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

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600 nm laser pulses of 20 fs (full width at half maximum) were monitored in the wavelength region of 420–560 nm. 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 (BB) in the inactive B-branch is inferred to dominate this effect. The excitation of BB at 600 nm leads to a transition into the S2 state (Qx band), which is followed by rapid internal conversion to the S1 state (Qy 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 S2 to S1 of BB. The evaluation of the kinetics leads to a value of 30 fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

► High rate of Qx− Qy relaxation of BB molecule in the RC of Rb. sphaeroides was determined.
► This rate was measured as a rate of an electrochromic bandshift of Car molecule.
► Car bandshift was not observed in experiments on isolated Car molecules.
► Effect due to the bandshift was absent in the carotenoid-less strain R-26.
► The bandshift was considerably less in the HM182L mutants of RCs.