The Journal of
Physical Chemistry
0 Copyright 1995 by the American Chemical Society
VOLUME 99, NUMBER 43, OCTOBER 26,1995
LETTERS Evidence for Coherent Energy Transfer in Allophycocyanin Trimers Maurice D. Edington, Ruth E. Riter, and Warren F. Beck* Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 Received: February 15, 1995; In Final Form: July 29, 1995@ We have performed two-color femtosecond pump-probe anisotropy experiments on allophycocyanin trimers, a photosynthetic light-harvesting system located at the core of. the phycobilisome of cyanobacteria. The phycocyanobilin (open-chain tetrapyrrole) chromophore dimer systems in allophycocyanin trimers exhibit an unusual ground-state absorption spectrum that has been attributed to molecular dimer systems with moderately strong dipole-dipole interaction strengths. Polarized two-color pump-probe transients, obtained with 80-fs 620-nm pump and 640-nm probe pulses, were fit by a global iterative reconvolution routine to a model consisting of dichroism-free and anisotropic functions. The anisotropy decays in a multiexponential manner from an initial maximum in the 0.58-0.7 range, with components exhibiting time constants of 10-30 fs, 280 fs, and 1 ps. The observation that the initial anisotropy is significantly larger than 0.4 evidences the initial presence of coherently excited molecular dimers in allophycocyanin trimers. The 10-30-fs anisotropy component reports the time scale for interexciton state relaxation, while the slower components report the time scale for localization of excitation on one of the dimer chromophores; the rate constant for localization is expected to be comparable with that estimated for Forster energy transfer between the two chromophores in a dimer. We suggest that the initial presence and decay of dimer electronic coherence might play a role in enhancing the energy-transfer function of allophycocyanin in the phycobilisome.
Introduction There are several examples of chromophore clusters in the known structures of photosynthetic light-harvesting proteins'-'2 and reaction c e n t e ~ s . l ~ -Very ' ~ recently, the crystal structure of the LH2 peripheral light-harvesting system from purple bacteria was determined by X-ray crystallographic method^'^*'^ to contain bacteriochlorophyll chromophore dimers arranged in a large "storage ring"; the LH1 light-harvesting system is thought to exhibit a similar arrangement of chromophores based on a low-resolution projection map obtained via electron micro~copy?~Whether or not coherent energy transfer between the chromophores in a cluster or ring structure in a photosynthetic light-harvesting system is of functional importance has attracted much interest in recent year~.~'-~O Chromophore dimer systems exist in trimers of allophy~ o c y a n i n , ~a' -light-harvesting ~~ system located at the core of *Abstract published in Aduance ACS Abstracts, October 1, 1995.
the p h y c o b i l i ~ o m e ~in~cyanobacteria. -~~ Although the structure of allophycocyanin is not yet known, a model based on the X-ray crystal structure of the related phycobiliprotein C-phycocyanin53698has been discussed by Beck and S a ~ e r .In~this ~ model, a C3 array of phycocyanobilin (open-chain tetrapyrrole) dimers is formed when allophycocyanin's a,@ monomer subunits aggregate into t r i m e r ~ . ~ I -The ~ ~ chromophore dimers are supposed to be composed of interacting phycocyanobilin chromophores in adjacent subunits; as shown in the inset of Figure 2, the ground-state absorption spectrum exhibited by allophycocyanin trimer preparations contains a partially resolved pair of bands which might arise from the two exciton states of a molecular dimer.3',32*39-4' The nature of energy-transfer processes in allophycocyanin trimers is currently being debated. Recent one-color and twocolor femtosecond pump-probe experiments by Sharkov and c o - ~ o r k e r s ~were ~ * ~interpreted ~ as showing that fast Forster energy t r a n ~ f e p -between ~~ the a80 and ,981 chromophores
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Figure 1. Pump-probe spectrometer used in two-color femtosecond anisotropy experiments. The input for this system was obtained from the amplified CPM laser described in the text. Symbols used: bs, 85%T/15%R dielectric beamsplittec SF14, Schott SF14 glass Brewster-angle dispersing prism pairs; eg, flowing ethylene glycol continuum cell; fp, 640-nm Fabry-Perot interference filter; p, calcite prism polarizer; delay, computercontrolled translation stage; pd, photodiode.
