Photosynthesis Research 10." 3 0 9 - 3 1 8 (1986) © Martinus N i j h o f f Publishers, Dordrecht - Printed in the Netherlands

309

REr'RINISNS OF CHLOROPHYLL FLUORESCERCE REVISITED: PROMPT EMISSION FROM PHOTOSYSTEM II WITH CLOSED REACTION CENTERS ?

OR

DEI~YED

D-4330

M~lheim

Exciton

decay,

G.H. SCHATZ AND A.R. HOLZWARTH

Max-Planck-lnstitut f~r Strahlenchemie, a.d. Ruhr, West Germany

Stiftstr. 34 - 36,

Kevwords: Charge recombination, Charge separation, Fluorescence kinetics, Kinetic model, Photosystem II.

Abstract. This paper proposes a model which correlates the exciton decay kinetics observed in picosecond fluorescence studies with the primary processes of charge separation in the reaction center of photosystem II. We conclude that the experimental results from green algae and chloroplasts from higher plants are inconsistent with the concept that delayed luminescence after charge recombination should account for the long-lived (approx. 2 ns) fluorescence decay component of closed photosystem II centers. Instead, we show that the experimental data are in agreement with a model in which the long-lived fluorescence is also prompt fluorescence. The model suggests furthermore that the rate constant of primary charge separation is regulated by the oxidation state of the quinone acceptor QA"

1. I n t r o d u c t i o n A hypothesis promoted by Klimov et al. (s~e (I,2) for summarizing reviews) postulates a charge recombination (P680- Pheo- QA > P680 Pheo QA ) between_the oxidized primary donor, P680 +, and the reduced primary acceptor, Pheo , in reaction centers (RC) of photosystem II (PS II) with a reduced first quinone acceptor, Q~. It was suggested that this charge recombination process is associated wi~h light emission from reexcited chlorophylls (Chl). This emission has been termed delayed (in contrast to prompt) fluorescence or recombination luminescence and was considered as origin of the 3 to 5 fold increase in Chl fluorescence yield which is associated with the reduction of Q~(3). The Klimov hypo~%esis has also been adopted to explain the slow (approx. 2 ns) component of time-resolved fluorescence studies (4,5). However, several recent experimental findings cannot be reconciled with this concept. In this paper, after summarizing the conflicting aspects, we present a kinetic model which permits a quantitative test of the compatibility of the Klimov hypothesis with experimental results of fluorescence decay kinetics.

2. R e s u l t s o f p i c o s e c o n d f l u o r e s c e n c e

studies.

Picosecond Chl fluorescence experiments with green algae and higher plant chloroplasts reported during the past years have commonly been analyzed in terms of a sum of 3 exponentials: a short-lived component (T(I/e)~50-150 ps), a middle component (T(I/e) ~ 450-750 ps) and a long-lived one (T(I/e)=1400-2300 ps) (see reviews (5-7)). From many studies screening the

[1631

310

[164]

