Dissertation

|1.5 Photosynthesis|
Figure 9: Molecular structure of important redox-active species involved in the initial charge
separation of PS II. Depicted parts to conform to the present text: OEC (Mn 4 Ca), tyr (Tyr z ), special
pair P680 (P D1 /P D2 ) and antenna pigments (Car and Chl), Pheo (Pheo D1 /Pheo D2 ), PQ (Q A /Q B ), and
cyt f (Cyt b-559). The protein surrounding is omitted for clarity. [20]
P700. Interestingly, chlorophylls exhibit two major absorption maxima in the blue and red of the
visible region. One between 600 and 700 nm, the so called Q y -band and the Soret Peak between
400 and 500 nm. Therefore, plants can use a major fraction of solar energy, except a part of green
and yellow light.
Nevertheless, ultrafast charge separation and electron transfer processes away from the special
pairs, finally start the true chemical steps. The key process for life on earth is the electron entering
the first redox shuttle. This happens by oxidative quenching of the excited P680* by nearby
pheophytin (Pheo) with formation of the strong oxidant P680 + and reduced Pheo − in PS II. In a
similar way excited P700* in PS I donates its excited electron to a nearby electron acceptor (A 0 , a
chlorophyll-a molecule, not depicted in figure 10), with formation of P700 + and A − 0 .
Regeneration of the photooxidized special pairs P680 and P700 happens almost simultaneously.
The missing electron in P680 + is recovered from water oxidized by a multiple reducible Mn 4 Ca
cluster also known as oxygen evolving complex (OEC). This complex sequentially extracts four
|11|