Directed picosecond excitation transport in purple photosynthetic bacteria

ChemicalPhysics 128 (1988) 227-235 North-Holland, Amsterdam

DIRECI’ED PICOSECOND EXCITATION TRANSPORT IN PURPLE PHOTWWNTHETIC BACl-ERL4 A. FREIBERG ‘, V.I. GODIK b, T. PULLERITS Band K. TIMPMANN a ’ Institute of Physics, Estonian SSR Academy of Sciences, 202400 Tartu, USSR b A.N. Belozersky Laboratory of Molecular Biology and Bioorganic Chemistry Moscow State University, 119899 Moscow. USSR Received 22 February 1988

Picosecond spectrally resolved fluorescent measmements together with the coupled kinetic rate equation model simulations of the data have been employed to determine the rates and pathways of heterogeneous excitation transport in the membranes of photosynthetic bacteria Rhodobacter sphaeroides and Chromatium minutissimum. Excitation transport from bacteriochlorophyll molecules B800 to B850, belonging to the same pigment-pigment-protein complex BSOO-850,proceeds in l-2 ps at room temperature and at 77 IL The intercomplex excitation transport from B850 to B875 takes about 10 ps for most excitations. In the minor part of BSOO-850complexes, the excitation transport to B875 complex takes much longer, about 50 ps. The macroscopic rate constant of excitation trapping by open reaction centres is shown to be the same for all the bacteria studied, although the number of antenna molecules per reaction centre differs significantly. This seems to be an indication of the intrinsic homogeneity of the long-wavelength bacteriochlomphyll band, which facilitates an additional localization of excitations in the vicinity of the reaction centre and, due to the shortening of the trapping time, increases the overall quantum yield of photosynthesis.

1. Introduction The main principles of the very efficient conversion of light energy into an electrochemical form by photosynthetic organisms continue to be the subject of intense investigations. Two directions of research have proved to be the most informative in this respect: ( 1) biochemical structure investigations, including the crystallization of the pigment-protein complexes of both reaction centres (RCs) [ 1 ] and the light-harvesting antenna [ 21 as well as their high-resolution X-ray analysis; (2) the elucidation of the physical mechanisms of the primary events of photosynthesis with the use of modem methods of physical experiment, in particular, high time (up to subpicosecond time range) and spectral resolution fluorescence as well as absorption spectroscopy. The photosynthetic apparatus is built in such a way that the functions of light harvesting and the conversion of the absorbed energy into an electrochemical form are performed by different types of membrane pigment-protein complexes, those of the so-called light-harvesting antenna complexes and reaction

centre complexes, respectively. Hence, one of the main problems of primary photosynthesis is: how to transport the light energy absorbed by the light-harvesting pigments to RC pigments with minimal losses. There have been a lot of theoretical and experimental papers (for the latest review, see ref. [ 3]), which suggest that the underlying mechanism of it is the resonance excitation transfer via chlorophyll singlet states. For the case of purple bacteria this point has recently been quantitatively settled [ 45 1. The purple photosynthetic bacteria Rhodobacter sphaeroides and Chromatium minutissimum under study in this work contain light-harvesting complexes of two types: BSOO-850 (designation by two major near-infrared absorption maxima of bacteriochlorophyll a (BChl) at 800 and 850 nm) and B875 (characterixed by a single absorption maximum 870890 nm). Both B800-850 and B875 complexes have been shown to contain two pigment-binding polypeptides with the molecular weight of 6 to 12 kDa. Each pair of the polypeptides binds two (B875) or three (BSOO-850) BChl molecules and respectively one or two molecules of carotenoids. The B875 complexes usually occur in intact membranes in a fixed

