J. Phys. Chem. 1996, 100, 17683-17689
17683
Interpretation of the Excited-State Structure of the Fenna-Matthews-Olson Pigment Protein Complex of Prosthecochloris aestuarii Based on the Simultaneous Simulation of the 4 K Absorption, Linear Dichroism, and Singlet-Triplet Absorption Difference Spectra: A Possible Excitonic Explanation?† Demet Gu1 len Physics Department, Middle East Technical UniVersity, Ankara 06531, Turkey ReceiVed: May 14, 1996; In Final Form: August 2, 1996X
An explanation of the excited-state structure of the FMO (Fenna-Matthews-Olson) complex of Prosthecochloris aestuarii is given from the point of view that the dipole-dipole interactions among the bacteriochlorophylls of the unaggregated C3 symmetric trimers observed in the cyrstalline state are mainly responsible for the key features of the “linear” absorption characteristics (absorption, linear dichroism, and singlettriplet absorption difference) in the Qy region.
I. Introduction In green sulfur bacteria the antenna that connects the chlorosomes (the peripheral antenna) and the photosynthetic membrane which hosts the reaction centers is an aggregated form of the well-characterized FMO (Fenna-Matthews-Olson) protein which was the first structurally resolved photosynthetic pigment-protein complex.1 The 3-D structure of the FMO complex, now at 1.9 Å resolution,2 consists of a trimer of identical monomeric subunits arranged in C3 symmetry. Largely β-sheet foldings of each subunit bind seven symmetry-inequivalent bacteriochlorophylls (Bchl) as shown in Figure 1. Despite all prerequisites, both experimental and theoretical, being well-satisfied, it has not yet been possible to find a sufficiently satisfactory solution to the excited-state structure of the FMO complex. Consequently any conclusions concerning its structure-function relationship remain partially unsubstantiated. Recently, substantial progress toward understanding the excited-state structure of the FMO complex has been reported by Pearlstein in two consecutive papers.3,4 The progress was due to simultaneous inclusion of the two aspects that are considered separately in the earlier simulations. These aspects are the possibility that each of the symmetryinequivalent Bchl’s might experience quite different “nonexcitonic” spectral shifts (“site energies”)5 and the possibility that the inter-subunit interactionssthe interactions between the monomers of the same trimersmay be important. The latter, which had been judged to yield no significant improvement over the monomer calculations in the absence of site shifts,6,7 has been shown to cause magnified spectral changes due to the rotational (C3) symmetry8 once the site shifts are allowed. While the possibility of having aggregated trimers in solution cannot be completely ruled out,9 “the trimeric minimum functional unit” hypothesis has been supported by a wide variety of experimental findings10-14 and will be accepted as the working model also in this writeup. * FAX: (90)-312-210-1281. e-mail:
[email protected]. Phone: (90)-312-223-0653. † Abbreviations: FMO, the Fenna-Matthews-Olson pigment-protein complex of Prosthecochloris aestuarii; Bchl, bacteriochlorophyll; LD, linear dichroism; CD, circular dichroism; ABS, absorption; STAD, singlet minus triplet absorption; fwhm: full width at half-maximum. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01405-0 CCC: $12.00
Figure 1. 2-D view of the Bchl’s of the monomeric subunit of the FMO complex from Prosthecochloris aestuarii. Bchl’s are labeled in the Fenna-Matthews1 numbering scheme. Two groups of the pigments can be visually identified: the porphyrin rings of 1, 4, and 7 are quite perpendicular to those of 2, 3, 5, and 6.
The point of departure of this study is the presumption that further confirmation on the advantage of simultaneous inclusion of the above-mentioned aspects and possible subsequent refinement of the excited-state structure should essentially be based on the analysis of the optical data that had not been employed by/available for the previous simulations. II. Model Considerations Re-evaluation of Some Previous Model Assumptions and Approximations. In the FMO complex there are no Bchl’s whose chlorin macrocycles are within van der Waals contact of each other. The shortest intra- and inter-subunit center-tocenter separations are respectively 11-12 and 24-25 Å. Furthermore no charge transfer states have been observed spectroscopically.15 It is therefore expected that the multipolar “excitonic” interactions are mainly responsible for determining the spectral characteristics of the complex. The strongest intra-subunit interactions (50-200 cm-1) are 1 order of magnitude stronger than the strongest inter-subunit ones which are of the same order as the weaker intra-subunit interactions. If the interaction matrix elements weaker than 50 cm-1 are excluded, the monomeric Hamiltonian has an almost perfect symmetric tridiagonal structure; each Bchl has the strongest interaction with the next one down the chain in the Fenna-Matthews numbering scheme.1 The Bchl 4-Bchl 7 © 1996 American Chemical Society
17684 J. Phys. Chem., Vol. 100, No. 44, 1996
Gu¨len
TABLE 1: Strengths, V (cm-1), and Shift Tendencies of the Strongest Intra-subunit Interactions in the Point Dipole and Monopole Approximationsa Bchl pair
Vb
shift tendency
1-2 2-3 3-4 4-5 5-6 6-7 4-7
171.9 (189.7) 55.2 (51.6) 100.9 (101.4) 119.4 (132.6) 158.4 (103.6) 61.0 (71.3) 115.2 (97.7)
red blue red red blue blue red
a See Figure 1 for the Bchl numbering convention. b The numbers in the parantheses are taken from ref 3 and correspond to the interaction strengths in the monopole approximation.
