Proc. Natl. Acad. Sci. USAVol. 92, pp. 12333-12337, December 1995Biophysics

Temporally and spectrally resolved subpicosecond energy transferwithin the peripheral antenna complex (LH2) and from LH2 tothe core antenna complex in photosynthetic purple bacteriaS. HESS*, M. CHACHISVILIS*, K. TIMPMANNt, M. R. JONESI, G. J. S. FOWLERt, C. N. HUNTERS, AND V. SUNDSTROM**Department of Chemical Physics, Lund University, P.O. Box 124, S-221 00 Lund, Sweden; tInstitute of Physics, Estonian Academy of Science, 202 400 Tartu,Estonia; and tRobert Hill Institute for Photosynthesis and Krebs Institute for Biomolecular Research, University of Sheffield, Sheffield S10 2UH,United Kingdom

Communicated by Ahmed H. Zewail, California Institute of Technology, Pasadena, CA, August 8, 1995

ABSTRACT We report studies ofenergy transfer from the800-nm absorbing pigment (B800) to the 850-nm absorbingpigment (B850) of the LH2 peripheral antenna complex andfrom LH2 to the core antenna complex (LH1) in Rhodobacter(Rb.) sphaeroides. The B800 to B850 process was studied inmembranes from a LH2-reaction center (no LH1) mutant ofRb. sphaeroides and the LH2 to LH1 transfer was studied inboth the wild-type species and in LH2 mutants with blue-shifted B850. The measurements were performed by using'100-fs pulses to probe the formation of acceptor excitationsin a two-color pump-probe measurement. Our experimentsreveal a B800 to B850 transfer time of -0.7 ps at 296 K andenergy transfer from LH2 to LH1 is characterized by a timeconstant of -3 ps at 296 K and -5 ps at 77 K. In theblue-shifted B850 mutants, the transfer time from B850 toLH1 becomes gradually longer with increasing blue-shift ofthe B850 band as a result of the decreasing spectral overlapbetween the antennae. The results have been used to producea model for the association between the ring-like structuresthat are characteristic of both the LH2 and LH1 antennae.

Organization and function of the core antenna complex (LH1)and the peripheral antenna complex (LH2) of purple bacteriahave been extensively studied in the past (1, 2, 29). A major stepforward in this work was taken very recently when the three-dimensional structure of a complex of the peripheral antennaabsorbing at 800 nm (B800) and at 850 nm (B850) from Rho-dopseudomonas (Rps.) acidophila was solved to high resolutionshowing a ninefold circular symmetry of a13 pairs (3). From thisstructure of LH2 and the similar circular structure of LH1 (4), ithas become clear that the pigment density in these light harvest-ing pigments is very high, leading to short intermolecular dis-tances and strong dipole-dipole interactions. Presently, twomodes of energy transfer are considered, incoherent F6rstertransfer or exciton state relaxation (5-8), and it is a challenge forfuture research to establish the levels of organization at which thetwo modes of energy transfer are operative. Until experimentsand theory have produced a unified description of the energytransfer dynamics, we have chosen to describe the energy transfersteps within LH2 and LH1 and between the complexes asincoherent Forster hopping. Early work on energy transfer dy-namics in photosynthetic purple bacteria (9-16) yielded infor-mation about the overall exciton lifetime in the antenna andprovided a time scale for energy equilibration within individualcomplexes (9, 10) and over the whole antenna (11, 16). Inparticular, the LH2 antenna is probably the most extensivelystudied complex. Picosecond absorption and fluorescence studiesperformed at room and low temperature on the LH2 pigment-protein complex of Rhodobacter (Rb.) sphaeroides revealed thatB800 -> B850 energy transfer is a very fast and temperature-

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dependent process (9-13). Most of the early experiments wereperformed by using picosecond one-color measurements toprobe the decay of the B800 excited state. More recently,femtosecond experiments (7, 17) yielded a relaxation time ofthe B800 excited state of 0.6 ± 0.1 ps at room temperature and2.4-2.6 ps at 77 K (8). Very similar results were obtained fromnonphotochemical spectral hole-burning measurements at 4 K(18-20).

