Both chlorophylls a and d are essential for the photochemistry in photosystem II of the cyanobacteria, Acaryochloris marina.
We have measured the flash-induced absorbance difference spectrum attributed to the formation of the secondary radical pair, P(+)Q(-), between 270 nm and 1000 nm at 77 K in photosystem II of the chlorophyll d containing cyanobacterium, Acaryochloris marina. Despite the high level of chlorophyll d present, the flash-induced absorption difference spectrum of an approximately 2 ms decay component shows a number of features which are typical of the difference spectrum seen in oxygenic photosynthetic organisms containing no chlorophyll d. The spectral shape in the near-UV indicates that a plastoquinone is the secondary acceptor molecule (Q(A)). The strong C-550 change at 543 nm confirms previous reports that pheophytin a is the primary electron acceptor. The bleach at 435 nm and increase in absorption at 820 nm indicates that the positive charge is stabilized on a chlorophyll a molecule. In addition a strong electrochromic band shift, centred at 723 nm, has been observed. It is assigned to a shift of the Qy band of the neighbouring accessory chlorophyll d, Chl(D1). It seems highly likely that it accepts excitation energy from the chlorophyll d containing antenna. We therefore propose that primary charge separation is initiated from this chlorophyll d molecule and functions as the primary electron donor. Despite its lower excited state energy (0.1 V less), as compared to chlorophyll a, this chlorophyll d molecule is capable of driving the plastoquinone oxidoreductase activity of photosystem II. However, chlorophyll a is used to stabilize the positive charge and ultimately to drive water oxidation.
Schlodder
Eberhard
E
Max-Volmer-Laboratorium für Biophysikalische Chemie, Technische Universität Berlin, Strasse des 17.Juni 135, 10623 Berlin, Germany. eberhard.schlodder@tu-berlin.de
Cetin
Marianne
M
Eckert
Hann-Jörg
HJ
Schmitt
Franz-Josef
FJ
Barber
James
J
Telfer
Alison
A
D002734:Chlorophyll / Q000378:metabolism*; D000077194:Chlorophyll A; D000458:Cyanobacteria / Q000737:chemistry / Q000378:metabolism*; D010777:Photochemistry; D045332:Photosystem II Protein Complex / Q000378:metabolism*
D045332:Photosystem II Protein Complex; D002734:Chlorophyll; C107509:chlorophyll d; D000077194:Chlorophyll A