Primary charge separation within P870* in wild type and heterodimer mutants in femtosecond time domain

Abstract

Primary charge separation dynamics in the reaction center (RC) of purple bacterium Rhodobacter sphaeroides and its P870 heterodimer mutants have been studied using femtosecond time-resolved spectroscopy with 20 and 40fs excitation at 870nm at 293K. Absorbance increase in the 1060-1130nm region that is presumably attributed to P(A)(δ+) cation radical molecule as a part of mixed state with a charge transfer character P*(P(A)(δ+)P(B)(δ-)) was found. This state appears at 120-180fs time delay in the wild type RC and even faster in H(L173)L and H(M202)L heterodimer mutants and precedes electron transfer (ET) to B(A) bacteriochlorophyll with absorption band at 1020nm in WT. The formation of the P(A)(δ+)B(A)(δ-) state is a result of the electron transfer from P*(P(A)(δ+)P(B)(δ-)) to the primary electron acceptor B(A) (still mixed with P*) with the apparent time delay of ~1.1ps. Next step of ET is accompanied by the 3-ps appearance of bacteriopheophytin a(-) (H(A)(-)) band at 960nm. The study of the wave packet formation upon 20-fs illumination has shown that the vibration energy of the wave packet promotes reversible overcoming of an energy barrier between two potential energy surfaces P* and P*(P(A)(δ+)B(A)(δ-)) at ~500fs. For longer excitation pulses (40fs) this promotion is absent and tunneling through an energy barrier takes about 3ps. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Spectral exhibition of electron-vibrational relaxation in P* state of Rhodobacter sphaeroides reaction centers

“…The idea of mixing occurring between the locally excited P* state and the state with charge-transfer character was used in the study by Khatypov et al (2012) to explain the near-infrared optical signatures of the primary photosynthetic events in the native Rba. sphaeroides RCs.…”

“…The oscillations are absent when P is excited by 40 fs pulses. In this case, a delayed increase in the long-wavelength slope of the P* stimulated emission band is observed (Khatypov et al 2012). This increase has been discussed in terms of the partial charge separation inside dimer P and the formation of the mixed P*(P A d?…”

“…The incoherent approximation of this model is applicable if the coherent effects in the population dynamics are negligible. The latter is true if the pump pulse is longer than 30-40 fs (Vos et al 1994a;Khatypov et al 2012). A set of the kinetics Eqs.…”

Incoherent Oscillations Accompanying Charge Separation in Photosynthetic Reaction Centers

“…In our model the * 2 P B A H A state is presumably associated with the P* stimulated emission band at 940 nm. This band is spectrally and temporally differ from the initially excited P* emission band at ~905 nm ( * 1 P in the model) and is clearly observed in native and mutant RCs at room and low temperatures [3,24]. In the experimental ΔA spectra the 940-nm emission band forms a long-wavelength tail of the broadband negative signal mainly consisted of the P absorption band bleaching at ~870 nm.…”

“…The nature of 940-nm emission band is not well understood. The visible and IR transient spectroscopy indicates that this band may be ascribed to the state with partial charge separation inside the dimer P [24][25][26]. From the other hand, the hole burning experiments at low temperatures show an absence of the P* conversion into another state on a time scale much shorter than 1 psec [27,28].…”