Nevertheless, despite these efforts, there is not yet a clearpicture of the total energy transfer dynamics of LH2. Similarly,there are only very preliminary results on the interactions andenergy transfer processes between LH2 and the LH1 core an-tenna. To obtain more definite information about the excitationdynamics within LH2 and from LH2 to LH1, we have performedtransient absorption measurements by using widely tunable-100-fs low-energy pulses from a sync-pumped optical paramet-ric oscillator (OPO). This allowed us to selectively excite variouspigments and monitor the dynamics by probing the decay of theinitially excited donor state or the appearance of excited acceptorpigments. To study the B800 -* B850 transfer within LH2, weused membranes prepared from a LH2-reaction center (RC)mutant ofRb. sphaeroides [DPF2(pRKEH2)] that lacks LH1 and,therefore, has LH2 as the only antenna complex (21). By com-bining the results for B800 -> B850 transfer obtained for thismutant with those for the wild-type (WT) Rb. sphaeroides, whichcontains both LH2 and LH1 complexes, we could also obtaininformation about the LH2 -> LH1 energy transfer step, whichso far has been difficult to observe with lower-time resolution.The LH2 -* LH1 transfer step was also studied in two mutantsofRb. sphaeroides where site-directed mutagenesis had been usedto produce blue shifts in the B850 bacteriochlorophylls (Bchls)within LH2 (22). When these altered complexes (B800-839 andB800-826) are synthesized along with the normal LH1-RC corecomplex, the effect is to produce an antenna where the spectraloverlap between the B850 and LH1 is altered. As a control, apseudo WT was used, in which normal LH2 complexes aresynthesized; because of the gene expression system LH2 isoverproduced in this strain, in comparison with the LH1-RCcomplex (see Fig. 3).

MATERIALS AND METHODSThe large number of coupled molecules in the intact antennamakes it necessary to use excitation densities as low as 1012photons per cm2 per pulse to avoid singlet-singlet or singlet-triplet annihilation. In the present work, we achieved this byusing a pump-probe spectrometer based on femtosecondexcitation pulses from a high repetition rate Ti:Sapphire laserand the independently tunable probe pulses from a synchronously

Abbreviations: WT, wild type; LH1, core antenna complex; LH2,peripheral antenna complex; RC, reaction center; BChl, bacteriochlo-rophyll; B800, B850, etc., peripheral antennas absorbing at 800 nm, 850nm, etc.; OPO, optical parametric oscillator.

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pumped OPO. The Ti:Sapphire laser was operated at a repetitionrate of 82 MHz producing - 100-fs pulses with an average outputpower of >2.0 W centered at 808 nm. This output was used topump the OPO and the pump light reflected from the entrancesurface of the nonlinear crystal (LBO) in the OPO (Q300-400mW) was used as excitation pulses in the pump-probe experi-ment. To prevent disintegration of the sample and accumulationof long-lived photo products, the repetition rate of the excitationpulses was reduced to 0.8-4 MHz with an acousto-optic pulseselector. The output pulses of the pulse selector were compressedto a duration of 80 fs and an essentially Gaussian time profile ofthe autocorrelation function, by using a pair of SF10 prisms. Thetime-bandwidth product of these pulses was -0.46, indicatingclose to transform limited Gaussian pulses. The OPO producestwo new output wavelengths, the signal and a slightly less pow-erful idler. The cross-correlation function of the excitation andprobe pulses was measured by using frequency mixing in a LiIO3

crystal and tumed out to be 200 fs. Probe wavelengths in the range830-880 nm were generated by frequency doubling the idler witha 2-mm angle-phase-matched BBO crystal at 5% efficiency. Theresidual radiation at 1.6-1.7 mm was cut-off by using a 1-cm cellwith distilledwater. Both beams were focused by a 10-cm lens intothe sample. Another experimental setup with picosecond timeresolution was used to examine the LH2 -* LH1 energy transfer

step in WT and B850 mutants of Rb. sphaeroides at 77 K Thismeasurement setup has been described in detail (2).A descriptionof the construction and some of the energy transfer properties ofthe LH2-RC mutant DPF2(pRKEH2) are described in ref. 21.Measurements were performed on samples with closed (oxidizedprimary donor, P+) RC at 295 ± 2 and 77 K

