, and E. MARGOLIASH. J. BioI.
Chern. 246, 1511 (1971).
1388
CAN. J. CHEM. VOL. 56. 1978
43. R. E. DICKERSON and R. TIMKOVICH. The enzymes. Vol.
XI. Edited by P. D. Boyer. Academic Press, New York,
NY. 1975. Chapt. 7.
44. N. SUTIN and 1. K. YANDELL. 1. BioI. Chem. 247, 6932
(1972).
45. F. S. MATHEWS. M. LEVINS. and P. ARGOS. 1. Mol. BioI.
64,449 (1972).
46. R. H. HOLM. Identification of active sites in iron-sulfur
proteins. 11/ Biological aspects of inorganic chemistry.
Edited by A. W. Addison, W. R. Cullen, D. Dolphin, and B.
R. James. Wiley-lnterscience, New York, NY. 1977. p. 71.
47. S. FERGUSON-MILLER, D. L. BRAUTIGAN, and E. MARGOLIASH. The electron transfer function of cytochrome c.
48.
49.
50.
51.
52.
11/ The porphyrins. Vol. VII. Edited by D. Dolphin.
Academic Press, New York, NY. In press. Chapt. 4.
G. A. KENNEY and D. C. WALKER. In Electroanalytical
chemistry. Vol. 5. Edited by A. 1. Bard. Marcel Dekker,
Inc" New York. NY. 1971. pp. 1-66.
J. R. MILLER. 1. Chem. Phys. 56, 5173 (1972).
R. H. FELTON. Primary redox reactions of metalloporphyrins. 11/ The porphyrins. Vol. V. Edited by D. Dolphin.
Academic Press, New York, NY. In press. Chapt. 3.
K. G. BRANDT, P. C. PARKS, G. H. CZERLINSKI, and G. P.
HESS. 1. BioI. Chem. 241,4180 (1966).
R. K. GUPTA, S. H. KOENIG, and A. G. REDFIELD. J.
Magn. Reson. 7, 66 (1972).