0301Y0104/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

228

A. Freiberg et al. /Directed picosecondexcitationtransportin bacteria

proportion corresponding to 20-30 antenna BChl molecules per RC. The known variability of the size of the photosynthetic unit due to growth conditions (incident photon flux and oxygen pressure) is the result of the variability of the number of B800-850 complexes. It is currently accepted that B875 complexes surround and interconnect several RCs, forming the so-called B875-RC complexes, while those of B800-850 are arranged peripherally, interconnecting a number of B875-RC complexes. As to the characteristic electronic excitation transport times between the complexes and different BChl molecules, the existing data are scanty and conflicting: the values from about 1 ns [ 61 to several picoseconds [ 5,7] have been suggested for the B850-875 excitation transfer in Rb. sphaeroides. The first direct measurement of the transfer kinetics and rates performed with the use of spectrally resolved picosecond fluorescence [ 8,9] and picosecond absorption [ 10,111 techniques, have also given contradictory results in a number of important respects. In particular, the time of excitation transfer from B850 to B875 was determined in refs. [ 10,111 to be about as large as the macroscopic time of excitation trapping by open RCs, which is known to be equal to 60 ps [ 4,5 1. According to our data [ 8,9 ] this time is several times shorter and does not exceed 10 ps for most excitations. In the present work, a more thorough investigation of picosecond dynamics of excitations in the heterogeneous light-harvesting antennae of purple bacteria Rb. sphaeroides and Chr. minutissimum has been performed, including a theoretical analysis and computer simulations of the investigated processes, based on the coupled kinetic rate equation model. This approach enabled the construction of a consistent picture of the directed excitation energy transfer in these bacteria both at room temperature and 77 K.

and glycerol 1: 3 v/v, for low-temperature measurements. Most of the room temperature experiments were made with the use of a flow cell [ 5 1. The decay kinetics of the fluorescence excited by picosecond light pulses from a synchronously pumped modelocked cw dye laser were measured in a reflection mode in the 780-1000 nm range with a synchroscan streak camera setup of 3 ps time and 4 nm spectral resolution, described in detail in ref. [ 12 1. The decay profiles were computer-simulated by a sum of exponentials

2. Materials and methods

3. Results and discussion

Purple bacteria Rb. sphaeroides and Chr. minutissimum (wild types, Moscow University collection) were cultivated and chromatophores isolated as described in ref. [ 51. Concentrated chromatophore suspensions were diluted before measurements by tris-HCl buffer, PI-I= 7.5, or by a mixture of the buffer

3.1. Fluorescence decay kinetics at 77 K

F(t)=

1 Ajexp(-t/Ti)

taking into account the 15 ps (full width at half maximum) apparatus response function. The terms Ai correspond to the initial amplitudes of the individual components, the decays of which are characterized by the lifetimes ri. Negative pre-exponential amplitudes represent the terms with exponential rise time. The uncertainty of the parameters derived from the analysis of the measured multi-exponential kinetics was about 30% for the lifetimes of about 10 ps, = 20% for those of several tens of picoseconds, and better than 10% for the lifetimes of about 200 ps. The accuracy of the determination of the amplitude ratio was 30%. The typical exciting light density was in the range 5 x 10*-l x 10” photons per cm2 per pulse, which excludes nonlinear processes but, due to the 82 MHz pulse repetition rate, provides the conditions of saturated (RCs photooxidized) photosynthesis, unless 10m3M of Na2S204 is added to the suspensions. Under the latter conditions the excitation trapping was shown to proceed very much like that for active photosynthesis with open RCs, the trapping time being only x 20% longer [ 4,5 1, presumably due to the reduction in the rate of charge separation induced by the electrostatic repulsion of negative charge on the primary quinone acceptor.

Previous studies [ 8,9] have revealed rather a complex dependence of the fluorescence kinetics of purple bacteria, containing three antenna BChl spectral forms, on the recording wavelength, when the selec-