on different allophycocyanin subunits is responsible for the range of 0.58-0.7,directly indicating the initial presence of subpicosecond evolution of the pump-probe absorptiondimer electronic coherence in allophycocyanin trimers. The difference spectrum and for the decay of anisotropy. In contrast, anisotropy decays with time constants of 10-30 fs, 280 fs, and Beck and attributed the instrument-response-limited 1 ps. These results suggest new interpretations (and functional decay of anisotropy or dichroism they observed in allophycosignificance)for the subpicosecond spectral evolution43exhibited cyanin trimers in one- and two-color pump-probe experiments by allophycocyanin trimers in pump-probe transient spectra. with picosecond duration pulses to interexciton state relaxat i ~ n P ~a radiationless -~~ transfer of population between the two Experimental Section molecular dimer exciton states.52 Femtosecond fluorescence Sample Preparation and Handling. Allophycocyanin trianisotropy experiments by Xie and c o - ~ o r k e r swith ~ ~ allophymers and C-phycocyanin hexamers were isolated from cultures cocyanin trimers from Spirulina also revealed a subpicosecond of the AN112 mutant of Synechococcus PCC 6301, as described depolarization; these results were discussed in terms of the previously.34*m*61 The a subunit of C-phycocyanin was isolated Forster energy transfer and interexciton state relaxation hypothand purified according to published procedures.62 Phycobilieses. protein preparations were concentrated over PM-30 membranes We have conducted new two-color femtosecond pump-probe in an Amicon ultrafiltration cell to obtain an absorbance of 0.4anisotropy experiments on allophycocyanin trimers that extend 0.8 when measured in a cell with a 1-mm path length. The the anisotropy measurement into the sub-lOefs regime. These samples were stored in the dark at 4 OC suspended in a 100 experiments were motivated by the recent work by Wynne and mkl sodium phosphate buffer solution at pH 7.0. For femtoHochstrasses3 and by Knox and GUlens2 that anticipates that second spectroscopy, phycobiliprotein samples were flowed the initial absorption or emission anisotropy observed in a through a fused-silica cuvette (1-mm path length) at room coherently excited molecular dimer will differ significantly from temperature (22"C). Permanent photobleaching of the samples that exhibited by an isolated dipole (or by an incoherently did not occur during our experiments, judging from a compariexcited dimer). An isolated dipole exhibits, at most, an initial son of the continuous absorption spectrum recorded before and anisotropy of 0.4,54*55 whereas a coupled dimer can exhibit an after exposure to laser excitation. anisotropy as large as 0.7 owing to quantum mechanical interference effects arising from its two exciton states?2~53*55*56 Nile blue (Exciton) was used as received. For control pumpprobe anisotropy experiments, nile blue was dissolved in The decay of anisotropy exhibited by a molecular dimer directly ethylene glycol to obtain an absorbance bf 0.4-0.8 when reports the decay of the off-diagonal elements of the density measured in a cell with a 1-mm path length. Nile blue samples matfix that describe coherenus between the exciton states?u3358 were held in a 1-mm fused-silica cuvette in femtosecond The most rapid phase of anisotropy decay decays with a time spectroscopy experiments. constant that reports the time scale for interexciton state Absorption spectra were obtained at room temperature with relaxation, with subsequent anisotropy decay expected with a a Hitachi U2000 spectrophotometer controlled by LabVIEW time constant similar to that for weak-coupling limit Forster (National Instruments) routines. Fluorescence spectra were energy transfer between the two chromophores in the obtained with an SLM Aminco-Bowman Series 2 spectrofluoHence, the latter time constant determines the rate constant for rimeter. 10calization~~*~~ of excitation on one of the chromophores of a dimer. Femtosecond Spectroscopy. A schematic diagram of the The key observation reported in this work is that the femtosecond laser spectrometer employed in the described subpicosecond anisotropy decay observed in allophycocyanin experiments is shown in Figure 1. The output of a standard trimers is multiexponential and exhibits an initial value in the colliding-pulse, mode-locked (CPM) laser oscillatofi3 was
