Femtosecond Energy-Transfer Dynamics between

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J. Phys. Chem. B 1998, 102, 881-887

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Femtosecond Energy-Transfer Dynamics between Bacteriochlorophylls in the B800-820 Antenna Complex of the Photosynthetic Purple Bacterium Rhodopseudomonas acidophila (Strain 7750)§ Ying-Zhong Ma,† Richard J. Cogdell,‡ and Tomas Gillbro*,† Department of Physical Chemistry, UniVersity of Umeå, S-901 87 Umeå, Sweden, and DiVision of Biochemistry and Molecular Biology, UniVersity of Glasgow, Glasgow G12 8QQ, UK ReceiVed: July 3, 1997; In Final Form: October 15, 1997

Femtosecond one-color pump-probe measurements were performed on the B800-820 light-harvesting antenna complex of Rhodopseudomonas acidophila, strain 7750, at room and low temperature (mainly 77 K). The isotropic and anisotropic decays obtained within the B800 band are clearly wavelength- and temperaturedependent. A fast isotropic decay component at room temperature was found to have a lifetime of 0.30 ps at 784 nm, 0.54 ps at 790 nm, and 0.75 ps at 800 nm. The interband energy-transfer time was found to be ∼0.75 ps at room temperature, which slows down to ∼0.90 ps at 77 K. The time-resolved anisotropy, obtained around 790 and 800 nm, exhibits a single-exponential decay feature with a depolarization time of ∼1.1 ps at room temperature and ∼450 fs at 77 K. Measurements within the B820 band showed a fast isotropic component having a lifetime 60-80 fs. The corresponding time-resolved anisotropy decay occurs in 100-130 fs at room temperature and ∼240 fs at 77 K. In view of the general similarity in the energy-transfer dynamics between the B800-820 complex and other LH2 antenna, we conclude that the markedly blue-shifted B820 band has no substantial effect on the interband transfer rate.

I. Introduction The primary processes of photosynthesis are initiated by photon absorption in different kinds of light-harvesting antenna, followed by a very rapid and efficient excitation energy transfer to the reaction center (RC), where the primary charge separation occurs.1,2 Among bacteria, algae, and higher plants, the purple bacteria are relatively simple and well characterized with known structures of RCs of certain species3-5 and recently two antenna complexes.6,7 In general, the light-harvesting system of purple bacteria possesses a core and a peripheral antenna, the so-called LH1 and LH2 complexes.1 LH1 surrounds the RC and has a lower singlet excited BChl a energy level than that of LH2. LH2 usually has two absorption bands: the higher energy one is usually located around 800 nm, and the other band is found at different wavelengths depending on the species and the strain of bacteria, as well as the growth conditions. According to the position of this long wavelength band, LH2 antenna are labeled as B800-820, B800-830, or B800-850 complexes. The purple bacterium Rhodopseudomonas (Rps.) acidophila, for instance, is capable of synthesizing different types of LH2 light-harvesting complexes with considerably different long wavelength absorption bands, depending on the light intensity used for growing the cells. Among the three wild-types of Rps. acidophila that have been isolated, i.e., strain 7050, 7750, and 10050, only strain 7750 shows an additional temperature effect.8 At 20-24 °C and moderate light intensities, only the B800820 complex is synthesized.8 A FT resonance Raman spec§ This work was presented at the Workshop on “Light-Harvesting Physics” held from 14 to 17 September 1996 in Birstonas, Lithuania. † University of Umeå. ‡ University of Glasgow. * Corresponding author. Telephone 46-90-165368, Fax 46-90-167779, e-mail [email protected].

