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Formation Dynamics, Decay Kinetics, and Singlet–Triplet Splitting of the (Bacteriochlorophyll Dimer) + (Bacteriopheophytin) − Radical Pair in Bacterial Photosynthesis
Author(s) -
Bixon M.,
MichelBeyerle M.E.,
Jortner Joshua
Publication year - 1988
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198800026
Subject(s) - chemistry , superexchange , bacteriochlorophyll , intersystem crossing , excited state , singlet state , singlet fission , photochemistry , photosynthetic reaction centre , triplet state , electron transfer , quantum yield , atomic physics , fluorescence , ion , physics , photosynthesis , biochemistry , organic chemistry , quantum mechanics
We explore the interrelationship between (i) the kinetic data for the primary electron transfer (ET) from the excited singlet state ( 1 P*) of the bacteriochlorophyll dimer (P) to bacteriopheophytin (H) in the reaction centers (RCs) of purple photosynthetic bacteria and (ii) the energetics and dynamics of the primary radical pair, P + H, i.e., its singlet‐triplet splitting rate, J , and triplet recombination rate, k T . We present a critical scrutiny of the ET mechanisms with regard to the role of the accessory bacteriochlorophyll (B). On the basis of the very weak temperature dependence of J , which sets a lower limit to the energy of (hypothetic) transient short‐lived ionic intermediate states, we argue against the applicability of sequential ET mechanisms involving P + B − H or P + BH − . The nonadiabatic/adiabatic ET mechanism is fraught with difficulties pertaining to a very low medium reorganization energy for the nonadiabatic crossing and an exceedingly high rate of the adiabatic crossing, together with a predicted order‐of‐magnitude enhancement of the low‐temperature (77 K) fluorescence quantum yield in high electric fields (5–10 mV/Å) which seem not to be borne out by experiment We propose that primary ET occurs via the unistep superexchange 1 P*BH r̊ P + H − , which is mediated by electronic coupling with P + B − H. The analysis of the competition between superexchange and thermally activated ET resulted in large values of the intermolecular electronic couplings at the equilibrium nuclear configuration of 1 P*BH. We suggest that the structural relaxation of one of the prosthetic groups (H) accompanying the primary ET results in the decrease of the intermolecular electronic coupling at the equilibrium configuration of P + BH − . It was established that the superexchange‐predicted kinetic data are consistent with the magnetic data for the radical pair. The small value of J for P + BH − originates from the cumulative effect of: (i) an essential, model‐independent cancellation between the triplet contribution, which is related to k T by a self‐consistency relationship, and the singlet contribution; (ii) the effect of configurational relaxation on the singlet contribution. Our analysis supports the superexchange mechanism for primary ET in bacterial photosynthetic RCs.