Transient absorption spectroscopy photosynthetic reaction centers

The next two chapters are devoted to ultrafast radiationless transitions. In Chapter 5, the generalized linear response theory is used to treat the non-equilibrium dynamics of molecular systems. This method, based on the density matrix method, can also be used to calculate the transient spectroscopic signals that are often monitored experimentally. As an application of the method, the authors present the study of the interfadal photo-induced electron transfer in dye-sensitized solar cell as observed by transient absorption spectroscopy. Chapter 6 uses the density matrix method to discuss important processes that occur in the bacterial photosynthetic reaction center, which has congested electronic structure within 200-1500cm 1 and weak interactions between these electronic states. Therefore, this biological system is an ideal system to examine theoretical models (memory effect, coherence effect, vibrational relaxation, etc.) and techniques (generalized linear response theory, Forster-Dexter theory, Marcus theory, internal conversion theory, etc.) for treating ultrafast radiationless transition phenomena. 

Kleima FJ, Gradinaru CC, Calkoen F, Van Stokkum IHM, Van Grondelle Rand Van Amerongen H (1997) Energy transfer in LHCH monomers at 77K studied by sub-picosecond transient absorption spectroscopy. Biochemistry 36 15262-15268 Kleinherenbrink FAM, Hastings G, Wittmershaus BP and Blankenship RE (1994) Delayed fluorescence from Fe-S type photosynthetic reaction centers at low redox potential. Biochemistry 33 3096-3105... 

The photophysical and electron transfer properties of bacteriochlorophylls (Bchl) and bacteriopheophytins (Bpheo) found in the reaction centers of photosynthetic bacteria have been directly associated with the mechanism of charge separation which underlies photosynthesis [1]. The appearance of the Bpheo anion (Bpheo ) within 3-5 ps after excitation of the special pair of Bchl (P) is well documented from transient absorption spectroscopy [2-4]. The 200 ps lifetime of Bpheo which is primarily determined by the electron transfer process to a quinone also has been established by picosecond changes in absorption [5,6], Thus, the general kinetic time scale for the primary processes in bacterial photosynthesis has been determined by the transient differences in electronic state properties. 

The kinetics of primary processes occuring in the photosynthetic reaction center (RC) has been studied so far almost exclusively by transient absorption spectroscopy in the femto- to nanosecond time range. Most of these studies were carried out furthermore on the reaction centers of purple bacteria like, e.g., R sphaeroides and Rs. rubrum (for a review see (1)). Despite a large number of transient absorption studies being carried out over the last decade, no agreement has been reached as to the nature of the primary processes and the identity of the intermediates. At present in particular two topics are highly controversial i) Is the accessory bacteriochlorophyll (BChl) a anion an intermediate in the electron transfer process (2), and ii) are... 

J. Breton, J.L. Martin, J.C. Lambry, S.J. Robles and D.C. Youvan, Ground state and femtosecond transient absorption spectroscopy of a mutant of Rb. capsulatus which lacks the initial electron acceptor bacterio pheophytin, in Reaction centers of photosynthetic bacteria, ed. M.E. Michel-Beyerle, Springer Series in Biophysics Vol. 6 (1990)... 

Here we present experimental evidence that the monomeric bacteriochlorophyll is required for triplet energy transfer from the primary donor to the carotenoid in photosynthetic bacterial reaction centers. Our approach is to use sodium borohydride to extract the monomeric bacteriochlorophyll from the reaction centers of the carotenoidless mutant Rb. sphaeroides R26 [3, 4]. The borohydride treated reaction centers are then reconstituted with the carotenoid, spheroidene [5], and the ability of the reaction center complex to carry out the primary donor-to-carotenoid triplet transfer reaction was examined by transient optical spectroscopy. Steady state optical absorption and circular dichroism (CD) measurements demonstrate diat spheroidene reconstituted into borohydride-treated Rb, sphaeroides R26 reaction centers is bound in a single site, in the same environment and with the same structure as spheroidene reconstituted into native Rb. sphaeroides R26 reaction centers. It is shown herein that the primary donor-to-carotenoid triplet transfer reaction is inhibited in the absence of the accessory bacteriochlorophyll. 

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