A. Freiberg et al. /Directed picosecondexcitation transport in bacteria

tive excitation of the shortest wavelength form is used. Let us present first the low temperature case, when energetically uphill excitation transfer is practically eliminated and, therefore, a more easily interpretable picture is expected. Fig. 1 shows three characteristic decay curves of picosecond fluorescence of Chr. minutissimum chromatophores excited by 772 nm light at 77 K. It can be seen that at about 800 nm fluorescence decay is well approximated by a single-exponential term with the decay constant of x5 ps. Fluorescence decay at 1,930 nm is also single-exponential with the constant of 190 ps, which is close to the values, previously determined for excitation quenching by closed photooxidized RCs at room temperature for both R. rubrum and Rb. sphaeroides [ 4,5]. Some fluorescence rise with the lifetime of about 10 ps is also observed in this spectral range. At intermediate wavelengths, 860<12<930 nm, the decay is more complex. For example, three exponentially decaying components are necessary to tit the data at 900 nm within the experimental uncertainty. The corresponding decay constants are: r2=8 ps, r3= 60 ps, r4= 190 ps. The double-exponential fit is

k,

Chr. minutissimum(

229

formally appropriate at 900 nm only if instead of the 190 ps component a much more short-lived one is suggested, which, according to the available data [ 45 ] is unreasonable. These three decay components of varied amplitudes are observed throughout the whole 860-920 nm range. The amplitude spectra for all the abovementioned components are shown in fig. 2. It follows that the most short-lived (5~s) fluorescence component belongs to B800 and the most long-lived one, to B875. Both the 8 and 60 ps lifetime components are evidently due to B850. It means that there are two speo trally slightly different pools of BSOO-850 complexes characterized by different rates of excitation transfer from B850 to B875. It should be stressed that the spectra in fig. 2 are not corrected for the lifetime-dependent reduction of the corresponding amplitudes by the deconvolution of experimental curves with the apparatus response function. If such a correction is taken into account, the amplitude A, of the 5 ps component increases 57 times, the A2 of the 8 ps component, 3-5 times, AS, only 30% and A4 remains practically unchanged. The deconvolution lifetime of 5 ps of B800 fluorescence (fig. 1) is resolution-limited and seems to be about three-fold overestimated, since the amplitude A, is three- to four-fold less than A4 even after the correction procedure. Model calculations, discussed below, also support this conelusion. It follows that the time Chr. minutissimum

0

300 Time

[PO)

Fig. 1. Picosecond fluorescence kinetics of Chr. minutissimum chromatophores at 800,900 and 930 nm (dotted curves) at 77 K. Solid curves represent the computer simulations of the experimental data: a single-exponential decay with ‘I,= 5 ps at rl= 800 nm: three-exponential decay with 72= 8 ps, r3= 60 ps, 7.,= 190 ps and amplitudes ratioofA2:AX:A4=4:3: 1 at,&900 nm; akinetits with 9 ps rise time and 190 ps decay time at 1= 930 nm. The apparatus response is also shown. Excitation light of 772 nm and of 0.4 W/cm2 density was employed. The monochromator bandwidth was 8 nm.

J

800

Wavelength

1000

900 (nml

Fig. 2. Amplitudes Ai of individual decay components of a picosecond time-resolved tluorescence spectrum of Chr. minutissimum chromatophores at 77 K: A,, for the 5 ps component; AZ, for the 8 ps component; A,, for the 60 ps component; A,, for the 190 ps component. Conditions of measurements as in fig. 1, except that the monochromator bandwidth was 4 nm.

A. Freiberg et al. /Directed picosecond excitation transport in bacteria

230

of excitation transfer from B800 to B850 is l-2 ps. The data of figs. 1 and 2 taken together indicate that about 75% of B850 excitations are transferred to B875 in about 8-10 ps, while the rest 25% of excitations in about 60 ps. Similar data were also obtained for Rb. sphaeroides chromatophores, which is illustrated by fig. 3. At L = 940 nm an exponential decay with the 220 ps decay constant and a slight fluorescence rise of 10 ps duration is observed when RCs are kept in a closed photooxidized state. When the RCs become open due to the addition of 1O-” M Na&O,, the fluorescence rise time remains unchanged, but the decay proceeds now, within about 60 ps, in close agreement with earlier data [ 5,8 1. At intermediate wavelengths, 860 and 880 nm, a triple-exponential decay is observed with ‘52= 9 ps, r3 = 50 ps, rd = 2 10 ps. Table 1 gives a more comprehensive representation of the results of the deconvolution of fluorescence kinetics for Rb. sphaeroides at 77 K in case of closed traps.

