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Advances in Photosynthesis Fundamental Aspects Part 3

Tham khảo tài liệu 'advances in photosynthesis fundamental aspects part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 52 Advances in Photosynthesis - Fundamental Aspects along the A branch probably due to a tyrosine residue strategically positioned instead of a phenylalanine present in the B branch Lia etal. 1993 . Once the special pair is excited it has been determined experimentally Fleming et al. 1988 that takes 3-4 ps for the special pair to ionize and produce a reduced bacteriopheophytin H in a reaction SP SP H . This reaction initiates an electron hop to a quinone Qa in about 200 ps and to a second quinone Qb if available. Initially the ionized quinol Q captures an introcytoplasmic proton and produces hydroxiquinol QbH which after a second ionization that produces QbH to form quinol QbH2. After any SP ionization a neutrality restablishment is required provided by the cytochrome cyt charge carrier. After SP ionization the cytochrome diffuses from the bc1 complex to a RC in order to replenish its neutrality SP SP within several microseconds Milano et al. 2003 . The first electron transfer step P P occurs in the RC within t 3 ps used for quinol QbH production Hu et al. 2002 . 2.5 Quinone-quinol cycling The RC cycling dynamics also involves undocking of QbH2 from the RC due to lower affinity among RC and this new product. Quinol starts a migration to the bc1 complex where enables the ionization of the cytochrome cyt charge carrier while a new quinone Qb molecule docks into the RC. The time before quinol unbinds and a new Qb is available has been reported within milliseconds Osváth Maróti 1997 to highlight quinol removal o as the rate limiting step Osváth Maróti 1997 if compared to special pair restablishment. Even though it has been reported that excitation dynamics change as a function of the RCs state Borisov et al. 1985 Comayras et al. 2005 at a first glance the several orders of magnitude difference among the picosecond transfer the nanosecond dissipation and the millisecond RC cycling seems to disregard important effects due to these mechanisms interplay. However the .