Letters amplified using two stages of double-open-confocal dye amplifiers,64both of which were pumped by a frequency doubled, diode-pumped, acoustooptically Q-switched Nd:YLF laser (Spectra Physics TFR with an LBO frequency-doubler assembly). As shown in Figure 1, amplified CPM pulses (2.252.50 pJ/pulse at a repetition rate of 3 kHz) were split by a dielectric beamsplitter to obtain pump (15%) and probe (85%) beams. The pump beam was injected into the pump arm of the modified Michelson interferometer after group-delay dispersion (GDD) compensation. The pump pulses (16 nJ/pulse at the sample position, adjusted in intensity by neutral density filters) exhibited 80-85 fs pulse widths (assuming sech* pulse shapes), as judged from autocorrelation measurements with a 300 pm-thick KDP crystal at the sample position. The spectrum of the pump pulses was centered at 620 nm and was 8-nm wide (fwhm), as recorded by the probe beam’s grating spectrometer (vide infra) with a 4-nm spectral band pass. The probe beam was obtained by generating a continuum in flowing ethylene glycol in a 3-mm flow cell. A three-cavity Fabry-Perot interference filter (10-nmfwhm band pass) with center wavelength of 640 nm (Omega Optics) was placed after the continuum cell to select the 640-nm probe wavelength. The wavelength-selected probe pulses (3 nJ/pulse at the sample position, adjusted in intensity by neutral density filters) were corrected for GDD and then were injected into the probe arm of the Michelson interferometer. The time-of-flight delay at the sample position between the pump and probe pulses was controlled by a computer-controlled translation stage (MellesGriot Nanomover, 0.333 fs/step) in the pump arm of the interferometer. The pump and probe pulses were linearly polarized by Glan laser calcite polarizers with extinction ratios of less than (Karl Lambrecht); the polarizers were rotated to set the relative polarization of the two beams 45’ apart at the sample position. The pump and probe beams were aligned parallel to each other and then were focused on the sample (or KDP crystal) by a 5-cm achromat. Pump-probe cross-correlation signals obtained with a 300 pm-thick KDP crystal at the sample position exhibited 120-fs (fwhm) widths. Robe light transmitted through the sample was analyzed by another Glan laser polarizer to select the polarization components of the probe pulses parallel and perpendicular to the polarization of the pump pulses. This method for obtaining the polarized pump-probe transients required for an anisotropy measurement is similar to that used by Galli et al.;57 in our experiments, however, separate runs were required for each polarization orientation. After the probe beam passed through a 0.27 m grating spectrometer (Spex 270m, 4-nm band pass), the intensity was measured by a photodiode detector (Newport) and gated integrator (SRS). Detection of pump-induced changes in the transmission of the sample was made by synchronously chopping the pump beam at 1500 Hz with a mechanical chopper (Palo Alto Research), which was phase-locked to the amplifier pulse-repetition frequency, and demodulating the gated integrator’s signal with a digital lockin amplifier (SRS 850). All instrumentation in the pump-probe spectrometer was controlled b y an Apple Macintosh Quadra 650 microcomputer using LabVIEW routines. ReSUltS
Figure 2 shows the two-color (620-nm pump, 640-nm probe) transients obtained at room temperature with allophycocyanin trimer preparations with the analyzer polarizer in the transmitted probe beam set parallel (upper transient, All) and perpendicular (lower transient, A l ) to the pump polarization. As shown in