troscopic study shows that the two 2-acetyl groups of the 820 nm absorbing BChl a molecules are free from hydrogen-bonding interactions in this complex.9 Furthermore, the primary X-ray diffraction data for crystals of the B800-820 light-harvesting complex from the strain 775010 show a very similar unit cell and space group comparable to the structure defined for the B800-850 complex from Rps. acidophila (strain 10050).6 Such a close similarity suggests that these two LH2 light-harvesting complexes have very similar structures. Thus, it may serve as an ideal natural antenna system to examine the effect of the interaction between the BChl molecules and protein environment and of spectral shift on the excitation energy-transfer processes. In this paper, we report detailed direct femtosecond timeresolved measurements on the B800-820 complex isolated from Rps. acidophila (strain 7750). The experiments were performed at several excitation and detection wavelengths and at both room and low temperature (mainly at 77 K), using the so-called onecolor pump-probe technique with a typical pulse length of 50 fs. Analysis of the isotropic and anisotropic kinetics obtained, in combination with information from the newly determined LH2 structure, enabled us to get new insights into the ultrafast intra- and interband energy-transfer and relaxation processes. II. Materials and Methods The B800-820 complex was prepared as described previously11 and stored in a freezer (-20 °C) until use. The sample was resuspended in 50 mM Tris-HCl (pH 8.0) to give a maximum absorbance of ∼0.5 around 800 nm in a 1 mm glass cuvette for the femtosecond pump-probe measurements. A rotating cell of 1 mm path length with glass windows was used in some room-temperature experiments to check for any possible sample damage due to laser illumination during the measurement. Since no observable difference in the kinetics was noted

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Figure 1. NIR steady-state absorption spectra at both room temperature (solid line) and 77 K (dashed line). These two spectra have been normalized at the maxima of the B800 band.

for the static and rotating samples, we will not specify the sample state for the kinetics reported in this paper. Lowtemperature experiments were performed using an Oxford Instruments liquid N2 cryostat. To obtain a good quality glass upon freezing, the sample was mixed in about 60% (v/v) glycerol. The steady-state absorption spectra were measured with a Beckman DU-70 UV/vis spectrophotometer. The femtosecond absorption measurements were performed using the one-color pump-probe technique as described previously.12 The maximum excitation intensity was less than 3 × 1014 photons pulse-1 cm-2, which was further attenuated three or five times with neutral density filters in order to confirm that the kinetics were free of excitation annihilation effects. The relative polarization of the pump and probe beams was varied between parallel, perpendicular, and magic angle (54.7°) by using a Berek’s polarization compensator in the pump and a Soleil-Babinet compensator in the probe beam. III. Results A. Steady-State Absorption Spectra. Figure 1A shows the NIR steady-state absorption spectra of the B800-820 complex at room temperature and 77 K. The B800 and B820 bands are strongly overlapping at room temperature with maxima at 799 and 818 nm, as observed previously.8 These two absorption bands are clearly separated at 77 K with maxima located at 797 and 821 nm, as a result of spectral band narrowing and shifts at low temperature. B. Isotropic Transient Absorption Kinetics. Isotropic decay kinetics were measured at both room temperature and 77 K at a number of wavelengths in the range from 784 to 840 nm. The kinetics obtained within the B800 band, as observed at 784, 790, and 800 nm at room temperature (Figure 2), initially show a fast recovery from photobleaching (PB) and/or stimulated emission (SE), followed by the excited-state absorption (ESA) of the B820 band (with positive ∆A). This type of transient absorption kinetics is commonly found for the B800 bands of different LH2 light-harvesting antenna of purple bacteria.12-15 The kinetics measured at longer wavelengths, as seen at 809, 819, and 840 nm, are dominated by PB and SE. In general, most of the kinetics could be well fitted to a twoor three-exponential decay function with the fitting parameters summarized in Table 1. At room temperature, the dominant and also the most accurately determined component within the B800 band was found to have a wavelength-dependent lifetime, which increases from 0.30 ps at 784 nm to 0.54 and 0.75 ps at

Figure 2. Isotropic decay kinetics measured within the B800 band at room temperature (symbols) and their best fits (solid lines). These kinetics have been modified in their relative amplitudes and offset for the sake of clarity.

