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temperature. Rotation around the C-C bond at the meso positions is
inhibited due to the steric constraint of peripheral substituents by the
/i-alkylation of pyrrole subunits or by introduction of bulky aryl
groups. Crowding of bulky substituents at the meso positions gives rise
to marked retardation of the rotation of large aromatic rings at
Chemically modified protoporphyrin IX
Ë // CHpCOO' CONH-CHCOO"
Figure 1. Chemical modification of protoporphyrin IX.
Ogoshi et al.
Convergent recognition groups
Figure 2. Convergent and divergent arrangements of recognition groups
fixed on the porphyrin framework.
the meso positions. For example, condensation of 2-hydroxy-naphthaldehyde
with pyrrole affords the suitable porphyrin host 5 which shows no
evidence of atropisomerization at elevated temperatures. Thus the product
porphyrin can tightly bind to benzoquinones through multihydrogen
The molecular recognition surface of the simple a,a,a,a-atropisomer
toward small organic substrates is estimated to be more than 80 A2, while
the recognition site of an antibody is estimated to be about 300 A2. Two
precursors such as the a,/?,a,/? and isomers have equivalent
interaction sites above and below the porphyrin plane. On the other hand,
both a,a,a,a and a,a,a,/? isomers constitute asymmetric interaction
spaces above and below plane. Strategies for synthesis of complementary
host porphyrins for a particular substrate depend on the number and
nature of interactions between the host-guest pair, and on the size and
shape of the guest molecule. If a metal ion is incorporated into the
porphyrin, one may expect that it will serve as the Lewis acid site and
reaction center for guest molecules having polar functional groups.
Free bases of the picket-fence type, a,a,a,a isomer, porphyrins have
at most four convergent interaction sites above the plane and the metal
ion may add a fifth effective interaction site for substrate. Metal
complexes of the a,a,a,a isomer allow the substrate to access the metal
through both functional group space and open space as shown in Figure 3.
In order to simplify the system, stepwise syntheses of multifunctional
porphyrins enable us to prepare porphyrins having more than three
functional groups and equivalent interaction sites above and below the
plane (see compound 7).15
A monomeric porphyrin host recognizes the structural features of a
guest molecule via several kinds of interactions. Reference porphyrin
host molecules are readily obtainable from similar synthetic routes.
Thermodynamic parameters of each interaction in the complex formed
between the complete host (H0) and guest (G0) can be estimated by
comparison with those of the reference host
a, u, (3,(3
Figure 3. Guest binding by various atropisomers of tetraaryl-porphyrins.
(Hi) and guest (Gj).16 For example, three interactions with amino acid
esters are estimated for the metal coordination and two kinds of hydrogen
bonding in the example shown in Figure 4.1718
Functional porphyrins are available for fixation of the large groups
possessing a binding site such as cyclodextrin, Kemp's acid, crown ether,
cryptand, steroid and calix[n]ar-ene (see Section II.J). The large cavity
of a cage group appended to a porphyrin framework can trap polar and
nonpolar organic substrates via hydrophobic and electrostatic
interactions, and by hydrogen bonding at the near
steric interaction +1.0 -0.1 koal/mol H
hydrogen bonding, -1.4 kcal/mol
coordination interaction, -4.0 kcal/mol
= porphyrin framework
Figure 4. Recognition of amino-acid ester by trifunctional chiral
porphyrin. The contribution of each interaction to the free-energy
changes were estimated by use of reference receptors.
46/Porphyrins and Metalloporphyrins as Receptor Models
proximity of porphyrin plane. For another example, large groups such as
calixarenes were linked at two functional groups on the porphyrin to
bring about a unique binding site to the guest molecule on the surface of
It is well known that incorporation of optically inactive protoheme
into the chiral apoprotein cavity induces characteristic Cotton effects
in the CD spectra in the Soret region. If guest molecules are chiral
natural products such as amino acids, sugars or nucleotides, multiple
interactions between the achiral porphyrin or its metal complex and the
chiral guest cause induced circular dichroism (1CD) in the Soret
region.21121 Measurement of the [CD provides important information about