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

The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
Previous << 1 .. 208 209 210 211 212 213 < 214 > 215 216 217 218 219 220 .. 240 >> Next

method.111 The association- and dissociation-rate constants al 25 in pH
6.9 aqueous solution were 1.9 x 10yM 's 1 and
1.9 x 10s'1, respectively. For complexation, a slacking-type
interaction between the -system of the porphyrin and the aromatic moiety
of the guest was operating.
Mizutani, Ogoshi and coworkers compared the association and
dissociation rates fora series of complexes between multifunctional zinc
porphyrins 167, 56 and Zn(OEP)] and amino-acid esters.112 The rate
constants were determined by 'H NMR relaxation time measurements. When
the association and dissociation rates were compared among the Zn(OEP)-
Leu-OMe complex (monolopic binding), the 56 Asp(OMe)-OMe complex
(ditopic binding), and the 67 Asp(OMe)-OMe complex (tritopic binding),
the association rate constants were 0.75 x l()y (monolopic binding), 1.2
x 104 (ditopic binding), and 16.0 x 10l)M 's_l (tritopic binding), and
the dissociation rate constants were
15.0 x 10^ (monotopic binding), 5.3 x UP (ditopic binding), and 3.6 x
lO^s-1 (trilopic binding). There is a large acceleration of association
and a small deceleration of dissociation. Therefore the additional
hydrogen bonding shifts the equilibrium toward complex formation without
suffering from the disadvantage of slow dissociation. These results
suggest that a combination of weak interactions leads to tight
complexation without retarding the kinetic processes, an ideal behavior
like that of the binding site of enzymes, where high specificity can be
achieved without suffering from a slow dissociation rate.
Conformational dynamics in molecular recognition has been studied for
several systems involving porphyrins.104 Atropisomerization of porphyrin
111 was controlled by adding quinones.21 In the absence of quinones, the
ratio of atropisomers was ,,,:.,,//:,,/?,//:,/?= 1.0 :
4.4:2.2:0.8. which is close to the statistical ratio of 1:4:2: 1. By
adding 2,3,5,6-letramethoxy-p-benzoqui-none, induced-fit-type binding
occurred and the proportion of isomer ,,a was increased up to 80%.
Ruffling of the pyrrole rings is another dynamic feature of poiphyrin
frameworks. A porphyrin bearing eight alkyl groups at the jl positions
and four aryl groups to the meso positions cannot be planar and is saddle
shaped due to the crowded substituents (see Chapter 6 by Senge). The rate
of ruffling was determined by use of racemization of a chiral porphyrins
112-119, in which the racemization proceeds via ruffling of pyrrole
rings.113 The rales were dependent upon the central metals as well as
upon the substituents at the meso positions. The rate increased in the
order: Zn<Cu<H2<Ni. which is the same as the order of
atropisomerization.114115 The ?>2 symmetric poiphyrin 120 is chiral and
exists as a racemate in solution.116 By adding
fry 'OH

>-NH N-
Ogoshi et al.
112: R =
117: R=


a M = H2 = Zn(ll) M = Cu(ll) d M = Ni(ll)
chiral mandelate, characteristic CD spectra were observed showing that
one of the enantiomers becomes stabilized. It is interesting to note that
after the mandelate was replaced with acetate, the Cotton effects
remained; in other words, the molecule memorized the chirality.
III. Electron Transfer Regulated by Molecular Recognition
Porphyrin and its derivatives play a key role in biological electron-
transfer (ET) systems. They are found in photosynthetic or respiratory
reaction centers involving the cascade of ET reaction processes. For
example, the high-resolution X-ray crystal structure of the bacterial
photosynthetic reaction center from Rhodopseudomonas viridis and
Rhodobacter sphaeroides reveals the unique assembly conformation of
porphyrin and quinone as cofactors in the protein, suggesting that the
distance and orientation of electron donor and acceptor are quite
important for effective ET reactions.117118 In mitochondrial membranes,
cytochrome peroxidase catalyzes the oxidation of ferrocyto-chrome c, in
which both proteins have the binding domain on the protein surface to
form a stable protein-protein complex.119 Furthermore, it is well known
that ubiquinone and cytochrome are cnicial electron carriers which
shuttle an electron between the special binding sites of membrane-bound
oxidoreductases in respiratory systems.120121 Therefore, in biological
systems, long-distance ET occurs not only between redox centers within a
Previous << 1 .. 208 209 210 211 212 213 < 214 > 215 216 217 218 219 220 .. 240 >> Next