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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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constant of w270ns.
40/Noncovalent Multiporphyrin Assemblies
3
(excited state of the special pair) and chemical energy (proton gradient
across the membrane). However, the initial event in photosynthesis is the
absorption of a photon by the light-harvesting antenna, prior to
excitation of the special pair. This event is followed by rapid
electronic energy transfer to the RC. Once the energy reaches the RC, it
is trapped and can no longer migrate to generate the special pair excited
state. As briefly discussed above, very fast deactivation of this state
occurs within the RC as an electron-hole pair is generated and followed
by charge separation. The function of the antenna as a light-harvesting
device is thus essential, either in green plants or bacterial
photosynthesis. In both types of living organisms, pigment-protein
complexes containing multichlorophyll or multi-bacteriochlorophyll
assemblies play an essential role. The beauty and the symmetry of these
ensembles is really breathtaking, as revealed by X-ray or electron
crystallography work from several groups.1516
The green plant light-harvesting complex has been studied by electron
crystallography of two-dimensional crystals, affording a structure at 3.4
A resolution.15 Chlorophyll a and chlorophyll b chromophores occupy the
center of the complex and two carotenoids are also found. Although a
convincing structural model has been proposed by the authors on the
ground of their electron diffraction study, the exact geometry of the
chlorophyll a/chlorophyll b assembly within its protein complex remains
to be determined.
If the arrangement of the chromophores in the plant light-harvesting
complex does not seem to be highly symmetric, the situation is very
different as far as the light-harvesting antenna complexes of
photosynthetic bacteria are concerned. The crystal structure
determination of the light-harvesting complex from Rhodopseudomonas
acidophila (LH2) has recently been carried out.16 As in the case of the
X-ray structure of the photosynthetic RC,7-8 this work represents a very
important step towards full understanding of natural photosynthesis. The
structure of the LH2 antenna complex was determined at 2.5 A resolution
and reveals a particularly beautiful and symmetric arrangement of
bacteriochlorophylls a (BCh a). The complete active assembly consists of
two subunits. Transmembrane helices of nine -apoproteins are packed side
by side to form a cylinder with a hollow part of radius 18 A. Nine
helices of /?-apoproteins are arranged radially with the "-apoproteins to
generate a large outer cylinder (radius ~ 34 A). A certain number of
carotenoids (13 or 14) are also found in the complex, but the BCh a are
the most relevant components to this chapter.
The BCh a molecules form two sets. A first disk-shaped group consists
of 9 BCh a molecules. These chromophores are well separated from one
another and absorb at 800 nm, which corresponds to very little or no
electronic interaction between the components. The other group of BCh a
molecules is formed by eighteen chromophores. Due to close proximity
between the chromophores, and thus strong electronic coupling, the
absorption spectrum of these BCh a molecules is bathochromically shifted
and centered at 850 nm. A view of the structure, restricted to the
chromophores, is given in Figure 3.
Figure 3. Disposition of the bacteriochlorophyll-a (BCh-a) chromophores
(restricted to the porphyrin macrocycles) in the LH2 antenna from
Rhodopseudomonas acidophila as determined by X-ray crystallography.,B The
protein helices have been omitted for clarity. The B850 BCh-a molecules
are perpendicular to the membrane and are quite densely packed, having
neighbor distances of 9-10 A.
The BB00 BCh-a molecules are parallel to the membrane, and are separated
by distances about a factor of 2 larger (18-21 A).
The results of interesting work on the functioning of the
photosynthetic light-harvesting complexes of bacteria, in the light of
the structure briefly discussed above, have recently been published.1718
It is noteworthy that relatively old work on green photosynthetic
bacteria has also afforded beautiful and complex structures, although
spatial resolution has not been sufficiently accurate to provide
nonambiguous three-dimensional maps of the protein-pigment complexes.19
Several other structural studies on multichromophore complexes have
been reported, either solved by X-ray diffraction or determined by
electron diffraction on two-dimensional crystals.20-22 Clearly, most of
the systems are closely related to photosynthesis. This trend is valid
for purely synthetic assemblies whose design and elaboration are mostly
guided by natural photosynthesis.
In the following discussion, we will focus on the construction of
multiporphyrin assemblies, whose components are held together by
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