Lifetime of the Excited State In Vivo: I. Chlorophyll a in Algae, at Room and at Liquid Nitrogen Temperatures; Rate Constants of Radiationless Deactivation and Trapping

Abstract

Using a mode-locked laser (lambda, 632.8 nm), fluorescence decay of chlorophyll (Chl) a in the green alga Chlorella pyrenoidosa, the red alga Porphyridium cruentum, and the blue-green alga Anacystis nidulans was measured by the phase-shift method under conditions when photosynthesis was not operative (3-(3,4-dichlorophenyl)-1,1-dimethylurea [DCMU] poisoning, or cooling to 77 degrees K). In the presence of 10(-5) M DCMU, the lifetime of Chl a fluorescence (tau) at room temperature is about 1.7 nsec in Chlorella, 1.0 nsec in Porphyridium, and 0.7 nsec in Anacystis. At 77 degrees K, tau is 1.4 nsec (for fluorescence at about 685 nm, F-685) and 2.3 nsec (for F-730) in Chlorella, 0.9 nsec (F-685) and 1.2 nsec (F-730) in Porphyridium, and 0.8 nsec (F-685 and F-730) in Anacystis. From the above measurement, and the assumption that tau(0) (the intrinsic fluorescence lifetime) for Chl a in all three algae is 15.2 nsec, we have calculated the rate constants of radiationless transition (that includes energy transfer to weakly fluorescent system I) processes competing with fluorescence at room temperature to be about 5 x 10(8) sec(-1) in Chlorella, 9 x 10(8) sec(-1) in Porphyridium, and 13 x 10(8) sec(-1) in Anacystis. At 77 degrees K, this rate constant for Chl a that fluoresces at 685 nm remains, in the first approximation, the same as at room temperature. From the tau data, the rate constant for the trapping of excitation energy is calculated to be about 1.2 x 10(9) sec(-1) for Chlorella, 2 x 10(9) sec(-1) for Porphyridium, and 2 x 10(9) sec(-1) for Anacystis. The efficiency of trapping is calculated to be about 66% (Chlorella), 68% (Porphyridium), and 60% (Anacystis). (It is recognized that variations in the above values are to be expected if algae grown under different conditions are used for experimentation.) The maximum quantum yield of Chl a fluorescence for system II (lambda, 632.8 nm), calculated from tau measurements, is about 10% in Chlorella, 6-7% in Porhyridium, and 5% in Anacystis under conditions when photosynthesis is not operative; the values at 77 degrees K appear to be very close to those with DCMU added at room temperature. ø for F-730 at 77 degrees K, however, is somewhat higher than for F-685. The predicted quantum yields of fluorescence for Chl a in intact cells (both systems I and II) at low intensities of 632.8 nm light are about 2-3, 1-2, and 1% for Chlorella, Porphyridium, and Anacystis, respectively.