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 .. 209 210 211 212 213 214 < 215 > 216 217 218 219 220 221 .. 240 >> Next

single molecule but also
between two different redox proteins or in a protein-cofactor complex.122
In the last two decades, a great number of covalently linked model
compounds have been reported, and results using them have supported the
theoretical ET mechanism.123 In particular, porphyrin-spacer-porphyrin,
por-phyrin-spacer-quinone and porphyrin-spacer-viologen have been typical
model compounds used to elucidate the Marcus mechanism of ET related to
distance or driving force between donor and acceptor.4-5-124125 In
contrast, model studies on noncovalently linked donor-acceptor systems
are also important for understanding biological ET,2-3-126-128 since
protein-protein or protein-cofactor recognition may dominate the overall
reaction of ET process. In this section, a variety of ET systems
employing noncovalently linked host-guest interactions are reviewed.
Recently, porphyrin units have been widely used as the model systems for
ET reactions. The reasons for choosing porphyrin or its derivative are as
follows: (1) porphyrin or its derivatives quite often act as electron
donors or acceptors in biological systems and have high photoreactivity;
(2) chemical and physical features of porphyrin units are easily
controlled through insertion of metal into the porphyrin core or by
substitution of functional groups at the meso- or ft-pyrrole position of
the porphyrin ring; (3) poiphyrins exhibit unique spectroscopic
characteristics in their UV-Vis and fluorescence spectra due to the
aromaticity of the large porphyrin ring; and (4) porphyrins comprise a
46 / Porphyrins and Metalloporphyrins as Receptor Models
Scheme 2
convergent complex
framework for host molecules, since it is possible to attach various
types of functional groups as binding sites at the porphyrin ring. These
advantages have encouraged us to construct new noncovalently linked
donor-acceptor pairs containing porphyrin units via molecular recognition
behavior to mimic the intracomplex ET in proteins.
It is well known from a series of ET model systems that orientation
and distance between donor and acceptor are essential factors. The design
of model systems can be divided into two approaches as shown in Scheme 2;
these involve convergent or divergent donor-acceptor binding through a
specific recognition process. Furthermore, it should be noted that the
type of interaction, such as hydrogen bond, salt bridge, hydrophobic
contact and/or charge-transfer, is essential for the molecular modeling
and the evaluation of ET rates. In addition, if the dissociation or
interconversion between donor and acceptor is fast, the overall ET
process may often depend upon the dynamics of donor-acceptor
complexation. Thus, it could be necessary to examine the various model
systems to understand the complicated biological ET processes.
Photoinduced singlet ET in noncovalent donor-acceptor systems is easily
monitored by fluorescence spectroscopy. In general, addition of acceptor,
guest molecule, to a solution of donor, porphyrin host molecule, reveals
significant quenching of the fluorescence derived from free-base
porphyrin or zinc porphyrin. Stern-Volmer plots which represent the
magnitude of fluorescence quenching as a function of electron-acceptor
concentration provide good information about binding affinity between
donor and acceptor, and forward ET property.129 The measurement of
fluorescence lifetime is useful for estimating the rate constant of
forward singlet ET from photoexcited porphyrin to electron acceptor. In
noncovalent assembly systems, it is possible to monitor biphasic
fluorescence decay profiles which contain two fluorescence lifetimes, rL
and rs.
Fluorescence intensity(f) = A| exp(-f/ij ) +As exp(-f/rs). (6)
where T[ and is are longer and shorter lifetimes, and AL and As are
fractional amplitudes, respectively. The longer and shorter components
can be assignable to the fluorescence lifetimes of a free and acceptor-
binding porphyrin. These lifetimes are independent of the concentration
of acceptor in the range of static-quenching processes. When is the same
as fluorescence lifetime of the porphyrin in the
divergent complex
absence of acceptor, the forward ET rate constant, kcx, is usually
calculated from the fluorescence lifetimes by130
= 1/rs - 1/, (7)
Furthermore, the ratio of two fractional amplitudes that depend upon the
concentration of acceptor leads to the binding constant between porphyrin
and acceptor; K = ASI (AL-[acceptor]).
Porphyrin cation radical species as intermediates in ET can be
directly detected by laser-flash photolysis or ESR spectroscopy. In
particular, transient absorption spectra of porphyrin-acceptor assembly
systems excited by laser pulses often show the characteristic broad band
Previous << 1 .. 209 210 211 212 213 214 < 215 > 216 217 218 219 220 221 .. 240 >> Next