TABLE 1: Decay Lifetimes and Relative Amplitudes of the Isotropic Transient Absorption Kinetics at Room (RT) and Low Temperaturesa λ (nm)

T (K)

784 790 800 809 819 840

RT RT RT RT RT RT

787 792 792 792 792 799 810 821 840

77 77 100 150 185 77 77 77 77

a

τ1 (fs)

A1

τ2 (ps)

A2

τ3 (ps)

A3

18 81 84 58

0.60 0.67 0.59 0.61

0.30 0.54 0.75 1.18 0.97 1.03

1.0 1.0 0.40 0.21 0.16 0.13

57 289 288 55 260 180

-0.58 -0.21 -0.05 0.12 0.25 0.25

184 440 280 174

0.22 0.27 0.33 0.30

86 64

0.45 0.48

0.64 1.09 0.91 0.86 0.58 0.90 0.89 0.93 0.87

0.78 0.73 0.67 0.70 1.0 1.0 1.0 0.20 0.07

37 640 220 190 363 333 260 130 210

-0.52 -0.51 -0.45 -0.37 -0.42 -0.52 -0.74 0.36 0.46

The components with negative amplitudes stand for ESA.

790 and 800 nm, respectively, as shown in Figure 2. The slow decay components with negative amplitudes represent the decay of the ESA of the B820 molecules, which proceeds in a time scale of roughly >100 ps, limited by the relatively short scan range used (50 ps). For a B800-820 preparation from the strain 7050, this slow decay has been determined to be >600 ps by picosecond time-resolved transient absorption experiments.16,17 The very fast decay component obtained at 800 nm and room temperature (see Table 1) is due to the coherent spike. Upon decreasing the temperature to 77 K, the ESA of the B820 BChl a molecules turned out to be more pronounced in the transient absorption kinetics observed within the B800 band, as seen at 787, 792, and 799 nm. The dominant decay component has a lifetime only slightly longer than the corresponding value obtained at room temperature. For instance, at 800 nm, the lifetime increases from 0.75 ps at room temperature to 0.90 ps at 77 K. The kinetics measured at 792 nm at several

B800-820 Antenna Complex of Rps. acidophila

Figure 3. Isotropic decay kinetics measured at 799 nm and 77 K (open circles) and the best fit (solid line). The residual of this fit is shown above.

temperatures between 77 and 150 K show that an additional fast component is required to get a better fit. This component has a temperature-dependent lifetime, which increases from 174 fs at 150 K to 440 fs at 77 K. As an example, Figure 3 shows the isotropic decay measured at 800 nm and at 77 K. The spectral evolution of the PB, SE, and ESA upon decreasing the temperature results in the apparent change in the kinetics pattern observed at 810 nm where the B800 and B820 bands are strongly overlapping (see Figure 1), at low and room temperature as shown in Figure 4. At 77 K, the kinetics are dominated by the ESA of the B820 with no initial bleaching due to the B800 molecules, while at room temperature a bleaching is observed instead. However, it should be noted that both the traces are dominated by the contribution from the B820 BChl a molecules. The kinetics measured at 809, 819, and 840 nm at room temperature as well as those at 821 and 840 nm obtained at 77 K show a very fast component with a lifetime of 60-80 fs, which exhibits only a small variation, as shown in Table 1, with the detection wavelength. The two other decay components were found to have time constants of approximately 1 and 50300 ps, respectively. The lifetime of the last component is again not well defined due to the short scan range. These isotropic decays are also weakly temperature-dependent. As an example, Figure 5 shows the isotropic kinetics obtained at ∼820 nm at both room temperature and 77 K, together with the best fits. C. Time-Resolved Anisotropy. The presence of ESA in the recovery kinetics observed within the B800 band, at both room temperature and 77 K, makes it difficult to obtain the anisotropy decay directly from the raw data. This is probably due to considerably different transition dipole orientations for the PB, SE, and ESA. Therefore, we calculated the anisotropy decays by indirect methods. Two methods have been used in this work to analyze the anisotropic decays obtained within the B800 band. The first method begins with the separation of the ESA from the experimental profiles by data analysis, since the contributions from PB/SE and ESA have different origins, from the B800 and B820, respectively. As in the case of the isotropic

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Figure 4. Isotropic decay kinetics measured at ∼810 nm at both room temperature and 77 K (symbols) and their best fits (solid lines). The kinetics have been modified in their relative amplitudes and offset for clarity.

