Page 582 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Reactions on Polymers 545

LH I

LH II RC LH II

FIGURE 16.10 Simplifi ed drawing showing two light harvesting II assemblies next to one light harvesting
I unit. The circles are polypeptides and the lines represent rings of interacting bacteriochlorophylls a (called
B850). In the middle of LH I, there is a protein called the reaction center (RC) where the primary photo-
induced electron transfer takes place from the special pair of bacteriochlorophylls b.

Energy migration (exciton)

s *
1
Energy transfer
A
s
o
Donor orbitals Acceptor

FIGURE 16.11 The exciton and energy transfer processes.

is converted to chemical energy via charge separation across the bilayer by means of an electron-
transfer reaction.
A special BCHl (P870) pair is excited either by the absorption of a photon or by acquiring this
excitation energy from an energy transfer from the peripheral antenna BCHl (not shown for simplic-
ity) triggering a photo-induced electron transfer inside the RC (36). Two photo-induced electrons are
transferred to a plastoquinone located inside the photosynthesis membrane. This plastoquinone acts

as an electron acceptor and is consequently reduced to a semiquinone and finally to a hydroquinone.
This reduction involves the uptake of two protons from water on the internal cytoplasmic side of the
membrane. This hydroquinone then diffuses to the next component of the apparatus, a proton pump
called the cytochrome bc1 complex.
The next step involves the oxidation of the hydroquinone back to a quinone and the energy
released is used for the translocation of the protons across the membrane. This establishes a proton
concentration and charge imbalance (proton motive force [pmf]). Thus, the oxidation process takes
place via a series of redox reactions triggered by the oxidized special pair BCHl (P870) which at
the end is reduced to its initial state. The oxidation process is ultimately driven, via various cyto-
chrome redox relays, by the oxidized P870. Oxidized P870 becomes reduced to its initial state in
this sequence. Finally, the enzyme adenosine triphosphate (ATP) synthase allows protons to fl ow
back down across the membrane driven by the thermodynamic gradient, leading to the release of
ATP formed from adenosine diphosphate and inorganic phosphate (Pi). The ATP fills the majority

of the energy needs of the bacterium.

16.9.2 GREEN SULFUR BACTERIA

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.
Green sulfur bacteria reaction centers are of the type I or the Fe-S-type (photosystem I). Here
1
the electron acceptor is not the quinine; instead chlorophyll molecules (BChl 663, 8 -OH-Chl a,

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