Rb. sphaer

3.2. Kinetic model We assume the following scheme of excitation transport between distinct light-harvesting antenna pigment pools and the RCs: B800 a,,

B850 (123 B875 - a’4 RC

The macroscopic time-independent rate constants of excitation transfer from pool i to pool j are denoted by Uij.Excitations are quenched by RCs directly only in the B875 pool. Trivial losses, such as fluorescence and intramolecular energy conversion, are introduced by time constants l/urir which were taken to be equal to 0.85 ns at 77 K [ 131 for all antenna molecules. Under low-intensity excitation conditions, the time dependence of the excited state concentration, n, ( I ), for each of the pools can be found from a system of three coupled kinetic rate equations:

oides rii(t)=

T =77K

1

i

[-a,jn~(t)+a,;n,(~)l (i=l,

-Ujin,(t)-~~,gU34n3(t)

2, 3) e

(1)

For simplicity, the term describing excitation characteristics is omitted. In the case of &pulse excitation the decay kinetics of excitations in each pool is given by n,(t)=

C CijeXp( -t/?;) i

,

(2)

where the amplitudes c,~are determined by the initial concentrations of the excited states and by the rate constants Ui> The dependence of fluorescence kinetics on the recording wavelength is described by 0

300

Time

[ps

I

Fig. 3. Picosecond fluorescence kinetics of Rb. sphaeroides chromatophores at 860, 880 and 940 nm (dotted curves) at 77 K. Parameters of approximation curves are at 1= 860 nm: TV= 8 ps, 7~=60ps..4,:A,=3.3;atL=880nm:7~=10ps,7,=45ps,~~=200 ps, Az:A3:A4=5:3: 1; at A=940 nm ( 10m2M NalS204 added): 7,=56 ps, 7,>2 ns, A.,:A5=30; at 1=940.nm (without additions): 7,,,,= 10 ps, 7.,=220 ps. Excitation light of 796 nm and of 0.25 W/cm’ density was employed. Monochromator bandwidth was 4 nm. The apparatus response function is also shown.

(3) where Fi(lz) is the fluorescence spectrum of the ith pigment pool taken from experiment. For comparison with the experimental data the theoretical curves I(& t) were convoluted with the apparatus response function. The amplitude spectra for each ?i value were obtained from the equations

A. Freiberg et al. /Directed picosecondexcitation transport in bacteria

C CoFi(l) * I

A,(A)=

(4)

To obtain stationary fluorescence spectra, time integration of eq. ( 3 ) should be performed: Z(n) = 7 Z(1, t) dt= 7 Fi(1) C rjcu e

(5)

i

0

There is one more simplification used in our model. The rate constants for the up-hill transfer Ujiare calculated based on the Forster approximation for pairwise rates of excitation transfer [ 14 ] as follows: zi

aji

=

a, z_ J

d v)Fj( v)

U

lJ4

dv

>

(6) Here Zi, Zj are the numbers of BChl molecules of the corresponding pools; F,(v) are normalized fluorescence spectra, such that JFi( V) dv= 1; ei( v) are the extinction coefficients in cm- I. A detailed analysis of the experimental multi-exponential fluorescence decay profiles, based on the above model, has been performed. It is concluded that a satisfactory approximation of the experimental data can be achieved only if the existence of not one but two different pools of B850 pigments is suggested both for Rb. sphaeroidesand Chr. minutissimum.The best fit to the experimental data is achieved when the ratio between the numbers of BChl molecules in the two pools is (3-4) : 1, the major pool of B850 molecules transferring their excitations to B875 in about 10 ps, the minor one in about 40-50 ps. The absorption maximum of B850 molecules transferring their excitations to B875 during the longer time should be by 5-7 nm red-shifted. Theoretical kinetic curves I(& t ) , calculated on the basis of these assumptions are in good agreement with the experimental data for all recording wavelengths both for Rb. sphaeroidesand Chr. minutissimum at 77 K. It is demonstrated in fig. 4 for Rb. sphaeroides with closed RCs. Together with the decay curves, a stationary fluorescence spectrum and the amplitude spectra of separate picosecond decay components are presented. The latter allow an easy tracking of rather