Figure 5. Isotropic decay kinetics measured at ∼820 nm at room temperature (RT) and 77 K (symbols) and their best fits (solid lines). The kinetics have been normalized at the initial maximum transient absorbance and offset for clarity with the horizontal dashed lines indicating the baselines of the kinetics.

kinetic fitting, all the parallel and perpendicular polarized kinetics could be fitted well to two- or three-exponential decay functions with a slow component describing the ESA recovery. The ESA component was subtracted from the measured parallel and perpendicular kinetics, according to the lifetime and amplitude obtained from the best fit. These corrected kinetics were used to calculate the anisotropy decay using the wellknown relation

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TABLE 2: Summary of Anisotropy Decay Lifetimes, Initial and Residual Values Obtained by the Two Fitting Procedures for the Depolarization Kinetics Measured at Room Temperature (RT) and 77 Ka λ (nm)

T (K)

τ (fs)

r(0)

r(∞)

790 800 819 840

RT RT RT RT

1140 (1090) 1150 (1090) 100 130

0.31 (0.30) 0.35 (0.40) 0.36 0.40

0.08 (0.0) 0.10 (0.03) 0.11 0.12

792 799 821 840

77 77 77 77

470 (210) 450 (330) 15 240

0.41 (0.28) 0.47 (0.37) 0.40 0.35

0.08 (0.16) 0.15 (0.26) 0.21 0.16

a

The results shown in parentheses are obtained using eq 1 in the separated fitting method.

r(t) )

∆A|(t) - ∆A⊥(t) ∆A|(t) + 2∆A⊥(t)

(1)

where A|(t) and A⊥(t) are the corrected transient absorbances with parallel and perpendicular polarizations, respectively. The anisotropy kinetics were fitted using a deconvolution program taking the autocorrelation function as the response function. The alternative and also more proper procedure employed a simultaneous fitting to the parallel and perpendicular kinetics according to the following relations:

∆A|(t) ) ∆Aiso[1 + 2r(t)] ∆A⊥(t) ) ∆Aiso[1 - r(t)]

(2)

where the isotropic decay was fitted first, and the anisotropy r(t) was assumed to be monoexponential. All these fitting results have been summarized in Table 2. For the anisotropy decay determined at 790 and 800 nm and at room temperature, these two procedures give identical lifetimes of approximately 1.1 ps while the initial and residual anisotropy values are different. Figure 6 shows the fitting curves for the anisotropy decay obtained at 800 nm. This wavelength-independent anisotropy decay time is very similar to that obtained for the B800 band of the LH2 preparations from Rb. sphaeroides and Rps. palustris at room temperature, also determined by femtosecond transient absorption experiment.14 At 77 K, the corresponding anisotropy kinetics calculated by these two methods exhibit faster decay times than those determined at room temperature, but with clearly different lifetimes. The lifetime obtained by the simultaneous fitting procedure is ∼450 fs at 792 and 799 nm, while the first procedure ends up with a shorter and also wavelength-dependent lifetime of 210-330 fs. The shorter lifetime indicates that the subtraction of the larger portion of ESA from the kinetics measured at 77 K, as used in the first fitting procedure of the anisotropy decay, distorts the anisotropy kinetics. Furthermore, as also seen in the anisotropic decays obtained at room temperature, the initial and residual anisotropy decays retained by these two methods are also different. These fitting curves are shown in Figure 7. We want to point out that somewhat similar shorter anisotropic decay times were also found recently in the B800 bands (790 and 800 nm) of intact chromatophores of Rb. sphaeroides and Rps. palustris at 77 K as fast dominating components.15 It is also seen in our recent experiments on the B800-850 of Rps. acidophila (strain 10050).12 The anisotropy decays observed at 819 and 840 nm, at room temperature and 77 K, can be fitted well with a singleexponential decay with an initial value of 0.4 or very close to

Figure 6. (A) Decay kinetics measured at 800 nm and room temperature with parallel (open circles) and perpendicular (open triangles) polarizations. The solid lines show the simultaneous fitting curves using eq 2. (B) The anisotropy decay (thin line) and its best fit (thick line) calculated using eq 1 in the separated fitting procedure.