influences of various parameters (e.g. redox potential, light intensity, inhibitor concentration) on these decay components, one result was unambiguous: the increase in the yield of the long-lived component upon reduction of Q~. The yield of the short-lived component was found to decrease upon reduction of Q~ in green algae (8). With spinach chloroplasts such behaviour of the rapid ~ecay was not always (9,10) so evident. Recently, the spectral resolution of fluorescence decay components allowed us to separate PS I- from PS II-emissions at room temperature in Chlorella (11) and Scenedesmus (12). Furthermore, the kinetic analysis for the resolution of more than 3 exponentials has been improved. The results c a n be summarized as follows: i. Emission from P S I is red-shifted with respect to that from PS II. It decays with ~(I/e)~80 ps. Both, amplitude and lifetime are not affected by the reduction of QA" ii. ~ii other fluorescence decay components, attributed to PS II, show identical spectral shapes, with maxima at 685 nm, strongly suggesting an origin from the same chromophore(s). iii. Contributions to the corresponding time resolved excitation spectra by chlorophyll b (11) show that the light harvesting complex (LHC) has no separate emission at variance with earlier interpretations (4,8,13). iv. With open centers (F) PS II emission is characterized by T, (I/e)=200-300 ps and T^~1/e)=500-650 pS, with closed centers (F ) by T3(1/e)=1150-1350 ps a~d ~4(1/e)= 2200-2400 ps. max v. T I ~nd T. can be attribufed to open and closed PS II -centers, respectively, and T~ and T3 to the corresponding states ~f PS II Acenters (7,12), as proposed in-(14). vi. A quantitative comparison (12) shows that the sums of the amplitudes (not yields) of the decay components in the F O state equal those in the Fma x state, i.e. (g1+g4)=(g?+g3). Thus, in any one of the extreme-stat~s [completely open or closed) the decay of each type of center (u or ~) can be characterized by a single exponential! In intermediate states all four of these PS II contributions should be present together (not yet resolved experimentally). Upon closing a reaction center, the increase in amplitude of the long-lived component is counterbalanced by the concomitant decrease in amplitude of the short-lived decay component in each type of center. It is proposed that upon reduction of QA the decay components with ~3 and T 4 replace those with T. and T . Such a pairwise complementary relationship b~tween the amplitudes ] of t~e shortand the long-lived components in open and closed centers, respectively, was already found in (8). However, owing to the limitations by the conventional three-exponential analysis at a single emission wavelength, the ~-center and the PS I contributions had not yet been resolved at that time. It was then recognized by Butler and coworkers (14) that the middle component of the data in (8) might well have reflected this second type of PS II centers. Their heterogeneous bipartite model (14,15) predicted a discrimination between the contributions by u- and ~-centers. We want to emphasize that the above mentioned pairwise complementary relationship between the amplitudes of the decay components represents the decisive experimental result which conflicts with the hypothesis of recombination luminescence by the following reasoning: The fast phases observed with open reaction centers are interpreted (8,11) (15,16) to reflect the overall energy migration / charge separation process. The mechanism o~ energy migration in the antenna is unlikely to be affected when the RC becomes closed (15). Therefore, the substitution of the fast by

[1651

311

the slow phase upon closing PS II (11) is assumed to reflect a diminished efficiency of primary charge separation in the closed RC. The alternative interpretation of the slow phase as a charge recombination process (I,2,4) necessarily requires a preceding efficient charge-separation. The latter should be indicated by the presence of both, the fast plus the slow phase under • F a x -conditions. This is in contrast to the above mentioned experlmen~s (11,12), whlch show the absence of fast PS If-phases under such conditions (within an accuracy of 5%). This qualitative argument will now be corroborated by a kinetic model which takes into account both processes, charge separation and charge recombination.

3.

Kinetic model: ~nergy_ p a r t i t i o n between c h l o r o p h y l l - e x c i t o n s and the r a d i c a l p a i r [P680- Pheo ]

For each of the PS II units (a and ~) we propose a model which assumes: (a) Chlorophylls of the core and of the LHC form a tightly coupled antenna domain. (b) Trapping of an exciton by P680 from the antenna chlorophylls is reversible (shallow trap) and reaches equilibrium within a time short compared to the overall exciton decay time. Thus, the probability for P680 being excited is statistically small (approx. 1/200 in PS II units), because inversely proportional to the antenna size. Under such a conditions, the apparent rate of primary charge separation is trap limited, i.e. approximately proportional to the probability of P680 being excited (17). This model is represented by the following scheme : k1

, A

I

k2 ) B

(

) C

(scheme A)

k_ I

k a = k d + kra d A

A denotes a strongly coupled domain of chloro~hylls (antenna chlorophylls plus P680), B is the singlet radical pair [P680- Pheo-], and C represents the product(s) of primary charge stabilization and/or loss processes (including triplet formation and decay to the ground state). Rate constants k represent: radiative decay (krad) , radiationless decay (kd) , primary charge separation (kl) , charge recombination to the excited sta£e (k ~) and processes of charge stabilization, triplet formation and recombination to the ground state (k2). The latter processes are assumed to be irreversible on the nanosecond time scale. In this model different states (open or closed) of the RC can be represented by different sets of rate constants. With open RCs a high yield of charge stabilization requires k21k1>>k_1. With closed RCs, electron transfer to Q~ (B to C) is blocked and the value of k 2 reduced as compared to open RCs. aThe model should meet the following criteria in the extreme states, F and F , of PS II units: (a) (almost) . . . o . a . a monoexponentlal decay klnet~cs; (b) 11~e~imes of about 200 ps and about 2 ns, respectively; and (c) about identical amplitudes of the corresponding decay components. The differential equations for this model are given in the appendix. The time dependence of A ~, the fluorescence emitter, results in: [A+](t) = [A+] ° (g exp(- t/Ta) + g=exp(- t/Tb)} (I) where the amplitude factors (gi) and lifetimes (To) are functions of the rate constants k (see appendix). Eq. (I) genezal~y describes a biexponential kinetics for a given set of rate constants, both for completely