231

a complex interplay between different decay components in dependence on the recording wavelength. 3.3. Room temperature fluorescence decay Of special importance is the ability of the abovementioned simplified model to describe fluorescence decay data at room temperature where the presence of the intense thermally activated up-hill transfer makes fluorescence kinetics extremely involved: at most wavelengths triple-exponential decay together with some fluorescence rise is observed. From the methodological point of view it seems rather impossible to obtain any unambiguous conclusion from the treatment of the experimental data under these conditions, unless some model assumptions are involved. We have applied the same kinetic model to the description of room temperature data as at low temperatures (taking into account the appropriate broadening and shift of the corresponding spectral bands). Fig. 5 shows a reasonably good agreement between the experimental and calculated data in the case of Rb. sphaeroides.The same holds for Chr. minutissimum.The whole set of fluorescence kinetic data for Rb. sphaeroidesat room temperature under the conditions of closed RCs is represented in table 2. It is important to note that quite a good fit to the experimental data both at 77 K and at room temperature is achieved under the assumption that the rate constants ail are almost temperature independent. The slight decrease in the measured value of r3 at room temperature as compared to the one at 77 K (from about 50 to.35 ps) is due to the inclusion of the uphill transfer from B875 to B850. It seems that in the temperature range of interest there are no substantial changes of intercomplex (intermolecular) distances in the membranes and of the mutual orientations of the transition moments of the corresponding BChl molecules. The usefulness of the model calculations involved is clearly seen also when we proceed to the interpretation of the data on the influence of the RC state on fluorescence kinetics both at 77 K and at room temperature. The data in figs. 6 and 7 show that our model scheme satisfactorily describes the main peculiarities of the kinetic curves at all wavelengths, including those in the vicinity of 860 nm. In our previous works [ 8,9], the analysis of the data was performed with-

A. Freiberg et al. /Directed picosecond excitation transport in bacteria

232

Rb.sphaeroides RCs

0

closed

300 Time

(PSI

800 Wavelength

900 (nml

Fig. 4. (A) Fluorescence decay kinetics (dotted curves) of Rb. sphaeroides chromatophores with closed traps at 77 K and simulated model curves (solid lines) suggesting that: ( 1) the time constant of excitation transfer from B800 to B850 is I ps; (2) the time constant of excitation transfer from B850 to B875 is 10 ps (main pool, 75% of B850 molecules), and 40 ps (remaining 25% of molecules); (3) the time constant of B875 excitation quenching by closed RCs is 260 ps. (B) Spectral distribution of the relative amplitudes of the separate lifetime (the numerical value ofwhich is written at the corresponding curves) components of the approximating curves obtained by the use of the above listed time constants. (C) The time-integrated fluorescence spectrum.

out any model assumptions, which did not permit the conclusive information to be obtained. Now it is clear that among the interpretations discussed in rcfs. [ 8,9] the one, suggesting that it is the B875 fluorescence lifetime that primarily depends on the RC state, is justified. The lifetimes of the other components are almost independent of the RC state.

4. Concluding remarks The study performed makes it possible to elucidate the presence and the nature of a directed flow of excitations from the short-wavelength BChl spectral forms to the long-wavelength one, and from the latter to RCs. A number of important kinetic parameters have been determined, which characterize heterogeneous excitation transport rates in the light-harvesting antennae of purple bacteria: (i) The intercomplex BSOO-850+B875 transfer rate is shown to be about 10 ps for most excitations. The minor part of B850 excitations, transported from B800-850 complexes to those of B875 in about 50

ps, is likely to belong to the part of the BSOO-850 complexes situated peripherally and unfavourably relative to B875. The failure to observe the short-lived kinetics of B850 excitation decay in the picosecond absorption measurements [ 10,111 is likely to be a result of the difficulties connected with the absence of reliable detailed information about the spectral manifestations of in vivo BChl excited states and specific coherent artifacts inherent to the method. (ii) The intracomplex B800+B850 transfer time has been estimated to be about l-2 ps at 77 K, in agreement with the data in refs. [ lo,11 1. Approximately the same rate of the transfer has also been measured at room temperature (the data in preparation ) . (iii) Excitation trapping by open RCs takes approximately the same time in purple bacteria with photosynthetic units of different sizes. Earlier studies with the purple bacterium R. rubrum have shown that the time of excitation trapping by open RCs in this bacterium, containing a single pigment-protein complex B880 and about 30 BChl molecules per RC, is