0.4 at time zero, as shown in Figure 8. The decay time at room temperature is around 100-130 fs, while it increases by a factor of 2 at 77 K, i.e., 240 fs. The depolarization time at room temperature is in excellent agreement with the 100/130 fs anisotropic decay determined by femtosecond transient absorption experiments on a LH2 complex from Rb. sphaeroides at 864 nm and at room temperature.18,19 It may also correspond to the fast depolarization component with a lifetime of 50-90 fs, observed recently in the isolated B800-850 complexes of Rb. sphaeroides by fluorescence up-conversion measurements.20 The extremely fast anisotropic decay at 821 nm, however, probably does not represent a meaningful dynamic process. The reason is that it is very close to the isosbestic point,16 and thus the kinetics measured with parallel polarization may be largely distorted by the coherent spike at initial times. The residual anisotropy values are wavelength-dependent and are higher at 77 K than those obtained at room temperature at similar wavelengths (see Table 2). IV. Discussion It is evident that the isotropic and anisotropic decay lifetimes obtained using the B800-820 complex do not exhibit substantial difference in comparison to other LH2 complexes,12,14,15,18-20 although there is a larger blue shift of the B820 band. It thus indicates that the intra- and interband relaxation and transfer processes are very similar in these LH2 complexes. In terms of the interband energy transfer, this similarity is consistent with the previous hole-burning work at even lower temperature.21,22 Furthermore, the similar energy-transfer dynamics also support the view that the B800-820 complex is structurally close to the B800-850 complex. As shown in Table 1, the fast isotropic decays observed within the B800 band are clearly wavelength- and temperature-

B800-820 Antenna Complex of Rps. acidophila

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Figure 7. Parallel (open circles) and perpendicular (open triangles) decay kinetics measured at 77 K at 792 nm (A) and 799 nm (B). The solid lines represent the simultaneous fitting curves using eq 2. The anisotropic decays (thin lines) and their best fits (thick lines) calculated using eq 1 in the separated fitting procedure are shown in (C) and (D). These two procedures of anisotropy analysis give clearly different decay lifetimes as shown in Table 2.

Figure 8. Parallel (solid lines) and perpendicular (dotted lines) polarized decay kinetics at 840 nm measured at room temperature (A) and 77 K (B). The corresponding time-resolved anisotropy decays (thin lines) and their best single-exponential decay fits (thick lines) are shown in (C) and (D).

dependent. So far, there is a large evidence indicating that the B800 band is dominated by inhomogeneous broadening.21-24 Thus, we would expect a temperature dependent intraband transfer: at room temperature both uphill and downhill transfer are possible whereas the uphill transfer would be partially inhibited at low temperatures, such as at 77 K. Since both the uphill and downhill transfer processes would cause depolarization due to the geometry of the Qy transition dipole moments

of the B800 molecules, the monoexpeonential anisotropy decay with lifetime of ∼1.1 ps, obtained within the B800 band at room temperature, should contain the contributions from both processes. The collective nature of the anisotropic decay implies that the depolarizatiom time should not be simply assigned as transfer time between BChl a molecules. At 77 K, the dominating downhill transfer could result in the excitation of the BChl a with lower energy of the first excited

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singlet state (S1), which may further undergo interband transfer. These subsequent processes were observed by van Grondelle and co-workers in LH2 from Rb. sphaeroides at 77 K, where a rise component of 440 fs was obtained upon excitation at 791 nm and detection at 810 nm.25 The dominating downhill transfer at low temperature also indicates that the depolarization would be different, in comparison to that at room temperature. Assuming that the these B800 molecules are organized in the same ring structure as found in the B800-850 complex (strain 10050),6 the anisotropy change due to a single step transfer can be estimated using the following relation26

r(∞) ) 1/5(3 cos2 θ - 1)

(3)