RESULTS AND DISCUSSIONB800 -- B850 Energy Transfer Within the LH2 Ring. By

using the LH2-RC mutant DPF2(pRKEH2) of Rb. sphae-roides, we take advantage of a simple genetically engineeredsystem for the study of B800 -* B850 energy transfer that helpsto eliminate possible interference from other energy transferprocesses but at the same time maintains the natural membraneenvironment of this pigment-protein complex. Fig. 1A summa-

rizes the experimental transient absorption traces at several probewavelengths in the range 830-875 nm, after excitation at 808 nm.At red wavelengths (875 nm), the excitation of B800 results in a

fast rise ofbleaching and a slower decay. By probing in the middleof the B850 band (-840 nm), we observe a very fast rise ofbleaching followed by a decay to a long-lived induced absorptionof the B850 pigment. Finally, at the blue-most wavelength (-830nm), only a rise of the absorption signal is observed.A global leastsquare fitting analysis, including convolution with the responsefunction, to the kinetics measured at all wavelengths yielded thefollowing lifetimes: T1 150 ± 50 fs, T2 0.68 ± 0.07 ps, and

T3 > 30 ps. The amplitude variations as a function of wavelengthfor the three time constants are given in Table 1. From earlierpicosecond measurements, we know that the longest lifetime T3is associated with the relatively slow quenching of the energy bythe closed RC (9, 11, 21). The much faster subpicosecondprocesses, therefore, reflect energy transfer within LH2. The factthat this energy transfer is characterized by kinetics more complexthan single exponential suggests that the simplest possible modelto describe the dynamic in LH2, a two-compartment model, isobviously not adequate to describe the actual situation. Frompicosecond (9-12), femtosecond (1), and hole-burning (18) ex-

periments, a very fast relaxation within B850 is known to exist.This very fast relaxation could be attributed to either spectralequilibration or exciton state relaxation, depending on whethereach B850 molecule is weakly or strongly coupled. For the presentdiscussion, we choose a simplified model and describe the relax-ation within B850 as incoherent energy equilibration over an

inhomogeneously broadened pigment spectrum consisting of two

A

B

0 2 4 6

Time (ps)

FIG. 1. (A) Two-color isotropic absorption measurements of theLH2-only mutant of Rb. sphaeroides performed by exciting at 808 rim andprobing in the wavelength range from 830 to 875 nm at 300 K (B)Simulated AA kinetics for various wavelengths inside the B850 spectralband, generated according to the kinetic model. (Inset) AA spectrum,composed from two components with a spectral separation of 55 cm-'.

components, blue-absorbing (B850b) and red-absorbing (B850r)pigments. This results in a simple kinetic model,

Table 1. Results of the global analysis for the LH2 only mutant

A, nm A1 A2 A3

830 - 1.0 1.24 -0.24835 - 1.0 1.13 -0.13840 - 1.0 1.38 -0.38850 -1.0 -0.38 1.38855 - 1.0 -0.63 1.63865 - 1.0 -1.32 2.32857 - 1.0 -1.23 2.22

Variation of amplitudes at common lifetimes: T, = 0.15 ± 0.05 ps,T2 = 0.68 ± 0.07 ps, and T3 > 30 ps. The given values have a standarddeviation of 10%.

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k, k2

B800 B850b = B850r,

that is associated to biexponential kinetics, with time constantsTi and T2 characterizing the energy equilibration among B800,B850b, and B850r. A fit of the kinetics generated by the abovemodel to the experimental data of Fig. 1A would require a

detailed knowledge of the difference absorption (AA) spec-trum of B850 and how the homogeneous spectra combine togive the observed inhomogeneous spectrum. However, onlythe approximate shape of this spectrum, characteristic of manyBChl pigment-protein complexes, is known (12). It displays anintense bleaching/stimulated emission in the red part of theabsorption spectrum and an induced absorption in the short-wavelength part. These two regions are separated by an

isosbestic point close to the absorption maximum (2, 12). InFig. 1B Inset, this schematic AA spectrum is displayed alongwith the two-component spectra of B850b and B85Or used todescribe the spectral inhomogeneity in a simple way. Thesimulated kinetics resulting from the energy equilibrationbetween these two spectral components are also displayed inFig. 1B. Qualitatively, the stimulation agrees very well with themeasured kinetics in Fig. 1A displaying a rise time thatessentially describes the B800 -+ B850 transfer in the red and

