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 .. 195 196 197 198 199 200 < 201 > 202 203 204 205 206 207 .. 240 >> Next

only (he amino group of melhionine was coordinated to cobalt. Al higher
pH (10.3), complexes with L-lysine consisted of the mixture of a-NIL-
bound species and i:-NH2-bound species. It is suggested that
electrostatic interactions between the carboxylate anion of the guest and
the pyridinium cation of the host stabilize the x-bound species al lower
pH.
3. The coupling constants between the a-proton and the ji-proton of amino
acids indicated that the conformation of amino acids bound to the cobalt
porphyrin was relatively fixed.
4. Analysis of the coupling constants according to a Karplus-type
equation revealed that the aromatic plane of phenylalanine and tryptophan
is parallel to the porphyrin plane, suggesting that the 7r-7i-stacking
interactions in water stabilize the complex.7h
These studies using water-soluble cobalt and zinc porphyrins showed
that coordinating interactions, n-n-stacking interactions and
electrostatic interactions determined the selectivity for amino acids and
the conformation of the porphyrin-amino-acid complexes.
C. RECOGNITION OF AMINO-ACID DERIVATIVES IN ORGANIC SOLVENTS
In organic solvents, hydrogen bonding serves as one of the driving forces
for complex formation. Rhodium(III) porphyrins 54 and 5577 and zinc
porphyrin 56" were prepared as hosts for amino-acid esters in organic
solvents that are able to bind amino-acid derivatives in a ditopic
fashion, namely by combination of a coordinating interaction and hydrogen
bonding. Zinc porphyrin 56 bound amino-acid esters with binding constants
ranging from 1100 to 10,900 M 1 in CHCI?. 'H NMR spectra of the 56-amino-
acid-ester complexes showed that the hydroxy proton of 56
is shifted downfield upon addition of amino-acid esters, indicating (hat
a hydrogen bond between the OH group of 56 and (he carbonyl group of
amino-acid esters was formed. Porphyrin 56 preferentially binds amino-
acid esters bearing bulky side chains, such as leucine methyl ester and
isoleucine methyl ester. The binding was analyzed via a ditopic binding
model in which the coordinating interaction between zinc and the amino
group as well as a hydrogen-bonding interaction between the phenolic OH
group and the carbonyl group are simultaneously operating and additively
contributing to the overall free-energy changes. To estimate the free-
energy changes attributable to each interaction, binding constants with
reference hosts 57 and reference guests were also determined. Analyses
demonstrated that the coordinating interaction is relatively constant ( -
AG = 14.2- 18.8 k.l / mol), while (he hydrogen-bonding free energy
increases with increased bulkiness of (he side-chain group ( - AG0 = 0.8-
6.3 kJ/mol). One possible explanation for this trend is that the bulky
side-chain group makes (he amino-acid ester rigid, which reduces the
conformational entropy loss upon binding./S
The amino-acid ester-zinc porphyrin 56 complexes showed characteristic
bisignate-induced Cotton effects in the Soret band (Figure 10a). It is
interesting to note that the induced Cotton effects diminish if the
hydrogen bonding between the porphyrin and amino-acid esters was
prohibited. For instance the complexes between melhoxyporphyrin 57 and
amino-acid esters show only weak Cotton effects in the Soret region and
they were not bisignate (Figure 10b). The magnitude of the value of Ai:
was increased with an increase in the hydrogen-bonding energy, suggesting
that the effective fixation of the bound guest induced stronger Cotton
effects. Therefore, the induced CD originates from the fixation of amino-
acid esters on (he porphyrin plane via ditopic binding (coordinating and
hydrogen-bonding interactions). Molecular orbital calculations
demonstrated that the coupling of the transition moments of the ester
group and that of the porphyrin chromophore can lead to the bisignate
CD.21 Effective fixation of amino-acid esters in the binding pocket of 56
can be sensitively delected by the bisignate Cotton effects.
A porphyrin host 67 having two hydrogen-bonding sites and a
coordinating site was prepared for additional recognition of a polar
side-chain./l; The porphyrin receptor 67 showed preference for amino-acid
esters with a polar .side-chain group, particularly dimethyl aspartate.
The value of - AG{) for Asp(OMe)-OMe in CHCI, was 27.6k.l / mol while
that for Leu-OMe was 24.2 kJ/mol (Table 2, entry 120,121).
Gadolinium(III) porphyrin 69 was used as a sensitive probe for
chirality of amino acids by taking advantage of Cotton effects in CD
spectroscopy.811 The gadolinium porphyrin had acetylacetonate as an axial
ligand. This porphyrin can extract amino acid from water. The 69-amino-
acid complexes showed bisignate Cotton effects in the Soret region, from
which the absolute configuration of amino acids can be deduced. Because
no acetylace-tonate was released either into the water phase or the
Previous << 1 .. 195 196 197 198 199 200 < 201 > 202 203 204 205 206 207 .. 240 >> Next