312

[1661

open and completely closed RCs, obviously is in contrast to experimental findings (see above, items iv. and v.). Nevertheless, this model possibly describes the real situation if one of the two exponential terms becomes so fast or so small in amplitude that it escapes experimental detection. This possibility is examined by the following numerical solutions. 4. ~ u m e r i c a l r e s u l t s U p p e r and l o w e r l i m i t s f o r v a l u e s o f t h e r a t e c o n s t a n t s a r e c h o s e n a s f t , l o w s : F o r k A = k d + k r a d we s u g g e s t t h e r a n g e 0 . 2 n s ! kA ! 0.8 ns . *

corresponding to the lifetimes of Chl in organic solvents (5-6 ns (18)) and in isolated Chl-protein complexes (19), respectively. In our model, the apparent rate constant for charge separation is limited by the trapping time of excitons correponding to the experimentally observed fast (= 200 ps) PS II fluorescence decay in open RCs. Therefore, we use the value k I = -I 5 ns . In a photosystem with an antenna of ~ 200 Chl/RC such a value for exci~on trapping would correspond to a time of ! I ps for the step P680 Pheo*P680-Pheo . This is in the same order of magnitude as the values of bacterial RCs. Plausible ranges for values of k 2 are considered to be 5 ns-1 < k 2 ! 20 ns -I for open and 0 ns -I ! k 2 ! 0.5 ns I for closed RCs, respectiveIy. Table I shows the data calculated for four qualitatively different cases of the model: -I In CASES A and ~ ( F and F_~_), the value of k . = 0.35 ns is chosen U a . -] corresponding to the suggested £1me constant of approx. 3 ns for charge recombination in closed RCs (I,20). For open centers (CASE A) an almost monoexponential fast fluorescence decay can be predicted assuming high charge stabilization rates. In this case the mentioned criteria are well met (+ signs in Table I). CASE B assumes that closing of the RC will affect only th~ rate con§rant of charge stabilization. Formation of the radical pair [P680 Pheo- QA ] is possible with high yields provided that [P680 PheoQ-] he still photochemically active as suggested by Klimov (see e.g. (I)). It is evident, that CASE B fails to fit the experimental data ( - in Table I), since the contributions by the fast phases remain dominant. Hence, the numerical results corroborate the qualitative argument against the recombination concept. Therefore, we propose that reduction of QA must also affect the rate of charge separatiOn,ak As shown in CASE C, ~enfold decrease of k I results in a much larger contribution by a 2.2 ns component and in an equivalent decrease of the fast phase. An additional increase of the charge recombination rate by a factor of 10 to 20 predicts fluorescence kinetics close to that observed. In this case the yield of radical pair formation is considerably lowered. This is not unreasonable, since the electric field due to the negative charge on QA may affect both rate constants, k I and k I" It has been reported that the Chl fluorescence yield can he reversibly increased from F n to Fm. x when an artificial membrane potential is applied by external electric T~eld pulses (21). The fluorescence yield calculated for CASE C is about tenfold increased over that for open RCs (CASE A), as expected for PS II units. For PS If. centers, this factor should be smaller as judged from th~ lifetime ratio ,~/,^ = 2. Altogether, the F /F -ratios result in an J . max o average of 4-5 known ~or photosynthetic membranes. The total fluorescence yield, 0~, in photosynthetic membranes is approximately 3% for open (22) and about 12% for closed centers. Similar values are predicted by our model

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314

[168]

(see Table I, CASES A and B). CASE D is adapted to a hypothesis by Breton (23). It assumes that the fluorescence kinetics at F reflect the charge stabilization rather than O