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All) or perpendicular (lower transient, A l ) to the pump beam’s polarization. Superimposed on the polarized pump-probe transients are fitted curves obtained by a global iterative reconvolution program; both transients were fitted simultaneously with the same model parameters. The model used to describe the dichroism-free function All 2AL was a, E:=,a, exp(-t/r,). where al = 0.71, 51= 100 fs, a2 = 0.09, 52 = 690 fs, a3 = -0.88, 53 = 72 fs, and a, = 0.20 (the amplitudes a, were normalized so that the decay terms (a, 0) summed to unity). The model employed for the anisotropy decay was r(t) = r, Cl=Ij3, exp(-t/$,), with PI = 0.32, $1 = 20 fs, BZ= 0.15, $2 = 280 fs, j 3 ~= 0.1 1, $3 = lo00 fs, and r, = 0.12. The fit used an instrument-response function derived from a zero-background pump-probe cross-correlation signal dbtained with a KDP crystal (of 300-pm thickness) at the sample’s position. Inset: Continuous ground-state absorption (solid curve) and steady-state fluorescence emission (dash-dot curve) spectra exhibited at ambient temperature (22 “C) by allophycocyanin trimers. Superimposed are spectra for the 620-nm pump and 640-nm probe pulses used in the femtosecond anisotropy experiments. The absorption spectrum was recorded with a 2-nm spectral band pass; the uncorrected fluorescence emission spectrum was obtained with excitation at 540 nm and a 4-nm spectral band pass for both the emission and excitation monochromators. The pump and probe spectra were recorded with 4-nm spectral band pass with the spectrometer and conditions used in the anisotropy experiments.
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the inset of Figure 2, the pump pulses excite the ground-state absorption spectrum of allophycocyanin trimers well to the blue of the sharper, 652-nm feature that has been previously assigned to the band arising from the lowest energy exciton state; however, previous work by Beck and Sauer suggested that the two exciton bands overlap exten~ively.~~ The probe beam at 640 nm was chosen to be to the red and off resonance with respect to the pump beam. In addition, the use of a monochromator in the transmitted probe beam prior to the photodiode detector (see Figure 1) ensured that the probe wavelengths actually detected were completely off resonance from the pump beam’s spectrum. Thus, the observed polarized pump-probe transients should not contain contributions from the coherence (four-wave mixing) feature65-69observed when the pump and probe pulses temporally overlap in the sample in one-color pump-probe experiments, and the observed transients seem to be consistent with this expectation. Both of the polarized pump-probe transients show an increase in probe-beam transmission owing to ground-state depletion and stimulated emission. Note especially that the rise of the All trace seems to occur well before the A 1 trace rises from the baseline, which suggests a large value for the initial anisotropy. To properly extract the anisotropy information contained in the polarized pump-probe transients shown in Figure 2, we adopted a method (and a notation) analogous to that described by Cross and Fleming70 for the analysis of polarized fluores-
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Probe Delay (fs) Figure 3. Two-color anisotropy decay (data points) obtained with allophycocyanintrimers at ambient temperature (22 "C), as calculated directly from the polarized pump-probe transients shown in Figure 2. The measured anisotropy decay R(t) is shown superimposed with several anisotropy curves employing the model described in the caption for Figure 2 but with the time constant 41 fixed and varied from 5 to 40 fs; the model for the curve marked NP (not present) omits the component with time constant 41but otherwise employed the same model used by the other curves. The thick solid curve shows the anisotropy decay calculated directly from the fitted polarized pumpprobe transients shown in Figure 2, where the time constant 41 was 20 fs.The inset shows the same anisotropy decay out to a probe delay of 3 ps with the 41 = 20 fs anisotropy curve shown superimposed.
