Fluorescence lifetimes of Chlorella pyrenoidosa

The fluorescence decay kinetics of in vivo chlorophyll measured using low intensity excitation

We report fluorescence lifetimes for in vivo chlorophyll a using a time-correlated single-photon counting technique with tunable dye laser excitation. The fluorescence decay of dark-adapted chlorella is almost exponential with a lifetime of 490 ps, which is independent of excitation from 570 nm to 640 nm. Chloroplasts show a two-component decay of 410 ps and approximately 1.4 ns, the proportion of long component depending upon the fluorescence state of the chloroplasts. The fluorescence lifetime of Photosystem I was determined to be 110 ps from measurements on fragments enriched in Photosystem I prepared from chloroplasts with digitonin.

A picosecond pulse train study of exciton dynamics in photosynthetic membranes

The fluorescence decay time of spinach chloroplasts at 77 degrees K was determined at 735 nm (corresponding to the photosystem I emission) using a train of 10-ps laser pulses spaced 10 ns apart. The fluorescence lifetime is constant at congruent to 1.5 ns for up to the fourth pulse, but then decreases with increasing pulse number within the pulse train. This quenching is attributed to triplet excited states, and it is concluded that triplet excitons exhibit a time lag of about 50 ns in diffusing from light harvesting antenna pigments to photosystem I pigments. The diffusion coefficient of triplet excitons is a least 300--400 times slower than the diffusion coefficient of singlet excitons in chloroplast membranes.

Picosecond time-resolved study of MgCl2-induced chlorophyll fluorescence yield changes from chloroplasts

The MgCl2-induced chlorophyll fluorescence yield changes in broken chloroplasts, suspended in a cation-free medium, treated with 3,-(3',4'-dichlorophenyl)-1,1-dimethylurea and pre-illuminated, has been investigated on a pico-second time scale. Chloroplasts in the low fluorescing state showed a fluorescence decay law of the form exp --At1/2, where A was found to be 0.052 ps-1/2, and may be attributed to the rate of spillover from Photosystem II to Photosystem I. Addition of 10 mM MgCl2 produced a 50% increase in the steady-state fluorescence quantum yield and caused a marked decrease in the decay rate. The fluorescence deday law was found to be predominantly exponential with a 1/e lifetime of 1.6 ns. These results support the hypothesis that cation-induced changes in the fluorescence yield of chlorophyll are related to the variations in the rate of energy transfer from Photosystem II to Photosystem I, rather than to changes in the partitioning of absorbed quanta between the two systems.

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part I. In the intact alga

The wavelength-resolved fluorescence emission kinetics of the accessory pigments and chlorophyll a in Porphyridium cruentum have been studied by pico-second laser spectroscopy. Direct excitation of the pigment B-phycoerythrin with a 530 nm, 6 ps pulse produced fluorescence emission from all of the pigments as a result of energy transfer between the pigments to the reaction centre of Photosystem II. The emission from B-phycoerythrin at 576 nm follows a nonexponential decay law with a mean fluorescence lifetime of 70 ps, whereas the fluorescence from R-phycocyanin (640 nm), allophycocyanin (660 nm) and chlorophyll a (685 nm) all appeared to follow an exponential decay law with lifetimes of 90 ps, 118 ps and 175 ps respectively. Upon closure of the Photosystem II reaction centres with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination the chlorophyll a decay became non-exponential, having a long component with an apparent lifetime of 840 ps. The fluorescence from the latter three pigments all showed finite risetimes to the maximum emission intensity of 12 ps for R-phycocyanin, 24 ps for allophycocyanin and 50 ps for chlorophyll a. A kinetic analysis of these results indicates that energy transfer between the pigments is at least 99% efficient and is governed by an exp --At1/2 transfer function. The apparent exponential behaviour of the fluorescence decay functions of the latter three pigments is shown to be a direct result of the energy transfer kinetics, as are the observed risetimes in the fluorescence emissions.

Picosecond fluorescence from photosynthetic systems in vivo

Picosecond time-resolved fluorescence emission from the pigments of intact photosynthetic systems and isolated pigment-protein fractions has been used to probe the mechanism of energy transfer and the organization of the pigments. The fluorescence kinetics of chlorophyll and the phycobilins of the red alga, Porphyridium cruentum, are governed by time-dependent kinetics, but the observed time dependence of the chlorophyll a fluorescence decay from dark-adapted Chlorella pyrenoidosa and spinach sub-chloroplast fractions is still open to conjecture. In contrast to the green plants containing only chlorophyll and carotenoids, Porphyridium shows distinct emission bands for each the pigments in the transfer sequence. The rate of energy transfer in vivo has the empirical form: dS/dt = -1/2S At-1/2, where S is the excited-state population of the donor pigment and A is the overall rate of energy transfer to the acceptor pigment. The kinetic analysis can describe closely the observed fluorescence risetimes and lifetimes of the photosynthetic pigments of Porphyridium. The extremely rapid rates of energy transfer, determined by this treatment, imply that exciton migration within each pigment bed of the phycobilisome is less extensive than in the chlorophyll-antenna systems. Changes in the fluorescence yield and decay kinetics of chlorophyll a and allophycocyanin in vivo can be induced at high excitation intensities by exciton-exciton annihilation.

Dynamics of excitions created by a single picosecond pulse

A theoretical analysis of bimolecular annihilation in finite domains is presented. A Pauli master equation is formulated for the case of varying incident delta function excitation sources. Expressions for the quantum fluorescence yield and its time dependence are derived. The relationship between the fluorescence yield and the number of hits per domain depends on two parameters: the rate constant of bimolecular exciton annihilation and the dimension of the domain in which this annihilation occurs. Recent experimental results imply that the exciton diffusion constant (D) is large (D approximately to greater than 10(-3) cm2 S-1) and that the photosystem II domains may contain as many as five photosynthetic units. An analysis of the time decay of the fluorescence indicates that, for a few hits per domain, the decay may be considered as exponential but for many hits it becomes non-exponential. Thus the fluorescence decay depends on the intensity of the excitation source and/or on the dimension of the domains. Conditions which change the effective size of the domain may change the shape of the fluorescence decay. Some biological consequences and experimental applications of this theory are presented.