A. Freiberg et al. /Directed picosecondexcitation transport in bacteria

233

Rb.sphaeroldes RCs

0

Ttme

closed

300

[psi

800

Wavelength

900 [nml

Fig. 5. Fluorescence decay kinetics (dotted curves) of Rb. sphaeroides cells with closed traps at room temperature and simulated model curves (solid lines). Approximation parameters as in fig. 4, except that the time constant of B875 excitation quenching by RCs is taken to be 135 ps. (The variability of the long-wavelength fluorescence lifetime from sample to sample was in the range 150-250 ps, see ref. [5]).(B)and(C)thesameasinfig.4. Table 1 Summary of the approximation parameters ( 1/e lifetimes and relative amplitudes) of the fluorescence kinetics of Rb. sphaeroides chromatophores with closed RCs at 77 K, excitation density at 796 nm is 0.25 W/cm* a)

average t values Tri,=

1 (nm)

r2 (ps)

73

850 860 870 880 890 900 910 920 940 960

12 8 12 10 5 9 12 13 10 9

45 60 40 45 40 55 70

9+2

(ps)

50f 10

74

(PS)

220 220 200 200 220 210 210 220 210

A2

A3

1.0 1.0 1.0 1.0 1.0 -1.0 -1.0 -1.0 -1.0 -1.0

0.15 0.3 0.4 0.6 1.0 0.3 0.2

A.4

0.1 0.15 0.2 0.25 1.0 1.0 1.0 1.0 1.0

210f20

lOf2

‘) The values of amplitudes in relative units are not corrected for reduction due to convolution with the apparatus response function. Negative amplitudes mean that the corresponding components are manifested as a fluorescence rise.

about 60 ps [ 4,5 1. The same value of the trapping time at uniform excitation of all antenna pigments has been measured for Rb. sphaeroides, whose pho-

tosynthetic unit size is about four times larger [ 5 1. This work shows that when the selective excitation of only the shortest-wavelength BChl form is employ&d,

A. Freiberg et al. /Directed picosecond excitation transport in bacteria

234

Table 2 Summary of the approximation parameters ( l/e lifetimes and relative amplitudes) of fluorescence decay kinetics of Rb. sphaeroides cells with closed RCs at room temperatures; excitation as in table I 7.

835 840 850 860 870 880 890 900 910 920 940 960

9 7 9 7 9 10 13 12 12 13 12 13

average 7, values T,,,, =

(PS)

1 (nm)

73

(PS)

74

(PS)

150 150 150 150 150 140 150 140 150 150 140 150

Sk1

.‘I?

A3

.44

0.4 0.1 0.1 -1.0 -1.0 -1.0 - 1.0 -1.0 -1.0 -1.0 - 1.0 - 1.0

0.5 0.8

1.0 1.0 I.0 1.0 1.0 I.0 I.0 1.0 1.0 1.0 I.0 I.0

1.0 0.8 0.4 0.1 0.03

150* 15

35f7

12 f 2

Rb.sphaeroides

0

300 Time

[PSI

800 Wavelength

900 [nml

Fig. 6. (A) Fluorescence decay kinetics (dotted curves) of Rb. sphaeroides chromatophores with open traps at 77 K and simulated model curves (solid lines). Approximation parameters are the same as in fig. 4. except that B875 excitation trapping time is taken to be 45 ps. (B) and (C) the same as in fig. 4.

the same trapping time is observed (figs. 6 and 7). This is the only essential point of controversy between our experimental data and the simple model scheme employed. The model predicts that if the time of B875 excitation trapping by open RCs is taken to

be 60 ps, as in R. rubrum, then the presence of additional BChl molecules in the B800-850 complexes should lengthen the overall excitation trapping time in Rb. sphaeroides and Chr. minutissimum up to 90 ps. We believe this disagreement to be a result of the