cence transients. This approach was used in the femtosecond fluorescence experiments of Xie and c o - w o r k e r ~and , ~ ~a similar method was used recently by Galli et al.57in their analysis of the anisotropy decay observed in magnesium tetraphenyl porphyrin. The measured intensities of the All and A l traces are assumed to represent a linear convolution of the true system response, all and a l , respectively, with the instrument-response function. The anisotropy function r(t) = (a&) - al(t))/(all(t) 2a~(t))describes the intensity relationship between all and a1 as a function of time. The all and a1 transients are simultaneously scaled by a dichroism-free function that describes the time dependence of the population in the excited state(s), K(t) = q ( t ) 2al(t), so that q ( t )= '/3K(t)[l 2r(t)] and al(t) = 1/3K(t)[l- r(t)].70371A global iterativereconvolution program was employed to fit the Ail and A 1 transients simultaneously in order to deconvolve the true r(t) and K ( t ) functions from the instrument-response function. The instrument-response function employed in the fitting procedure was derived from a zerobackground pump-probe cross-correlation signal measured at the sample position with a KDP crystal of 300-pm thickness. The smooth curves in Figure 2 describe a representative fit to the two transients obtained using this procedure. The model used for the K(t) function was a, C;=,a, exp(-tlz,), where = 0.71, ~1 = 100 fs, a2 = 0.09, ~2 = 690 fs, a3 = -0.88, ~3 = 72 fs, and a, = 0.20 (the amplitudes ai were normalized so that the decay terms (a, > 0) summed to unity). The nondecaying amplitude a, allows for the long lifetime (-1.5 ns) of the phycocyanobilin SIstate in allophycocyanin preparation^.^^ The r(t) function was modeled by r, Z;=I/31exp(-t/@,), with = 0.32, $1 = 20 fs, p2 = 0.15, $2 = 280 fs, p3 = 0.11, $3 = 1000 fs, and r, = 0.12. Figure 3 shows the measured anisotropy function R(t) = (A&) - Al(t))/(All(t) 2Al(t)) as calculated point-by-point directly from the All and A1 traces shown in Figure 2. The smooth curves drawn through the anisotropy data points describe several R(t) functions that are intended to show the contribution of the anisotropy component with the shortest time constant ($1) to the system response. In accord with the general findings of
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Cross and Fleming,70 the largest observed r(t) values are encountered on the rising edges of the polarized pump-probe transients shown in Figure 2. If the rising edge is excluded from the anisotropy analysis, one can adequately describe the remaining anisotropy decay using a model entirely lacking the decay component with time constant 41. Such a model is marked with the smooth solid curve labeled NP in Figure 3. The effect of the finite instrument response function (which is govemed by the -80-fs-wide pump and probe pulses) is shown in Figure 3 to cause a decrease in R(t) as the time constant 41 is made shorter, with the amplitude /?I fixed at 0.32. This result is also fully consistent with the results obtained by Cross and Fleming.70 The data are enclosed by the curves corresponding to a time constant 41in the 10-30-fs range. There is, however, a considerable degree of correlation between the values estimated for the amplitude /?I and time constant 41for the fastest anisotropy decay component; as the time constant 41is made smaller, the amplitude /?I required to describe the data increases. Given the currently available signaynoise ratio, this correlation results in a substantial uncertainty for both quantities. A reasonable description of the rising edge of the polarized pumpprobe transients, however, requires the presence of a substantial anisotropy component of intensity /?I ranging from 0.20 to 0.32, making the initial anisotropy r(0) lie in the range 0.58-0.70, well in excess of the value of 0.4 expected in the absence of dimer coherence. We performed several control experiments to ensure that the large 40) observed in allophycocyanin trimer preparations can be distinguished from an experimental artifact. Two-color pump-probe experiments conducted with nile blue solutions using the optical procedures and conditions used subsequently during the same experimental session with the allophycocyanin samples exhibited initial anisotropies no larger than 0.4 (results not shown). In addition, phycocyanobilin chromophores in isolated a subunits of C-phycocyanin do not exhibit initial anisotropies r(0) larger than 0.4 in one-color pump-probe experiments at 620 nm (results not shown); the a subunits of C-phycocyanin and allophycocyanin contain just one phycocyanobilin chromophore, so these systems contain chromophores in the isolated dipole limit.