,

the charge separation process. In this case, the decay of A is limited by k~ and all preceding steps have to be much faster. Assuming that the c~Iculated fast decay phase (~ 20 ps) is too rapid to be detected experimentally, this model would indeed fit the observed lifetimes. However, several severe problems remain: the predicted fluorescence yield would be far too low and almost invariable. Furthermore, the apparent time for charge separation (= 20 ps) is surprisingly short: it is shorter than the expected first passage time which is required for an exciton to pass through a 200 Chl/RC antenna towards the trap, as extrapolated from recent studies on the P S I antenna (16). In addition, the time corresponding to the elementary step of charge separation ought to be shorter than approximately 0.1 ps. We have tried to find also rate parameters describing the fluorescence decay kinetics attributed to 8-centers (CASE .E). They are quite well mimicked by introducing two changes with respect to the ~-centers: an increase in k A for spillover of excitation energy from 8-centers towards PS I (24), and a decrease in k I for a lower photochemical efficiency. 5. D i s c u s s i o n The combination of CASES A and C, which describe best the observed fluorescence kinetics, can be characterized as follows: excitons are equilibrated rapidly (too fast to be resolved by current single photon timing instruments) between antenna- and RC-Chl. In open RCs equilibrated excitons are mainly used for photochemistry and decay with kl, the limiting apparent rate constant of charge separation. The primary ~adical pair is stabilized at a rate probably faster than that for apparent charge separation. Thus, most of the excitation energy is very rapidly (k~ ! k I) converted into free energy of stabilized products. Consequentlg, the relative radical pair concentration will not exceed 35 % at any time (see RP in • max Table I). In closed RCs the yield of charge separation is most likely reduced by reduction of k I and increase of k 1" Due to the lower probability of charge separation, the disappearance of'the antenna excitons is no longer controlled by the k I process but rather by intrinsic deactivation processes for antenna Chl (k~). We note that already a tenfold decrease of k.] suffices to increase the-fluorescence yield by a factor of almost 5 (e.g. from 2.0 % to 9.6 %, see Table I). Our calcul~tions_show furthermore, that in closed RCs relative concentrations of [P680- Pheo ] in the range of 5-10% (e.g. 5.3% or 8.8%) may be obtained. Its maximal value is reached a~ 400-§00 ps after excitation (e.g. 393 ps or 586 ps). The decay of [P680 Pheo ] is controlled by Tb = I/~ b (see eq A.4). Experimentally observed processes as e.g. spin polarized triplet formation (25&26) and photoinduced accumulation of Pheo-, both proceeding via [P680-PheO-QA-], can be accommodated by this model. Pheo- formation is reported with+yields between 0.002 and 0.01 (27,28) corresponding to a competitive P680- reduction for about 1/10 to 1/50 of the maximal radical pair concentration, RPmax, in CASE B. Based on the radical pair lifetime of approx. 2 ns this suggests a P680 + reduction within 20 ns ! T(I/e) ! 100 ns under conditions of low redox potential (E ! -430 mV). In our model, as expressed in scheme A, the relative [P680 + Pheo-] concentration depends on the antenna size. This is a consequence of the

[1691

315

dependence of the excitation probability for P680 on the number of Chl taking part in exciton equilibration. The apparent rate constant of charge separation, kl, shows the same dependence. Reduction of the antenna size would lead to a reciprocal increase in k I and, as is evident from the data in Table I, to higher temporary [P680 + Pheo-] concentrations. In the extreme case of isolated RCs, the probability of radical pair formation approaches values close to I, even when the RCs are closed. Therefore, high yields of radical pair formation and significant recombination may be expected only in isolated RCs and in particles with a very small antenna. For photosystems in vivQ, however, these processes appear to be insignificant. This indicates that there is no correlation of the slow fluorescence decay component from algae and chloroplasts from higher plants with a process requiring radical pair formation, be it reexcitation of Chl or of Pheo (29). It is suggested that a large antenna may serve two principal purposes: optimized light capturing properties by high absorption cross section, and protection of the RC against loss processes by enabling efficient photochemistry at low levels of radical pair formation. We also emphasize the fac t that the rise of [P680 + Pheo-] (B) is not associated with a rise of Chl (A) in any of the model cases (only positive values of amplitudes gi ) . Recently, a 200 ps risetime of a 1.2 ns fluorescence decay has been reported and taken as evidence for recombination luminescence in Chlorella (30). This result was obtained after subtraction of the fluorescence kinetics with open RCs from those with (partially) closed RCs, while the individual kinetics did not show any rise term. We doubt that such subtraction of exciton kinetics arising from two different RC-states can provide information about processes characteristic for one of these states (Fma ) " In our model the socalled variablp = ~ ma - F o , does .not. originate from a special fluorescence yield, Fva fluorescing species, whlc~ would a~d to the emlsszon from open RCs as they become closed. Rather, the increased fluorescence yield is due to the increase of the average lifetime of the same emitters! Therefore, F is vat. an operational term from CW measurements and does not represent an zaentifyable part of picosecond time resolved fluorescence. The modified bipartite model (14,15) and our model have in common that both explain long-lived fluorescence components without using the Klimov mechanism, since both include excitation transfer from P680 back to the strongly coupled antenna. The differences are the following: the bipartite model refers to exciton partition between antenna- and RC-Chl; it does not include the reversibility of primary photochemistry. It results in a biexponential decay kinetic for each type of center in any one of the states, F^ and F (15), and it fails to simulate the decrease in amplitude ~f the f a ~ X p s II phase upon closing the RC. If extended to account also for charge recombination, more complex, i.e. threeexponential, kinetics would be expected. Our model ~ocuses on the primary photochemical steps of energy partition between P680 and the radical pair. Antenna properties are still important; they are included not as explicit parameters but as an implicit probability function for the excitation of P680. With parameters accounting for photophysical as well as photochemical properties of PS II RCs, this model describes apparently monoexponential exciton decay kinetics and thus meets the criteria given above. One of the possibilities to test the different cases of the model is to determine the activation energies of the rate limiting step (k I ) . It is expected to be positive for any contribution by recombination luminescence,