blue wings of the B850 spectrum. In the center of the spectrum,close to the isosbestic point of the slow B850 decay, there is asubstantial amplitude of the fast 150-fs component character-izing the energy equilibration within B850. Finally, we empha-size once more that this simple two-component model of theB850 spectral inhomogeneity is not intended to give a detaileddescription of the B850 equilibration. Nevertheless, at roomtemperature where this equilibration is very fast and charac-terized by an essentially wavelength-independent time con-stant, it reproduces the main features of the dynamics. Whenthe same equilibration process is modeled at low temperature(<77 K), a much more sophisticated description is required,because then the equilibration dynamics are strongly nonex-ponential as a result of energy localization on the pigments inthe red part of the intensity distribution function. It is inter-esting to compare the results obtained at 850 nm (Table 1) withthose reported by Shreve et al. (17) for the same wavelength.Their measurements suggested a shorter energy transfer time(-0.4 ps), and the discrepancy was interpreted as a result ofexcitation annihilation within B850, shortening the rise time ofB850 excitations. From the present results, it is obvious that inaddition to intensity-dependent nonlinear processes, such asexcitation annihilation, spectral equilibration strongly contrib-utes to the dynamics measured in B850. To establish thelifetime for B800 -> B850 transfer, it is consequently importantto monitor the B850 kinetics over a wide range of wavelengths.LH2 -- LH1 Energy Transfer in WT Rb. sphaeroides. Little

is known about the energy transfer between the LH2 and LH1antenna pigment systems of Rb. sphaeroides; although there isstructural information for the LH1 and LH2 components, thenature of the contacts between them is unknown. The dis-tances involved can, however, be calculated from the rate ofB850 -> LH1 energy transfer. Accordingly, we have under-

taken the measurement of this process with the two-colortechnique. Having established the value of the B800 -> B850

energy transfer time as 0.68 ± 0.07 ps at room temperature, wecan use the measurements on the WT species to obtaininformation about the LH2 -> LH1 (B850 -> B875, where

B875 is the pigment of LH1 absorbing at 875 nm) process. Fig.2 shows the results at three probe wavelengths within the B850and B875 absorption bands. Lifetimes and amplitudes wereagain obtained with single-wavelength and global analyses, andthe global analysis results are presented in Table 2. At all threewavelengths, the kinetics are characterized by three lifetimes,0.3 ± 0.05 ps, 3.3 ± 0.3 ps, and >30 ps. At 840 and 850 nm,

<

880nm

/1

850nm

Rb. spIhaeroides

e"\\ ;exc=808 nmii

840 nm

0 4Time ( ps)

FIG. 2. Measured kinetics ofWT Rb. sphaeroides at 300 K probedat three wavelengths (840, 850, and 880 nm) after exciting into theB800 band at 810 nm.

the formation and equilibration of the B850 excitations willgive a major contribution to the measured kinetics throughexcited state absorption and bleaching of the B850 pigments.This is shown by the respective negative and positive signs ofthe kinetic curves of Fig. 2 at these wavelengths. The rise timeanalyzed as only one lifetime turned out to be similar ("0.3 ps)to that observed for the LH2-only mutant at the same wave-lengths and, therefore, was interpreted to describe the B800B850 and B850b B850r energy transfer processes. Whenthese processes are described with a single exponential life-time, a value slightly shorter than the 0.7-ps time constant ofthe B800 -- B850 transfer was obtained as a consequence ofthe fast equilibration within B850, as was discussed above forthe LH2-only mutant. The >30-ps lifetime, as for the LH2-onlymutant, is associated with quenching of the antenna excitationsby the RC. In addition to these two lifetimes, there is an -3-pslifetime at 840 nm characterized by an increased excited stateabsorption and at 850 and 880 nm characterized by an in-creased bleaching. From the spectral properties of B850 andB875, this is the kinetic pattern expected for B850 -> B875energy transfer (2, 12). At 880 nm, in the red wing of the LH1absorption band, we expect to observe a major rise-timecomponent of the B875 photobleaching/stimulated emissionas a result of B850 -* B875 transfer. This is corroborated bythe kinetic curve for 880 nm in Fig. 2, which has a 3.3-psrise-time component. When probing at 880 nm, the measuredsignal will, in addition to B875 bleaching, also have contribu-tions from stimulated emission of B850 and blue-absorbingB875 pigments. Thus, kinetics measured at these wavelengthswill exhibit lifetime components reflecting the B800 -* B850transfer and the fast energy equilibration within both B850 andB875. This will give rise to the observed kinetic components onthe few hundred femtosecond time scale (see Table 2). Thus,the present results verify and substantiate previous suggestionsthat the energy equilibration between LH2 and LH1 occurswith an -3-ps time constant (2). This lifetime is also consistent

Table 2. Variation of amplitudes for the WT Rb. sphaeroideskinetics at common lifetimes

A, nm Ai A2 A3840 - 1.0 1.14 -0.14850 - 1.0 0.16 0.84880 -1.0 -1.16 2.16

Common lifetimes: T1 = 0.3 + 0.05 ps, T2 = 3.3 ± 0.3 ps, and T3 >30 ps. The given values have a standard deviation of 10%.