A. Freiberg et al. /Directed picosecond excitation transport in bacteria

233

Rb.sphaeroldes

\

300

0 Time

(psi

800 Wavelength

900

tnml

Fig. 7. (A) Fluorescence decay kinetics (dotted curves) of Rb. sphaeroides cells with open traps at room temperature. Solid lines are simulated model curves with the same approximation parameters as in fig. 6. (B) and (C) the same as in fig. 4.

oversimplification of the model, that does not take into account an intrinsic spectral inhomogeneity of the B875 absorption band. Indications of such inhomogeneity have been revealed in several recent works [ 11,15- 17 1. The spectral inhomogeneity of the type under investigation seems to promote an additional localization of excitations in the vicinity of RCs and, due to the shortening of the trapping time, the increase of the overall quantum yield of the primary charge separation in RCs. Acknowledgement The authors are indebted to Dr. V. Sundstriim for several valuable discussions and for providing the preprint of ref. [ 111 before publication, to Professor K. Rebane for reading the manuscript and helpful remarks, to Professor A.Yu. Borisov for stimulating suggestion and support, and to Dr. S.G. Kharchenko and I.V. Bukhova for the preparation of chromatophores. References [ 1] H. Michel, 0. Epp and I. Deisenhofer, EMBO J. 5 ( 1986) 2445. [2] W. Wolfram, Z. Wacker, M. Leis, W. Kreutz, I. Shiozawa, N. Gadon and G. Drews, FEBS Letters 182 ( 1985) 260.

[ 3 ] R. van Grondelle, Biochim. Biophys. Acta 8 11 ( 1985 ) 147. [4] A. Freiberg, V.I. Godik and K. Timpmann, in: Advances in photosynthesis research, ed. C. Sybesma (Nijhoff, The Hague, 1984) p. 45. [ 51A.Yu. Borisov, A.M. Freiberg, V.I. Godik, K-K Rebane and K.E. Timpmann, Biochim. Biophys. Acta 807 ( 1985 ) 22 1. [6] K.L. Zankel, in: The photosynthetic bacteria, eds. R.K. Clayton and W.R. Sistrom (Plenum Press, New York, 1978) p. 341. [7] P. Sebban and I. Maya, B&him. Biophys. Acta 772 (1983) 436. [8] V.I. Godik, K.E. Timpmann, A.M. Freibeig, A.Yu. Borisov and K.K. Rebane, Dokl. Akad.,Nauk USSR 289 ( 1986) 714. [9] V.I. Godik, A.M. Freiberg,X.E. Timpmann, A.Yu. Eorisov and K.K. Rebane, in: Progress in photosynthesis research, Vol. I, ed. J. Biggins (Nijhoff, The Hague, 1987) p. 41. [lo] V. Sundsttim, R. van Grondelle, H. Bergstr6m, E. Akesson and T. Gillbro, Biochim. Biophys. Acta 851 (1986) 431. [ 111 R. van Grondelle, H. Bergstrom, V. Sundstriim and T. Gillbro, Biochim. Biophys. Acta, submitted for publication. [ 121 A. Freiberg, Laser Chem. 6 (1986) 233. [ 13 ] P. Sebban, G. Jolchine and I. Moya, Photochem. Photobiol. 39 ( 1984) 247. [ 141 Th. Forster, Ann. Physik 2 ( 1948) 55. [ 15 ] H.J.M. Kramer, J.D. Pennoyer, R. van Grondelle, W.H.J. Westerhuis, R.A. Niedermann and J. Ames& Biochim. Biophys. Acta 767 ( 1984) 335. [ 161 A. Freiberg, V.I. Godik and K.E. Timpmann, in: Progress in photosynthesis research, Vol. 1, ed. J. Biggins (Nijhoff, The Hague, 1987) p. 44. [ 17 ] A.Yu. Borisov, R.A. Gadonas, R.V. Danielius, AS. Piskarskas and A.P. Razjivin, FEBS Letters 138 ( 1982) 25.