[170]

316

while being close to zero for prompt fluorescence. Chl fluorescence yields have been observed as a function of temperature at F 0 (2,31,32), F_a x (32) and FO" , after light induced quenching of Fma x upon photoaccumula~lon of Pheo- (2,31). Results were contradictory: In (31) and (2) an activation energy of 0.04-0.08 eV was found, while in (32) and (33) it was concluded that there is no evidence for such an activation energy. More information should be obtained from measurements of nanosecond and picosecond absorption changes, which could directly monitor the radical pair. The only such repor~ (20) §bowed a 4 ns decay with a difference spectrum attributed to [P680- Pheo ] in closed RCs. It was observed in a PS II particle (TSF 2a) of small antenna size. But the decay phase was associated with a low yield of radical pairs. It was assumed to reflect the same process as a slow component of fluorescence decay measurements presented in the same paper, but obtained with a less purified PS II preparation (TSF 2, with a typically threefold larger Chl/RC ratio than TSF 2a (34)). In view of these facts a straightforward correlation of the two kinetics should deserve more careful experiments. Appendix The differential equations for the kinetic system in scheme A are: *

*

d[A ]/dt = -(kA+kl)[A ] + k 1 [ B ]

(A.I)

d[B]/dt = k1[A ]-(k_1+k2)[B ]

(A.2)

With the initial boundary values for t=O [A ]o = [Ao]

and

*

where

[B]o = 0 they have the following solutions (35) *

[A ] = [Ao]{ga exp(-~at) + gb exp(-~bt)}

(A.3)

[B] = [Ao]{kl/(~a-~b)}{-exp(-~at)

(A.4)

+ exp (-~ht)}

ga = (X-~b)/(~a-~b)

(A.5)

gb = (Ta-X)/(~a-~b)

(A.6)

7a = I/2 {(X+Y) + [(X-Y) 2 + 4 klk_1] I/2)

(A.7)

~b

(A.8)

I/2 {(X+Y) - [(X-Y) 2 + 4 klk_1 ]I/2}

Ta = I/X a

(A.9)

Tb = 1/X b

(A.IO)

X = kA + k I

(A.11)

Y = k_1 + k 2

(A.12)

The fluorescence yield is given as: w

@F = I/[Ao]

[ [A ] kra d dt o

(A.13)

[171]

317

With eq. (A.3) and assuming krad to be constant one obtains (A.14)

0 F = krad[gaX a +gbTb ]

The maximal value of the relative concentration of the radical pair Pheo ], RPmax, is obtained after setting d[B]/dt = 0 as: kl

[B]max -

LtVb/~a) [

~b /

(~a-~b)

~a/(~a-~b )] (~b/~a)

[P680+

(A.15)

~a-~b at the time tmax = in(la/~b)/(~a-~b)

(A.16)

AcknowledQement We are grateful to Dr. H.-P. Schuchmann for translating reference (33), published in Russian. We thank Prof. K. Sauer for critically reading the manuscript and Prof. K. Schaffner for his interest and support.