The C9 symmetry of the B800 ring indicates that the angle θ between the transition dipole moments of the nearest-neighboring molecules is 40°. Calculating r(∞) using this value gives a terminal anisotropy of 0.16, which agrees well with residual anisotropy determined at 799 nm (see Table 2). The even lower residual value obtained at 792 nm indicates that it contains, on average, the contributions of more than one-step transfer. The additional fast isotropic component obtained at 792 nm also indicates the presence of intraband transfer. At 77 K, this component has a lifetime of 440 fs (see Table 1), which is very close to the anisotropic decay time of 470 fs, determined at the same wavelength and temperature. It is pertinent to mention here that limited intraband transfer within the B800 ring, prior to transfer to the B850 molecules, was also seen in previous studies.27,28 The alternative interpretation for the shorter anisotropy decay time within the B800 band at 77 K compared to that at room temperature could imply a difference in the mechanism of depolarization; i.e., it arises from an excitonic relaxation instead of excitation hopping. The excitonic states involved could be some high-lying excitonic levels of the B820 that spectrally overlap with the B800 band. Assuming that the B820 molecules form a similar structure as the B850 pigments of the B800850 complex (strain 10050),6 the close spatial arrangement of these molecules would result in strong coupling between the BChl a molecules within the protomer, as well as in the proximal protomers. The corresponding coupling values were roughly estimated to be ∼340 and ∼380 cm-1, respectively.12 Such a coupling may result in the formation of excitonic states. In a simplest case (ignoring diagonal and off-diagonal energy disorder and considering only the nearest-neighbor interactions), the energy levels of the one-exciton state, Ek, can be expressed as12,29,30

Ek ) E0 ( [M12 + M22 + 2M1M2 cos(2πk/N)]1/2 (k ) 0, 1, 2, ..., N - 1) (4) where M1 and M2 represent the pairwise interactions between the BChl a molecules within the same and different protomers, respectively, N is the number of protomers, and E0 is the excitation energy of the S1 state of an isolated BChl a. The one-exciton state consists of two branches of excitonic levels, arranged symmetrically with respect to E0. Each branch is comprised of one nondegenerate and four doubly degenerate excitonic levels. Since the dipole moments for the two BChl a molecules within the same protomer are not exactly antiparallel, the excitonic states in the higher-lying branch can thus also carry a certain dipole strength. Numerical estimation showed that these excitonic levels span ∼1400 cm-1, and several doubly degenerate excitonic states overlap with the B800 band.12 Inclusion of the site inhomogeneity, as found in the hole-burning

experiments,23 may remove the degeneracy and further cause the transitions to the higher-lying excitonic states to be optically allowed. Similar width of the excitonic manifold was also obtained by Sauer et al. in a more rigorous calculation.31 The corresponding excitonic scheme for the B820 could be different due to an increase in E0 and/or a change in the coupling between molecules. The variation of E0, M1, and M2 could occur as the result of difference in hydrogen-bonding,9 protein environment,32 and even the spatial arrangement and orientation of BChl a molecules.33 Furthermore, these excitonic states may also be coupled with the S1 state of the B800 band. This coupling has been suggested to be a possible mechanism for the additional B800 decay channel identified for the B800-850 complexes of Rps. acidophila and Rb. sphaeroides.24 In addition, those higher-lying excitonic states of the B820 may also serve as excellent acceptors for the interband energy transfer from B800. Such an excitonic model for the acceptor states of the interband transfer is supported by very recent holeburning work on various B800-850 complexes34 as well as theoretical studies.35-37 As seen in the present study, the energy transfer from B800 to B820 takes ∼0.75 ps at room temperature as measured at 800 nm (Table 1). This value is very similar to the transfer time found in other LH2 complexes,12,14,15,38 although the longer wavelength absorbing band is greatly blueshifted in the B800-820 complex. At 77 K, this transfer takes 0.90 ps, only slightly shorter than the interband transfer time in the B800-850 complexes of Rb. sphaeroides39 and Rps. acidophila (strain 10050),12 obtained in low-intensity pumpprobe experiments. This transfer time is consistent with the results obtained in the previous subpicosecond (∼500 fs pulse length) pump-probe experiments on the B800-820 complex from the strain 7050 at 77 K, where a wavelength-dependent decay lifetime of 0.7 ps at 800 nm and 1.0 ps at 804 nm was found.13,40 Hole-burning experiment further shows that this transfer slows down to 2.00 ( 0.10 ps at 4.2 K.21,22 The similarity of the interband energy transfer at 77 K in the B800820 and B800-850 complexes strongly excludes the possibility of vibrationally relaxed monomeric B800 to B820 transfer as suggested previously,21 since the large difference in the spectral overlap between the donor and acceptor in these two cases should result in a significantly different energy-transfer rates in view of the Fo¨rster mechanism. Moreover, the involvement of some vibronic modes of the B820 (B850), which has been proposed to be the mechanism for the interband transfer in LH2,21,23,41 has been shown to be unlikely.35 The presence of these differently polarized excitonic levels may also account for the fast isotropic and anisotropy decays observed within the B820 band, as a result of relaxation between the excitonic states. In a recent study on a LH2 complex of Rb. sphaeroides using 12 fs pulses, Nagarajan et al. identified a rapid anisotropy decay time of 20 fs from an initial value larger than 0.4.42 These authors attributed this to dephasing of excitonic states, in view of the presence of the orthogonally polarized degenerate excitonic states.43,44 V. Concluding Remarks The B820 band in this B800-820 complex is largely blueshifted when compared to the structure defined B800-850 complex of the same species (strain 10050) and other LH2. This shift was suggested to be due to the absence of the hydrogenbonding interactions for the two 2-acetyl groups of BChl a molecules9 and may be also due to the difference in protein environment32 and even the spatial arrangement and orientation of BChl a molecules.33 As evident from this detailed study,