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with the idea that the total energy equilibration over the wholeantenna system is basically complete within <10 ps (2).LH2 -> LH1 Energy Transfer in Rb. sphaeroides Mutants

with Blue-Shifted B850 Absorption. The LH2 -> LH1 transferstep was further examined by measuring the overall rise timeof B875 photobleaching after excitation of B800 in a series ofRb. sphaeroides mutants with gradually blue-shifted B850absorption bands and two controls, the true wild type and one

pseudo WT in which LH2 is overproduced with respect to LH1.The absorption spectra of samples under investigation are

illustrated in Fig. 3 and show how the absorbance of the B850band can be genetically controlled, while the B800 and B875bands remain constant. The construction and spectroscopiccharacterization of these mutants have been described (22),and the B800 -- B850 (8839, B826) energy transfer has been

studied with 100-fs pulses at room temperature and at 77 K(8). The present measurements were carried out at 77 K tofavor only unidirectional down-hill energy transfer processes.

Fig. 4 presents the results of measurements of the photo-bleaching/stimulated emission rise times for reconstituted WTLH1 in the wavelength range 889-920 nm, after excitation at800 nm. The measured apparent rise time is seen to increasefrom <1 ps at 889 nm to a constant level of -5 ps forwavelengths >900 nm. At the red wavelengths (>900 nm), thephotobleaching/stimulated emission of B875 is mainly de-tected. The -5-ps rise time observed at these wavelengths,therefore, reflects the formation of the B875 excited state (seealso below and Table 3). When the rise time of LH1 excitationsis probed at shorter wavelengths (889-900 nm; Fig. 4), themeasured apparent rise time is shortened to (and even below)a value limited by the resolution of the measurement system.;Fhis is a result of the growing contribution of the fast energytransfer step between B800 and B850 and subpicosecondenergy equilibration processes within B850 and B875, alsorevealed as an increase of the amplitude of a subpicosecondcomponent at shorter wavelengths in the femtosecond data forWT Rb. sphaeroides at room temperature (Table 2). In thefollowing measurements of LH1 rise times for the blue-shiftedLH2 mutants, a probe wavelength of 905 nm was selected toavoid the fast equilibration dynamics and unambiguouslyprobe the LH2 -> LH1 transfer step. The results of thesemeasurements are summarized in Table 3.To estimate the rate of the energy transfer step B850

B875 from the measured rise times of B875 excitations, a

200

I1

I 5(0-L......

i*o2-j

...... ...scudeSt A~~~~~v|(

\VI\vClencttl (1111' )

FIG. 3. Absorption spectra at 77 K for WT Rb. sphaeroides andmutants with specifically mutated B850 complexes, used in picosecondmeasurements.

6-

5-

4-

0."'

3-

2-

I1-

0-

8 890 895 900 905 910 915 920

Wavelength (nm)

FIG. 4. Rise times of LH1 excitations in the pseudo-WT mutant ofRb.sphaeroides as a function of probe wavelength, measured at 77 K withpicosecond pulses. Excitation was at 800 nm into the B800 pigment.

two-step kinetic model with forward rate constants k1 and k2and backward rate constants k-1 and k-2 was used.

k, k2B800 =± B850 -± B875

k, k-2

In this model we neglect the spectral inhomogeneity of thepigments, because equilibration within each pigment (-200 fs,see above) is much faster than the transfer steps betweendifferent pigments (from above we know that B800 -* B850 is-0.7 ps at room temperature and -2 ps at 77 K and B850 >B875 in the WT species is -3 ps). The energy transfer steps inthis model are in addition practically irreversible for allmutants (ki/k-i >> 10; only for the B826 mutant a somewhatsmaller ratio of -15 is estimated for the B800 -- B826 step),since the energy gaps between different spectral bands (370-820 cm-1) are much larger than the value of kT (54 cm- 1) at77 K (similar kl/k-i ratios were also obtained from Forsterspectral overlap calculations). In this case, the time depen-dence of B875 excited states is characterized by standardbiexponential kinetics, and with the known forward transferrates ki (8), we can obtain the rates k2 from a fit to themeasured kinetics of formation of the B875 excited state. Inthis procedure, convolution with the measured cross-correlation function of the excitation and probe pulses wasperformed. The results for all Rb. sphaeroides mutants underinvestigation are summarized in Table 3. We conclude that thetransfer time for the wild-type species is in good agreement

