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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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explained in terms of covalent bonding of the carboxylate to the zinc
ion, two hydrogen bonds, and steric constraint between the amino acid
residue and strapped groups of the host. Binding of the substrate from
the N-alkyl side is blocked and controlled by asymmetric-ring deformation
at the strapped site due to the N-alkyl group/'4 The chiral N-substituted
porphyrins were used as guest molecules for polypeptides or apoenzyme/15
This highly enantioselective recognition by proteins provides a method
for optical resolution of particular substrates/'1'
Coordination complexes with a metal ion sandwiched by two porphyrin
ligands were reported to exhibit dynamic features upon molecular
recognition (Figure 9).67 These double-decker poiphyrins are thus
important host frameworks for modeling of, for instance, allosteric
effects/18
II. Host-Guest Chemistry of Porphyrins in Molecular Recognition
In this section, the porphyrin framework is considered as a scaffold for
constructing a host molecule for recognition of a specific molecule. The
rigid framework of porphyrins is ideal for use as the basic unit
structure for the host molecule, on which various recognition groups with
varying interaction natures are lixed. Extended conjugated n electrons
serve as good probes for detecting intermolecular interactions via
various spectroscopic means. A number of spectroscopic techniques such as
NMR, UV-Vis, circular dichroism, magnetic circular dichroism,
fluorescence, 1R and Raman have been successfully employed to obtain
useful information.
A. RECOGNITION OF AMINES AND DIAMINES
The most thoroughly studied host-guest system with a porphyrin-based host
is the complex formed between amines and metalloporphyrins. The
stoichiometry of the host-guest complex and the binding affinity will
depend on the nature of the metal, the basicity of the amine, the
reaction media and the pH in the case of water. Porphyrin-amine
equilibria in water were studied by Pasternack el al.m [5,10,15,20-
tetrakis(yV-methylpyridinium-4-yl)porphyrina-tolCo(lIl) (27) binds
pyridine in water, with the binding constants and the rate constants of
complex formation depending upon pH. At acidic pH, pH < 4, the first
replacement of water by pyridine,
(P)Co(I-bOb -!- py ^ (P)Co(HiO)(py) +
(I)
proceeds with a quite large association constant, K> 10s M _l. The
association constant for the second replacement,
(P)Co(py)(H,0) + py ^ (P)Co(py)2 + H,G (2)
was smaller, = 4.8 x 104M ~1 at 25 C. At neutral pH, many species such
as (P)Co(OH)(H20), (P)Co(OH)(py), (P)Co(H20)(py), and (P)Co(pyh are
present and the determination of equilibrium constants becomes difficult.
Combining the equilibrium and kinetic data, the equilibrium constant of
Me H
OMe
//
OMe
-Me
Figure 9. Double-decker porphyrins
()()() + py ^ (P)Co(OH)(py) +
(3)
was determined to be 9 x 10' M '.
For the binding in organic solvents, zinc porphyrins are used in a
number of studies, where a 1:1 complex is predominantly formed and a
single equilibrium can describe the host-guest system.
(P)Zn + L ^ (P)Zn L, L = amine (AG0, AH", and AS1
(4)
287
288
Ogoshi et al.
Ph
Ph
28: M = Zn 29: M = H2 30: M = Co(ll) 31: M = Fe(ll)
32
33: X =
Hereafter the values of AG, /7" and AS0 refer to the standard state of
1 M of each species. We also use notation, AB, to represent a host-guest
complex between compound A and compound B. Table I summarizes the binding
free energy for complex formation for various amine-metallo-porphyrin
complexes. Monomeric porphyrins bind amines with - AG values in the
range of 10-25 kJ/mol, while dimeric porphyrins bind ditopic guests with
a larger - AG of 30-43 kJ / mol.
Although the coordinating interaction between zinc and the amino group
is the major driving force for complex formation, other interactions also
make significant contributions to the overall free-energy changes.
Restriction of the binding space by introducing steric substituents leads
to selectivity for amines. Ogoshi et al. reported cyclophane porphyrins,
49 and 50, that exhibit size-selective binding of pyridine derivatives.70
Owing to the restricted space for an axial ligand, the porphyrin was able
to distinguish axial-ligand bulkiness. Thus, porphyrin 49 with a longer
bridge
binds 4-benzylpyridine in the binding pocket of the bridged side, while
porphyrin 50 with a shorter bridge cannot accommodate 4-benzylpyridine.
The importance of London dispersion forces for binding was demonstrated
by Imai and Kyuno.71 By using a capped porphyrin 42, a 13-fold
enhancement of azetidine binding was observed in toluene at 25 C
compared with the reference zinc porphyrin 43 which has no capping. This
binding enhancement can be attributed to London dispersion forces between
the azetidine CH2 moiety and the bridging groups of the porphyrin.
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