Discussion The results described in this paper indicate that the phycocyanobilin dimers in allophycocyanin trimers exhibit a strong enough dipole-dipole interaction that they are initially coherently excited upon absorption of a photon. This conclusion is based upon the observation that the initial anisotropy r(0) observed in a two-color pump-probe experiment lies in the range 0.58-0.70, which is significantly larger than the value expected if the dimer chromophores were uncoupled. This situation may not be unique to allophycocyanin; it may be that the function of dimers and larger clusters of chromophores in light-harvesting proteins is somehow enhanced by the initial presence of dimer electronic coherence. In previously reported one-color anisotropy experiments on the bacteriochlorophyll a protein isolated from Prosthecochloris aestuarii,I2 Struve and co-workers observed an initial anisotropy of -0.6, suggesting the initial presence of coherence in that system. The present results indicate that allophycocyanin trimers exhibit a multiexponential anisotropy decay, with the fastest decay component in the 10-30-fs regime and slower components in the 280-fs and 1-ps regime. The slower decay components are comparable to those observed previously (by Sharkov and c o - ~ o r k e r via s ~ pump-probe ~~~~ experiments and by Xie and c o - w o r k e r ~via ~ ~fluorescence up-conversion experi-
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Letters ments) and might be explained by fast Forster energy transfer between the chromophores of an uncoupled phycocyanobilin dimer. The 10-30-fs component, however, decays on a time scale that is much too short for it to be explained by energy transfer?I especially given the results obtained by Sauer and of Forster energy-transfer rates between S ~ h e ein r ~calculations ~ the chromophores in the phycocyanobilin dimers of the related phycobiliprotein, C-phycocyanin. The theory for molecular dimers discussed by Wynne and Hoch~trasse$~ and by Knox and Gulen52would account for the anisotropy decay in terms of dephasing of the initial dimer electronic coherence prepared by the absorbed photon. The analysis of the molecular dimer theory by van Amerongen and S t r u ~ eshows ~ ~ that a complex multiexponential anisotropy decay would be expected if a spectrally broad femtosecond laser pulse excites the two exciton states of a molecular dimer, with the initial decay to r(0) 5 0.4 characterized by a rate constant 2 r = y f, where y and y' are the rates for downward and upward interexciton state relaxation. Subsequent anisotropy decay would occur with a rate constant comparable to that expected for Forster energy transfer in the uncoupled dimer limit. The end product of the dephasing would be localized chromophore states. I recently performed calculations of the Matro and fluorescence anisotropy decay for a chromophore dimer like that in C-phycocyanin in which effects owing to vibrational relaxation and dephasing were included. The initial very rapid decay we observed in allophycocyanin's anisotropy decay is generally consistent with the behavior indicated by the calculated anisotropy decays. Owing to coherent molecular vibrations and to coherent energy transfer between the chromophores of the dimer, the calculated anisotropy decays exhibited oscillatory components. Using a simplified Redfield theory, Matro and Cina showed that coupling of the chromophore dimer to a thermal bath causes an efficient damping of the anisotropy oscillations. The anisotropy decay we observed in allophycocyanin trimers using pump-probe methods does not suggest the presence of underdamped oscillatory components at the present signdnoise ratio; however, the deviation of the anisotropy data in the 0-125-fs probe-delay region of Figure 3 from the superimposed trends predicted by the sum-of-exponentials model suggests the possible presence of an oscillatory component that might be resolved with better time resolution. Being that the anisotropy decay observed in allophycocyanin trimers is best explained by dephasing of dimer electronic coherence, the initial 10-30-fs anisotropy decay component reports the time scale for interexciton state relaxation according to the theory discussed above. A similar time scale for interexciton state relaxation may be present in the purple bacterial light-harvesting protein LH2. Sundstrom and cow o r k e r ~ ~previously ~*~' observed a highly polarized transition from net photobleaching io net excited-state absorption with a 520-fs time con8tant in LH2. They assigned this transition to an interexciton state relaxation process involving the exciton states of a bacteriochlorophyll cluster. Given what is now known about the structure of LH2,I8 it may be possible to describe the interexciton state relaxation in terms of the subunit bacteriochlorophyll that form the LH2 ring structures.'