Table 3. Measured rise times of absorption changes at 905 nmafter excitation at 800 nm for WT Rb. sphaeroides and several B850blue-shifted mutants at 77 K

Spectralinterval

Calculated betweenB850-*B875 B800--B850 B875

Measured energy energy andrise time transfer transfer B850,

Sample (r), ps time, ps time,* ps nm

Pseudo WT 5.3 ± 1.4 4.6 + 1.5 2.4 26WT 5.9 ± 2.1 5.2 ± 2.3 2.4 34B800-839 6.6 + 1.5 6.2 + 1.6 1.8 42B800-826 7.6 + 1.4 7.0 ± 1.6 0.8 58

B850 -- B875 energy transfer times are calculated from these data.*From ref. 21.

I -.

I I I I I

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with the results obtained above with better time resolution, whichshows that our deconvolution procedure for the picosecond datayields reliable results. The B850 -- LH1 transfer times for themutants become gradually longer with increasing blue shift of theB850 band. This effect is a result of the decreasing spectraloverlap for LH2 -- LH1 energy transfer in the series B850, B839,and B826. The effect is analogous to the spectral-overlap-controlled B800 -- B850 (B839, B826) processes previouslystudied, within blue-shifted LH2 complexes (8).A Model for the Association ofLH2 and LH1 Complexes. By

assuming that LH2 -- LH1 transfer arises from a Forster typehopping mechanism, we can use the 5-ps transfer time at 77 K(3.3 ps at room temperature) to calculate the distance from aB850 pigment within a ring to the nearest neighbor pigment inLH1. The model in Fig. 5 is based on the recent crystallo-graphic data ofLH2 (3) and LH1 (4). The overall arrangementof the LH1 model is based on the 16a/16,3 ring structuredetermined for the LH1 complex of Rhodospirillum rubrum(4), but the arrangement of the Bchls is essentially thatdescribed for the LH2 complex of Rps. acidophila (3). In eachcase allowances have been made for the sequences of respec-tive a and ,B polypeptides for the Rb. sphaeroides complexes. Asa reference energy transfer time, we have used the -35 psmeasured for the trapping of energy from LH1 by the RC(23-25), which was inferred to correspond to a distance of 35-40 A between the B875 Bchls and the special pair of the RC.[Although a faster antenna to RC energy transfer has beensuggested (e.g., ref. 26), we believe there is substantial time-resolved (23-25, 27) and steady-state (28) data proving thatthis energy transfer step is relatively slow and the rate limitingstep in the overall energy transfer.] The time of -5 ps at 77 Kfor B850 -- B875 transfer should, therefore, correspond to27-32 A, given the R6 dependence (R is the donor-acceptordistance) of the Forster energy transfer rate and assumingsimilar orientation of the chromophores involved. This dis-tance is depicted in the LH2-LH1 ring model of Fig. 5. In thisestimation of the LH2-LH1 distance, we have in addition,guided by the model, assumed that three B850 molecules at theLH2-LH1 contact site can each transfer energy to three LH1molecules. The distance is, however, only a weak function ofthe number of interacting molecules. Since the high-resolutiondata of McDermott et al. (3) for LH2 show that the B850 Bchlsare, at most, 15 A from the outer edge of the complex, then the27- to 32-A BChl-BChl distance between LH2 and LH1 rings

36A0

0 \

o\\0

FIG. 5. Schematic diagram of an (a13)9 LH2 ring (Left) next to an(a13)16 LH1 ring (Right). The rings are arranged to reflect the distancesinferred from energy transfer calculations, based on the data in refs.3 and 4. The scale bar shows the diameter of the ring of a-helices asreported in ref. 3; the complexes are viewed perpendicular to themembrane plane, the BChls are represented by the bars, the a-helicesare represented by the solid circles, and the f3-helices are representedby the open circles.

calculated from our measurements is entirely consistent withthe structural data (see Fig. 5).

This research was financially supported by The Swedish NaturalScience Research Council and EEC Research Grants SCI*-CT92-0796and ERBCHBGCT930361. M.R.J. and C.N.H. acknowledge financialsupport from the Biotechnology and Biological Sciences ResearchCouncil and the Welcome Trust. K.T. acknowledges receipt of trav-eling grants from the Royal Swedish Academy of Science.

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Biophysics: Hess et al.

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