The rate of Qx→Qy relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift

Abstract

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600nm laser pulses of 20fs (full width at half maximum) were monitored in the wavelength region of 420-560nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (B(B)) in the inactive B-branch is inferred to dominate this effect. The excitation of B(B) at 600nm leads to a transition into the S(2) state (Q(x) band), which is followed by rapid internal conversion to the S(1) state (Q(y) band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S(2) to S(1) of B(B). The evaluation of the kinetics leads to a value of 30fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Measuring the Ultrafast Correlation Dynamics between the Qxand QyBands in Chlorophyll Molecules

“…In the case of Chl a in pyridine, where the Q x relaxation lifetime is reported to be around 107 fs, the effect of the lifetime broadening on the analysis can be neglected. As for Chl-e6, although the internal conversion lifetime from the Q x state in methanol is not yet known to the author, we hypothesize that it is between the ranges of ∼50–100 fs as reported in most of the Chl analogue systems. ,,, Therefore, it is assumed here that this effect is also negligible.…”

“…As for Chl-e6, although the internal conversion lifetime from the Q x state in methanol is not yet known to the author, we hypothesize that it is between the ranges of ∼50−100 fs as reported in most of the Chl analogue systems. 1,3,14,22 Therefore, it is assumed here that this effect is also negligible.…”

Assignment of the Q-Bands of the Chlorophylls: Coherence Loss via Qx − Qy Mixing

“…The Q x → Q y relaxation times for chlorophyllides are known to vary considerably with both chemical composition and environment3839404142. These times can be determined from a complete spectral assignment.…”

“…100–200 fs39 or 100–400 fs depending on environment40, 4800 fs in Prosthecochloris aestuaril 41; processes with lifetimes as short as 30 fs have also been reported42.…”

Functional interpretation of the role of cyclic carotenoids in photosynthetic antennas via quantum chemical calculations

“…The system of electronic excited states of carotenoids, chlorophyll and oxygen form a dynamic interacting system [13][14][15]. Energy of the carotenoids S1 excited state is represented as a continuum of values (light blue box) that it takes for the various carotenoids.…”