* If interexciton state relaxation on the 10-30-fs time scale actually does occur in allophycocyanin trimers, then one might be able to observe an ultrafast red-shift in time-resolved pumpprobe absorption-difference spectra that evidences the relaxation of population from the upper to the lower exciton state. The experiments reported by Sharkov and c o - w o r k e r ~did ~ ~not
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provide sufficient spectral coverage and time resolution to test this prediction. The population-only function K(t) we obtained from the polarized pump-probe transients shown in Figure 2 is dominated by 70-fs rise and 100-fs decay components that suggest spectral evolution on a time scale faster than that detected by previous work, but these components are still slower than that detected via the anisotropy function r(t). Note that the 640-nm probe pulses are located near to an apparent isosbestic point in the ground-state absorption spectrum (see the inset of Figure 2) marking the overlap of the sharper, 652nm component with the broader, 620-nmcomponent; if these two components represent the overlapping lower and upper exciton bands, respectively, then a 10-30-fs K(f) component arising from interexciton state relaxation might not be observed in our experiments. The slower 280-fs and 1-ps anisotropy decay components observed in allophycocyanin trimers in this work occur over a similar time scale to that of the -500-fs time scale for blueto-red spectral redistribution that was detected in degenerate pump-probe and transient absorption spectroscopy experiments by Sharkov and c o - w ~ r k e r s . ~ ~The * ~ present ~ , ~ ~ anisotropy results imply that the observed spectral evolution accompanies a dephasing process that results in lo~alization.5~9~~ As suggested previously by Struve and c o - w ~ r k e r localization s ~ ~ ~ ~ ~ may cause distinctive changes in the pump-probe absorption-difference spectrum. Because a consequence of localization is a redistribution of oscillator strength back to the chromophore or site states from the initial distribution of oscillator strengths in the exciton states,59 we suggest that this dephasing process may have important implications in the control of Forster excitation transfer between light-harvesting proteins in the photosynthetic antenna.
Note Added in Proof. Since the submission of the revised version of this paper, we learned that the X-ray crystal structure of allophycocyanin trimers from Spirulina platensis has been determined by Huber and co-workers (Brejc; et al., J. Mol. Biol. 1995, 249, 424-440). The structure is consistent with the model discussed in this paper. Acknowledgment. This work was supported by grants from the Petroleum Research Fund (administered by the American Chemical Society), the Searle Scholars Program (the Chicago Community Trust), and the United States Department of Agriculture. Additional support came from a Cottrell Scholars Award to W.F.B. (from the Research Corporation). M.D.E. was supported by a graduate fellowship from the Packard Foundation. We would like to thank Professors Walter S.Struve (Iowa State University) and Villy Sundstrom (Lund University) for helpful conversations and encouragement. We thank Professor W. M. LeStourgeon (Department of Molecular Biology, Vanderbilt University) for the use of his spectrofluorimeter, and we thank Professor Joe Brechem (Department of Molecular Physiology and Biophysics, Vanderbilt University) for help with his global iterative convolution program. References and Notes (1) Fenna, R. E.; Matthews, B. W. Nature 1975, 258, 573-577. (2) Fenna, R. E.; Ten Eyck, L. F.; Matthews, B. W. Biochem. Biophys. Res. Commun. 1977, 75, 751-756. (3) Matthews, B. W.; Fenna, R. E.; Bolognesi, M. C.; Schmid, M. F.; Olsen, J. M. J. Mol. Biol. 1979, 131, 259-285. (4) Tronrud, D. E.; Schmid, M. F.; Matthews, B. W. J. Mol. Biol. 1986, 188, 443-454. (5) Schirmer, T.; Bode, W.; Huber, R.; Sidler, W.; Zuber, H. J. Mol. Biol. 1985, 184, 251-217. (6) Schirmer, T.; Huber, R.; Schneider, M.; Bode, W.; Miller, M.; Hackert, M. L. J. Mol. Biol. 1986, 188, 651-616.
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