Lifetime of the excited state in vivo. II. Bacteriochlorophyll in photosynthetic bacteria at room temperature

Lifetime of the excited state (tau) of bacteriochlorophyll (BChl) in photosynthetic bacteria, measured with a mode-locked argon laser (oscillating at 488 nm; mode locked at 56 MHz) as light source, ranged from 0.3 to 2.5 nsec. These tau values are reported with a precision of +/-0.1 nsec. The value of tau at high exciting light intensity (I) was two to three times that at low intensity. For young cultures of green bacterium Chloropseudomonas ethylicum, tau ranged from 0.5 (low I) to 1.0 nsec (high I); for those of the purple bacterium Rhodospirillum rubrum, from 0.4 (low I) to 1.0 nsec (high I); and for those of the BChl b-containing Rhodopseudomonas viridis, from 1.0 (low I) to 2.5 nsec (high I). These data provide information regarding the efficiencies of the photochemical process in these bacteria. Quantum yield (ø) of BChl fluorescence, calculated from ø = tau/tau(0) (where tau(0) is the intrinsic lifetime of fluorescence), ranges from 2-6% at low intensities to 6-14% at high intensities.

Migration of electronic energy from chlorophyll b to chlorophyll a in solutions

Absorption, emission, and fluorescence excitation spectra of pure solutions of chlorophyll a (Chl a) and chlorophyll b (Chl b) in diethyl ether and of equimolecular mixed solutions of the two pigments, were determined at room temperature as functions of concentration (in the range from 5 x 10(-6) M to 4 x 10(-3) M) and of wavelength of the exciting light (in the regions 380-465 and 550-650 nm). The efficiency of energy transfer from Chl b to Chl a, derived from these data, was found to depend on the wavelength of exciting light. Furthermore, the transfer efficiency calculated from sensitization of Chl a fluorescence by Chl b was substantially smaller than that calculated from quenching of Chl b fluorescence by Chl a. Both these effects are tentatively explained as evidence of superposition of a "fast" energy transfer (taking place before the Boltzmann distribution of vibrational energy had been reached) upon the "delayed" transfer, which takes place after vibrational equilibration. The first-named mechanism is made possible by overlapping of the absorption bands of the two pigments; the second, by overlapping of the emission band of Chl b and the absorption band of Chl a. The first mechanism can lead to repeated transfer of excitation energy between pigment molecules, the second only to a one-time transfer from the donor to the acceptor. Both mechanisms could be of the same, second-order type, with the transfer rate proportional to r(-6). An alternative is for the fast mechanism to be of the first order, with the transfer rate proportional to r(-3), but spectroscopic evidence seems to make this alternative less probable.

The relationship between P-680 and C-550

The published reports of flash-induced absorbance changes in the 680-690 nm spectral region, which have been attributed to bleaching of the primary reaction center chlorophyll of photosystem II (PSII) P-680, are discussed in light of what is known about the primary electron acceptor of PSII, C-550. The question of whether the fluorescence yield changes, which accompany the photoreduction of C-550, might influence the measurements of chlorophyll bleaching is examined. The responses attributed to P-680 and their relationship to C-550 indicate that, if the absorbance measurements are valid, P-680 probably functions as the primary electron donor to PSII rather than as a photochemical sensitizer of the primary redox reaction.

Interactions between ferredoxin and chlorophyll in a monolayer system

Ferredoxin (Fd) is surface active; both its area/molecule, A, and surface potential, ΔV, (at an air-water interface) vary with the pH of the subphase. From the surface isotherms (at pH 7.7) A and ΔV are 167 A2 and 170 ± 20 mV, respectively, when the surface tension is 10 dyne/cm.

Fd and chlorophyll a (Chl) appear to form a mixed monomolecular film. At pH 7.7 a maximum interaction between Fd and Chl is observed when the monolayer contains a mole ratio of Fd/Chl ≅ 2. Irradiaion of Chl-Fd films, in nitrogen atmosphere, results in a bleaching of Chl and an increase of ΔV. The quantum yield for this bleaching is estimated to be 0.4.

Light-induced electron-transfer reactions between chlorophyll a and hydrogenated pteridine derivatives in solution

Preliminary studies of the formation of cationic 2 - amino - 6, 7 - dimethyl - 4 - hydroxy - 5, 6, 7, 8 - tetrahydropteridine radicals in alcoholic solution are reported. The results indicate that the same type of radical is formed either by oxidizing agents like hydrogen peroxide and iodine, or in a photoinduced reaction with chlorophyll a. A similarity between the epr signal of the light-induced cationic pteridine radical and the photoinduced epr signal II in photosynthesis is observed.