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318

References 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21 22 23. 24 25. 26 27 28 29. 30. 31. 32. 33. 34. 35. 36.

Klimov,W and Krasnovsky,AA (1982) Biophys. 27:186-198 KlimoviW (1984) in Adv. in Photosynthesis Research; Proc. 6th Int. Congr. Photosyn. I :131-138 Duysens,LNM and Sweers,H (1963) Microalgae and Photosynth Bact :353-372 Haehnel,W, Nairn,JA, Reisberg,P, and Sauer,K (1982) Biochim. Biophys. Acta 680:161-173 Karukstis,KK and Sauer,K (1983) J. Cell. Biochem. 23:131-158 Holzwarth,AR (1985) in Encyclopedia Plant Physiology: Photosynthetic Membranes Eds: LA Staehelin and CJ Arntzen, in press, Springer, Berlin Bolzwarth,AR (1986) in Topics in Photosynthesis Ed: J Barber, in press, Elsevier, Amsterdam Baehnel,W, Holzwarth,AR, and Wendler,J (1983) Photochem. Photobiol. 37:435-443 Karukstis,KK and Sauer,K (1983) Biochim. Biophys. Acta 725:246-253 Karukstis,KK and Sauer,K (1983) Biochim. Biophys. Acta 722:364-371 Holzwarth,AR, Wendler,J, and Haehnel,W (1985) Biochim. Biophys. Acta 807 :155-167 Kaminski,J (1985) Acta Phys. Pol. A 67:679-700 Nairn,JA, Haehnel,W, Reisberg,P, and Sauer,K (1982) Biochim. Biophys. Acta 682:420-429 Butler,WL, Magde,D, and Berens,SJ (1983) Proc. Natl. Acad. Sci. USA 80:7510-7514 Berens,SJ, Scheele,J, Butler,WL, and Magde,D (1985) Photochem. Photobiol. 42:59-68 Gulotty,RJ, Mets,L, Alberte,RS, and Fleming,GR (1985) Photochem. Photobiol. 41:487-496 Pearlstein,RM (1982) in Photosynthesis I: Energy Conversion by Plants and Bacteria :293-331 Kaplanova,M and Parma,L (1984) Gen. Physiol. Biophys. 3:127-134 Lotshaw,WT, Alberte,RS, and Fleming,GR (1982) Biochim. Biophys. Acta 682:75-85 Shuvalov,VA, Klimov,VV, Dolan,E, Parson,WW, and Ke,B (1980) FEBS Lett. 118:279-282 Meiburg,RF, van Gorkom,HJ, and van Dorssen,RJ (1983) Biochim. Biophys. Acta 724:352-358 Latimer,P, Bannister,TT, and Rabinowitch,E (1956) Science 124:585-586 Breton,J (1983) FEBS Lett. 159 :I-5 Butler,WL (1978) Ann. Rev. Plant Physiol. 29:345-378 Rutherford,AW (1983) in The Oxygen Evolving System of Photosynth. :63-69 Evans,MCW, Atkinson,YE, and Ford,RC (1985) Biochim. Biophys. Acta 806:247-254 Klimov,W, Klevanik,AV, Shuvalov,VA, and Krasnovsky,AA (1977) PEBS Lett. 82:183-186 Diner,BA and Delosme,R (1983) Biochim. Biophys. Acta 722:452-459 Breton,J (1982) FEBS Lett. 147:16-20 Mauzerall,DC (1985) Biochim. Biophys. Acta 809:11-16 Klimov,W, Allakhverdiev,SI, and Pashchenko,VZ (1978) Dokl. Akad. Nauk. SSSR. 242:1204-1207 Mathis,P (1984) in Adv.in Photosynthesis Research; Proc. 6. Int. Congr. Photosynth. 1:155-158 Voznyak,VM, Sevdimaliev,RM, and Kim,VA (1984) Stud. Biophys. 99:59-66 Klimov,W, Ke,B, and Dolan,E (1980) FEBS Lett. 118:123-126 Demas,JN (1983) in Excited State Lifetime Measurement. :I-273 Borisov,AY and Ii'ina,MD (1971) Biochem. USSR 36:693-695