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 .. 182 183 184 185 186 187 < 188 > 189 190 191 192 193 194 .. 240 >> Next

1. A large number of different functional groups able to interact with
guest molecules can be incorporated at the peripheral meso and pyrrolic
/^-positions.
2. The large macrocyclic aromatic structure of the porphyrin system
enables one to monitor small chemical changes due to intermolecular
interaction by use of molecular spectroscopic measurements such as
electronic absorption, NMR and circular dichroism.
3. Metalloporphyrins act as the catalytic reaction center and/or the
Lewis acid site to bind functional groups of guest molecules.
In this chapter, we will concentrate on the subjects of synthetic
strategies for: (1) porphyrins and the capability of the products for
molecular recognition of small organic substrates via intermolecular and
noncovalent interactions;
(2) effective electron-transfer reaction regulated by intermolecular
interaction2-5; and (3) self-assembled porphyrin to construct
supramolecular system for the guest.6 The structures of porphyrins and
their metalloporphyrin derivatives have attracted chemists due to their
suitability as a host framework for organic guest molecules.
B. CHEMICAL MODIFICATION OF NATURALLY OCCURRING PORPHYRINS
For monomeric host porphyrin framework, we may start from natural
porphyrins such as protoporphyrin IX and its derivatives. In earlier
times, the 3- and 8-vinyl and 13- and 17-propionates of protoporphyrin IX
were subjected to simple chemical modifications like reduction, addition
and condensation. The first chemical modification for synthesis of heme
protein models reported by Lautch et al. is condensation of an
oligopeptide with the two propionates.7-8 In order to mimic the proximity
of the heme, terminal His or Cys of oligopeptide linked to the 13- and
17-propionates of heme can coordinate to the heme iron as axial ligands.
Reconstitution of apoproteins or apoenzymes with various synthetic
hemes has been widely investigated in order to elucidate the effect of
structural factors of the heme on the protein functions. If protoheme
chemically modified at the 13- and 17-propionates forms a stable
reconstituted
46/Porphyrins and Metalloporphyrins as Receptor Models
281
heme with apoprotein, terminal functional groups are directed to interact
with guest molecules such as methyl-viologen and benzoquinone
derivatives.9 In fact, reconstitution of chemically modified porphyrins
linked with spacer groups having many carboxylate terminals affords a
stable protein. Assembly of carboxylate groups localized at the surface
of the protein changes the character of the protein surface and shows
strong affinity for positively charged electron acceptors such as methyl
viologen and cytochrome c. Efficient electron transfer from
photochemically excited porphyrin takes place in such a host-guest
complex. This suggests the importance of molecular recognition between a
reductase and oxidase couple. In place of site-directed mutagenesis of
heme enzymes, this kind of reconstitution using well-designed chemical
modification for the prosthetic group may provide an alternative approach
to understanding the biological function of molecular recognition (Figure
1, Protoheme). A myoglobin dimer was prepared by using a bis-porphyrin
bridged at the propionic acid groups (see compound l).i0
Phenylboronic acids were linked with each of the two propionates of
protoporphyrin IX through amide bonds as shown in Figure I, compound
2.11-1- Strong interaction of monosaccharide with a boronic-acicl-
appended porphyrin causes deaggregation of the sugar-porphyrin complex
from the aggregated form. This aggregation-deaggregation process brings
changes in color and fluorescence intensity that depend upon the extent
of association and structure of the sugars.11 Protein reconstitution with
a heine appended with
boronic acids produced a stable hemeprotein after addition of D-fructose
which acts as a specific cofactor to the boronic acid sites.
C. SYNTHETIC STRATEGY FOR MULTIFUNCTIONAL PORPHYRINS AS
BUILDING BLOCKS FOR RECEPTORS
Recent work with unnatural porphyrins has prompted the synthesis of these
multifunctional hosts which are more efficient than those derived from
naturally occurring porphyrins. Functional groups for interaction with
guest molecules can be fixed on the porphyrin framework in convergent
and/or divergent approaches (Figure 2). If convergent functional groups
are required to be fixed perpendicularly to the porphyrin plane,
synthesis of the inesv-inyl porphyrins similar to (he precursors of the
picket-fence-type of porphyrin is one of the most promising synthetic
pathways. The convergent functional groups could be fixed by condensation
of pyrrole with the arylaklehyde having functional groups at designated
places on aryl groups. The reaction product is usually obtained as a
mixture of four atropisomers that are usually interconvertible at ambient
Previous << 1 .. 182 183 184 185 186 187 < 188 > 189 190 191 192 193 194 .. 240 >> Next