Identification of the First Steps in Charge Separation in Bacterial Photosynthetic Reaction Centers of Rhodobacter sphaeroides by Ultrafast Mid-Infrared Spectroscopy: Electron Transfer and Protein Dynamics

Abstract

Time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy in the region between 1600 and 1800 cm(-1) was used to investigate electron transfer, radical pair relaxation, and protein relaxation at room temperature in the Rhodobacter sphaeroides reaction center (RC). Wild-type RCs both with and without the quinone electron acceptor Q(A), were excited at 600 nm (nonselective excitation), 800 nm (direct excitation of the monomeric bacteriochlorophyll (BChl) cofactors), and 860 nm (direct excitation of the dimer of primary donor (P) BChls (P(L)/P(M))). The region between 1600 and 1800 cm(-1) encompasses absorption changes associated with carbonyl (C=O) stretch vibrational modes of the cofactors and protein. After photoexcitation of the RC the primary electron donor P excited singlet state (P*) decayed on a timescale of 3.7 ps to the state P(+)B(L)(-) (where B(L) is the accessory BChl electron acceptor). This is the first report of the mid-IR absorption spectrum of P(+)B(L)(-); the difference spectrum indicates that the 9-keto C=O stretch of B(L) is located around 1670-1680 cm(-1). After subsequent electron transfer to the bacteriopheophytin H(L) in approximately 1 ps, the state P(+)H(L)(-) was formed. A sequential analysis and simultaneous target analysis of the data showed a relaxation of the P(+)H(L)(-) radical pair on the approximately 20 ps timescale, accompanied by a change in the relative ratio of the P(L)(+) and P(M)(+) bands and by a minor change in the band amplitude at 1640 cm(-1) that may be tentatively ascribed to the response of an amide C=O to the radical pair formation. We conclude that the drop in free energy associated with the relaxation of P(+)H(L)(-) is due to an increased localization of the electron hole on the P(L) half of the dimer and a further consequence is a reduction in the electrical field causing the Stark shift of one or more amide C=O oscillators.

Light-harvesting and structural organization of Photosystem II: From individual complexes to thylakoid membrane

“…Although the very fast energy transfer from CP43/ CP47 to RC in the trap-limited model as found by Holzwarth and coworkers [7] can indeed not be explained by (generalized) Förster theory, it is on the other hand questionable whether the ultrafast primary charge separation in combination with very slow charge recombination (implying a very large initial drop in free energy) in the transfer-to-the-trap-limited model proposed by Raszewski and Renger [66] is realistic. At least in isolated RC complexes this has never been observed [6,7,48,[69][70][71][72][73][74]. On the other hand, the results on PSII membranes could not be explained with the timeresolved results on isolated RC's [42].…”

Overall energy conversion efficiency of a photosynthetic vesicle

“…In the RC, the excitation quickly settles onto the so-called special pair BChls (Small, 1995;Damjanovic´et al, 2000), where it induces the transfer of an electron (Pawlowicz et al, 2008;Jordanides et al, 2004). This transfer proceeds stepwise to reach a quinone molecule, Q, attracted into the RC from the quinone/quinol pool of about 900 molecules (Cartron et al, 2014).…”

The petite purple photosynthetic powerpack

“…These signals arise from the shift in frequency of a vibrational mode as the RC evolves from one state into the next. Although many of these signals involve modes internal to the bacteriochlorins, this form of spectroscopy also reveals protein groups that undergo frequency shifts on the formation of a particular state, and so has the potential to throw new light on protein dynamics on the timescale of charge separation (see [49] and references therein).…”