TEMPERATURE-DEPENDENCE OF THE INITIAL ELECTRON-TRANSFER KINETICS IN PHOTOSYNTHETIC REACTION CENTERS OF CHLOROFLEXUS-AURANTIACUS

Absorbance changes in the near infrared were measured in photosynthetic reaction centers of the thermophilic, gliding, green bacterium Chloroflexus aurantiacus over the temperature range from 80 to 320 K, using 605-nm excitation pulses with an autocorrelation width of about 0.8 ps. Spectra collected during the first 1.5 ps after excitation show stimulated emission from the excited singlet state P*, absorbance increases at 762 and 802 nm, and absorbance decreases in the 865-nm band and at 783 and 815 nm. The absorbance changes associated with the formation of P* suggest that the electronic transitions in the ground-state spectrum are strongly mixed. Spectra collected up to 90 ps after the excitation pulse show the decay of the stimulated emission, and bleaching at 750 nm and 815 nm, reflecting the transfer of an electron from the bacteriochlorophyll dimer (P) to the initial electron acceptor(s). The electron-transfer reaction is slower in C. aurantiacus than in reaction centers of purple bacterial species; P* decays with a time-constant of 7.1 +/- 0.5 ps at 296 K and 9.0 +/- 0.1 ps at 320 K. At lower temperatures, the kinetics are non-exponential. The decay of the stimulated emission at 80 K can be fit either with a biexponential function with time constants of 3.0 +/- 0.1 and 32 +/- 12 ps, or with a stretched exponential function [exp - (t/tau-0)alpha] with tau-0 = 4.1 +/- 1.2 ps and alpha = 0.45 +/- 0.06. Several models that might account for the nonexponential decay kinetics of P* are discussed. A model based on rapid interconversion among conformational states in P* provides a simple interpretation of the temperature dependence of the kinetics.