B800-820 Antenna Complex of Rps. acidophila this spectral shift does not have a significant influence on the excitation energy-transfer processes in the complex. The interband transfer occurs in ∼0.75 ps at room temperature and ∼0.90 ps at 77 K and is comparable to the values determined in other LH2 complexes. Wavelength-dependent isotropic decay components were found within the B800 band at room temperature, with lifetime of 0.30 ps at 784 nm, 0.54 ps at 790 nm, and 0.75 ps at 800 nm. The additional isotropic component obtained at 792 nm at low temperatures represents downhill transfer between the B800 molecules. The temperature-dependent constants were found to 440 fs at 77 K, 280 fs at 100 K, and 174 fs at 150 K. Furthermore, the anisotropy kinetics, obtained at about 790 and 800 nm, exhibit a single-exponential decay of ∼1.1 ps at room temperature and ∼450 fs at 77 K. This faster depolarization at low temperature is proposed to originate from either a difference in the energy-transfer pathway (downhill transfer) under low temperature or an excitonic relaxation. The excitonic levels involved may be those higher-lying states of the B820 molecules, which spectrally overlap with the B800 band. The anisotropy decays measured within the B820 band are found to occur in ∼100-130 fs at room temperature, while it slows down to ∼240 fs at 77 K. Acknowledgment. This research was supported by the Swedish Natural Science Research Council, the British Research Council, and the Human Science Frontiers Program. Y.-Z. Ma is grateful to the Kempe Foundation for financial support. We thank Dr. A. N. Macpherson for a critical reading of the manuscript. References and Notes (1) van Grondelle, R.; Dekker, J. P.; Gillbro, T.; Sundstro¨m, V. Biochim. Biophys. Acta 1994, 1187, 1-65. (2) van Grodelle, R. Biochim. Biophys. Acta 1985, 811, 147-195. (3) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180, 385-398. (4) Allen, J. P.; Feher, G.; Yeates, T. O.; Komiya, H.; Rees, D. C. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 5730-5734. (5) Chang, C.-H.; El-Kabbani, O.; Tiede, D.; Norris, J.; Schiffer, M. Biochemistry 1991, 30, 5352-5360. (6) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517-521. (7) Kroepke, J.; Hu, X.; Muenke, C.; Schulten, K.; Michel, H. Structure 1996, 4, 581-597. (8) Gardiner, A. T.; Cogdell, R. J.; Takaichi, S. Photosynth. Res. 1993, 38, 159-167. (9) Sturgis, J. N.; Jirsakova, V.; Reiss-Husson, F.; Cogdell, R. J.; Robert, B. Biochemistry 1995, 34, 517-523. (10) Guthrie, N.; MacDermott, G.; Cogdell, R. J.; Freer, A. A.; Isaacs, N. W.; Hawthornthwaite, A. M.; Halloren, E.; Lindsay, J. G. J. Mol. Biol. 1992, 224, 527-528. (11) Cogdell, R. J.; Hawthornthwaite, A. M. In The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, 1993; Vol. 1, pp 23-42. (12) Ma, Y.-Z.; Cogdell, R. J.; Gillbro, T. J. Phys. Chem. B 1997, 101, 1087-1095. (13) Hess, S.; Visscher, K. J.; Pullerits, T.; Sundstro¨m, V. Biochemistry 1994, 33, 8300-8305.

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