Books
in black and white
Main menu
Share a book About us Home
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
Previous << 1 .. 217 218 219 220 221 222 < 223 > 224 225 226 227 228 229 .. 240 >> Next

values were comparable with the kinetic data in native cytochrome /-zinc
myoglobin complex reported by Hoffman and coworkers.m The present system
demonstrates the first example of a long-range ET reaction within the
noncovalently linked protein-protein complex via synthetic binding domain
bound to the prosthetic group.
myoglobin rMb(159)
L_Zn
-coo~
-coo
coo-
I. SURVEY OF ET MODEL SYSTEMS
The construction of noncovalently linked electron donor-acceptor pairs
has been shown to be an important and challenging task, since in
biological systems molecular recognition behavior and the subsequent
specific complex structure may control the overall ET reaction in
multiple ET pathways. In the last decade, methodologies for construc-
r(COOH)"
COOH),
(),,
rMb(157)
j-(COOH)4
rMb(158)
tion of donor-acceptor linkage via molecular recognition have been
clarified and a variety of simple ET models formed by the accumulation of
weak interactions have provided important insights on intracomplex or
interprotein ET processes. Future work in this field could involve the
construction of higher-order ET models which mimic the biological system
and elucidation of ET mechanisms through noncovalently linked systems.
IV. Self-Organized Porphyrin Systems
In porphyrin chemistry, the concept of molecular recognition has been
introduced only recently. In spite of this relatively short history, the
concept is now essential to the construction of biomimetic systems
containing porphyrins. As shown in the preceding section, various types
of porphyrins having so-called binding sites have been designed based on
the concept of molecular recognition using noncovalent interaction
between substrates and binding sites. These model systems have
demonstrated that molecular recognition indeed plays a definitive role in
the determination of selectivities and, sometimes, reactivities of these
synthetic porphyrins. The main function of molecular recognition in these
artificial systems is to lix an appropriate substrate near the porphyrin
active site to carry out further chemical processes such as redox
reactions. These processes are essentially dynamic, and usually are
involved as one of the kinetic steps. Nature, however, also utilizes the
functions of molecular recognition in more static situations to construct
bioactive molecules. In the simplest case, the porphyrin molecules in
enzymes are usually incorporated into the proper position in the proteins
by using noncovalent interactions. Such complementary interactions
between proteins and chromophores may become more important for the
construction of large scale bioactive devices containing multiporphyrin
assemblies as an energy and/or electron transmitter. A typical example of
such a complex multiporphyrin assembly is seen in the photosynthetic
system,IXS |86 which is one of the most important targets of biomimetic
chemistry. Although the total structure of this tremendous system is not
fully known, the expected
i
326
Ogoshi et al.
B850
Figure 16. Model of purple bacterial photosynthetic unit (Reprinted with
permission from Pullerits, .; Sundstrom, V. Acc. Chem. Res., 1996, 29,
381).
porphyrin network is impressive enough to show the importance of
multiporphyrin assemblies.187 In the photosynthetic pigment complexes
shown in Figure 16, we can see many types of porphyrin assemblies found
in nature, for example, the dimeric assembly for charge separation, the
chain type assembly for electron or energy transfer, and the large-scale
array type assembly for antenna function. In order to mimic each part of
these multiporphyrin assemblies, various types of artificial
multiporphyrin systems have been developed and investigated. In this
section, we will describe self-organized porphyrin assemblies and related
multiporphyrin systems.
A. COVALENTLY LINKED MULTIPORPHYRIN SYSTEMS
Before discussing self-assembling systems, it is necessary to roughly
survey some typical examples of covalently linked multiporphyrin systems
because they provide prototypes of artificial multiporphyrin systems and
some of them may be useful as components for further large-scale
assemblies.
Among the various types of covalently linked multiporphyrin system,
the face-to-face porphyrin dimer has been attracting intense attention,
because the dimer system had been predicted to play essential roles in
the initial charge-separation process of photosynthesis, and bacterial
photosynthesis,1 17 in particular. Several rigidly bridged porphyrin
dimers had been prepared in order to investigate characteristics of this
special configuration, though even the porphyrin dimer linked with a
flexible alkyl chain tends to prefer a face-to-face conformation due to
relatively strong stacking interaction between two porphyrin surfaces.46
There are two types of bridging methods available to enforce the face-to-
Previous << 1 .. 217 218 219 220 221 222 < 223 > 224 225 226 227 228 229 .. 240 >> Next