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interactions should be the construction of a biomolecular assembly to
understand highly ordered biological systems. This section focuses on
artificially created ET models formed by molecular-recognition events on
protein surfaces. The topics of native protein-protein
pairing and the models by site-directed mutagenesis are not discussed
1. Cytochrome ñ Binding Porphyrins
The ET between cytochrome ñ and porphyrin is a suitable model for
elucidation of biological ET within the protein complex formed by
electrostatic self-association. Although there were several preliminary
works involving the use of protoporphyrin170 or
tetrakis(sulfonatophenyl)porphyrin,171 Rodgers, Zhou and coworkers first
reported the ET reaction within a stable 1: 1 complex between cytochrome
ñ and uroporphyrin 153.172-173 The electrostatic interaction was
monitored by UV-Vis studies, which led to the ionic strength-dependent
association constant; #, = 95-1.1 x 104M ~ 1 at 4-60 mM ionic strengths.
Intracomplex ET rate constants were determined by monitoring typical
transient absorbance changes of triplet states and the ET products as
shown in Table 10. The thermal recombination process is consistent with
predictions of semi-classical theory of ET reactions with reorganization
energy of A = 0.70eV, whereas the photoinduced forward ET exhibits no
inverted region in the range of 0.3-1.29eV driving force due to the
contribution of the coordinated solvent mode.
Groves and coworkers have recently reported a well-organized ET model
where electrons move from a membrane spanning the Mn(II) porphyrin 154 to
46 / Porphyrins and
Metalloporphyrins as Receptor Models
Table 10. Rate Constants for the Photoinduced Forward ET and Charge
Recombination Reactions in Uroporphyrin-Cytochrome ñ Complexes at 25 C"
Reaction Ä f(V)
^lorWcjrd ($ )
'Zn-cyt ñ h Fe(lll)-Up(CN)^ ---* Zn-cyt ñ + Fedl)-Up(CN); 0.30
9.] > 104
'Zn-cyt ñ + Mn(III)-Up ---" Zn-cyt r' + Mn(II)-Up 0.42
1.5 ê 104
'Zn-cyt c' -t Fe(III)-Up --->Zn-cyt r' -l-Fe(ll)-Up 0.45
5.5 x 10"
'Up !- Mn(lll)-cyt c --- Up" 1 + Mn(ll)-cyt c 0.86
9.1 - 10''
'Zn-tlp \ Mn( 111 )-cyt ñ --- Zn-Up' 4 Mn((l)-cyt ñ 0.99
8.2 ê 10''
'Up -1- Fe( (ll)-cyt ñ --- Up'1 + Fe(lll )-cyt c 1.06
1.7 x Ê)'1
'Zn-Up ! Fc'(l(()-cyt <¦ --- Zn-Up' -l- Fe( U)-cyt r 1.29
2.0 x ]()''
Reaction Ä E (V)
Zn-Up'' -l Fe(ll)-cyt < --- Zn-Up -I-Fe((ll)-cyt c 0.47
8.5 - Ê)"
Up' -f Fc(ll)-cyt ñ---Up + Fed 11)-cyt ã 0.60
3.7 õ Ê)1'
Zn-Up' H- Mndl)-cvt ñ --- Zn-Up i MndlU-cyt < 0.77
2.1 ¦ Ê)1,
Up' -h Miidl)-cyl ã --- Up-I Mn(lll)-cy! < 0.80
1.9 ó 10"
Zn-cyt <¦' J- Fe(U)-Up --- Zn-cyt ( + Fcdll)-Lip 1.23
1.4 > Ê)1
Zn-cyt ( ' + Mndl)-Up --- Zn-cyt c + Mn(lll)-Up 1.26
5.5 v 101
Zn-cyt c' I- Fe(ll)-Up(CN): ---* Zn-cyt ñ -i- Fedll )-Up(CN); 1.38
6.0 > Þ'
l'c\l r. cvtnchrnmc < , Ãð. uropnpiphv ïï.
chrome ñ via a trianionic zinc porphyrin 155 which is located on the
surface of the membrane.1 UA ^ Cytochrome r bound to (he organized lipid
membrane via electrostatic interactions with an association of 5 x 10*'
M_l. The thermal ET rale from Ihe Mn(ll) porphyrin in the vesicle to
cytochrome ñ was found to be first order and independent of the length of
the imidazole tail of the zinc porphyrin, with a rate constant of l(V s
1; the rate constants depended upon the lipid length. These results
supported the stable ternary complex formation, and ET occurred through
multiple pathways in this membrane ensemble's model of the respiratory
2. Interprotein ET Models Formed by Chemical Modification of Myoglobin
Myoglobin is one of the most popular oxygen-storage hemoproteins and is
well characterized in terms of both primary and tertiary structure,
thereby facilitating its use as a building block for model proteins.
Furthermore, the heme as a prosthetic group in myoglobin can replace the
native heme with an artificial metalloporphyrin. because the prosthetic
group is bound in the protein matrix via multiple noncovalent weak
interactions.176177 In 1995, Hayashi, Ogoshi and coworker prepared a
novel myoglobin rMb( 156)
di|u!niil<n ||)ho>|)h,ilul\ ê ii< il me (I'I'K ) :nul èèòòüèï
li'lmsph.ii ijvk hnlim. 11>MÃ< ) or JiI:iui\l|ilil"'!'li;i!uh'k'li('!iiu
i|)| 14 )
reconstituted with an artificial zinc porphyrin 156, as a specific
interface for cationic methyl viologen (MV), bearing an assembly of eight