Since about 100 light-harvesting chlorophylls (antenna
chlorophylls) are associated with PS I RC, excitation
transfers among them prevent one from observing the
elementary steps of the charge separation [16–18]. However,
a special preparation of PS I RC from spinach
with a significantly reduced number of antenna Chls
has been developed, which are designated P700-enriched
RC [19,20]. The P700-enriched RCs retain the original
electron-transfer capability from P700* to A0 [21–24].
About 90% of the original antenna Chls, secondary electron
acceptor phylloquinone and carotenoids are removed
during the enrichment process [25–28]. Phylloquinone or
other quinones with similar molecular structures and redox
potentials can be reconstituted into the original phylloquinone
binding site [21,22] which recovers the electron
transfer to the electron acceptor iron sulfur center (Fx)
[22,29]. One of the most unique features of P700-enriched
RC is that there are almost no so-called ‘red absorbing’
chlorophylls in their absorption peak redder than P700.
Preferential excitation of P700 is possible, by which we
can directly observe the primary processes from P700*
خب اين يكيش
The main focus of this paper, is to review the recent
progress of the primary photophysical and photochemical
studies of PS I RC with a special emphasis on the lessons
learned from the P700-enriched complexes of a higher plant
(spinach). It is also our aim to compare these with the purple
bacteria, of which the structure and functions have been
studied mo
شرمنده اينم اون يكيش:43:
In this scheme, the primary charge-separated state
PCBChlL− is not accumulated as a dominant transient. It
is interesting to know whether or not such electron transfer
dynamics of PB are universal among a variety of photosynthetic
RCs. In PS I RC, there has not yet been sufficient
spectroscopic data to elucidate the primary charge separation
from P700
بعد اينم اون يكيش
One of the clearest differences from the PB system is
the efficient exchange of excitation energy between the
special pair and the other pigments in the electron transfer
system [16]. The apparent dynamics of the primary charge
separation in the PS I RC are strongly dependent on the
contribution of excitation transfers (Chl* P700* and
Chl*!P700*). These processes complicate the analysis
of the kinetics in PSI RC. The primary charge separation is
probably from the electronically excited P700 (P700*) to the
intermediate acceptor chlorophyll, A (Fig. 1). However, this
reaction has been too rapid to be observed spectroscopically
(vide infra). This is followed by the sequential charge shift
reactions from A to A0−, A0− to phylloquinone (2-methyl-
3-phytyl-1,4-naphthoquinone, Q) and then to the iron-sulfur
complex (4Fe–4S clusters), FX, FB, and FA [5,17,18]. The
accumulation of P700CA0− or the photobleaching of A0−
has been observed to occur with an apparent time constant
of 3–14 ps including the energy transfer time among antenna
chlorophylls and to P700 [23,24,32–34]. The apparent
time constant seems to vary depending on the numbers of
antenna Chls in the PS I RC complex
خب فك كنم اين آخريشه.gif "N Aggressive (21)")
.gif "N Aggressive (17)")
The multi-step electron transfer from P700 to phylloquinone
somewhat resembles that in the purple bacterial RC
complex (Fig. 1). In the purple bacterial RC, the donor
is a bacteriochlorophyll dimer (P), and the acceptors are
bacteriochlorophyll, bacteriopheophytin (H), ubiquinone
(or menaquinone) (QA) and ubiquinone (QB). The electron
transfer from A0 to Q in PS I is comparable with that of
H to QA in PB, since the molecular structure and size ofthe cofactors are similar to each other. However, the rate
constant of the former reaction is 10 times greater, and the
energy gap is about half in comparison with those of the
latter [36]. The difference between them may reflect the
difference in the geometries of the functional groups and/or
the structure of the protein matrix
.
خب در آخر هم بازم ازتون خواهش ميكنم يه يا علي بگين كار اين بيسواد رو را بندازين استاد بد جوري پيله كرده.gif "N Aggressive (15)")
نوشته شده توسط maryam_cn
.gif "N Aggressive (41)")
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