(≈100 cm-1 ) interaction is the only exceptional tridiagonalitybreaker. Table 1 summarizes the strengths of the strongest pairwise interactions and their shift tendencies. An important implication of that specific structure is that the characteristic absorption peaks, at 793, 803, 812, and 825 nm, should envelop excitonic transitions of relatively limited delocalization. The delocalization will be more limited if the site energies are stretched over a broad interval. It is therefore no real challenge to simulate the absorption spectra (ABS) by moving around the site energies in the directions suggested by the shift tendencies of the important pairwise interactions. Further fine-tuning follows easily if in addition the excitonic line widths are let free as adjustable parameters as in refs 3 and 4. The challenge has been to produce the main features of the ABS and the circular dichroism (CD) simultaneously by using line widths that are realistically close to the experimentally observed values. Most of the excitonic line widths that yield the “best” simultaneous ABS and CD fits are far broader (100350 cm-1) than implied by the hole-burning measurements (7080 cm-1) of Johnson and Small (J-S).11 It would certainly be not only more realistic but also advantageous to include the wealth of experimental line width information to get much better control over the most critical adjustable parameters, the site energies. At present, there is neither a direct experimental nor a reliable theoretical method that can be employed for a convincingly accurate determination of the site energies. Through a series of tests and by reviewing the trends in the earlier simulations,3,4,9 one would not be totally wrong to project that the CD simulations are very sensitive to several factors. One is the aggregation state of the sample: CD simulations can be easily misleading due to longer range chiral interactions between the adjacent trimers. Another one is that CD is much more dependent than the linear spectrasABS, linear dichroism (LD), singlet-triplet absorption difference (STAD)son the approximation selected for inputting the transition dipole moment direction information. It should also be noticed that the CD spectrum is somewhat dependent on the solvents,16,17 and as noted earlier,9 it is quite possible that Bchl-Bchl interactions and/or Bchl site energies might have been disturbed by the solvent. For these reasons, it is probably safer not to rely completely on the CD as the fit partner of the ABS. Instead, it may be more reasonable to select the “linear” absorption spectra mentioned above as the simultaneous fit partners. This is an approach that has not been tried in the past and is the one that will be used in this writeup. Details of the exciton calculations can be found in ref 9. The interactions between the Bchl’s will be treated in the point dipole approximation. While the point monopole approximation18 may
Figure 2. 4 K ABS, STAD, and LD spectra of the FMO complex of P. aestuarii. Data are taken from VM.12
be considered to be more “correct”, in this first attempt to a simultaneous fit of the ABS, LD, and STAD, the point dipole approximation is used. Spectral widths will simply be introduced by dressing the stick spectra with the symmetric Gaussians. Unless otherwise noted any simulation related to the earlier Pearlstein simulations will be carried out within the monopole approximation using the parameters reported in refs 3 and 4. The 4 K ABS, LD, and STAD Spectra. The 4 K ABS, STAD (change in the Qy region (singlet) absorption upon triplet formation), and LD: the absorption difference in the directions parallel and perpendicular to an experimentally defined alignment axis, data of van Mourik (VM) et al.,12 are shown in Figure 2. Earlier LD measurements of Whitten et al. (300 K) and Swarthoff et al. (300 and 100 K) can be found in refs 19 and 20. Simulations of this work will however be based on the VM data. The STAD spectrum with features comparable in strength over the entire Qy band is suggestive that the triplet formation affects most of the Qy-region characteristics of the complex. All three independent LD spectra have the same very strong LD signature around 812 nm, whereas no strong LD activity on either the red or the blue wings of the Qy band are observed.
FMO Complex of Prosthecochloris aestuarii
J. Phys. Chem., Vol. 100, No. 44, 1996 17685
Figure 3. Directions of the individual Bchl Qy transition dipole moments with respect to the C3 symmetry axis. The numbers in the paranthesis are the values of the angles that the dipole moments make with the C3 symmetry axis. The Qy direction is defined along the NBND diagonal in the Brookhaven Protein Data Bank format.
The details of the 100 K Swarthoff vs the 4 K VM data on the red-most side are somewhat different. The former has a negative feature peaking around 835 nm. So far no transition around this wavelength has been identified by any other spectroscopy. It is probably an artifact. Comparison of the Whitten et al. data (which uses electric field alignment) with the other two (which use the gel compression technique) indicates that in these two techniques the FMO aggregates are oriented differently, probably in directions more or less perpendicular to each other. It is difficult to access the degree of ordering in the sample. The general consensus however is that, in the gel compression technique, the short axes of the flat disklike aggregates, like FMO and LHCII of green plants,21 are oriented more or less perpendicular to the experimentally defined alignment axis. The short axes of the FMO trimers are along the C3 symmetry axis. With this convention, negative (positive) LD signals are associated with the electronic transition dipole moments more parallel (more perpendicular) to the C3 symmetry axis and the characteristic 812-nm signal can be correlated with a transition (or a set of transitions) which is quite parallel to the C3 axis. Problem Identification. Although the dipole moments giving rise to LD spectra are of excitonic nature, the analysis of the individual transition dipole moment directions of the monomeric Bchl’s shown in Figure 3 has been extremely elucidating in identifying the general problem associated with the current best fits of Pearlstein, in searching a tentative solution, and in suggesting a better-controlled strategy for future calculations. The FMO complex turns out to be very cooperative and instructive by providing a very distinctive dipole moment structure: the Qy dipole moment directions of three of the pigments (Bchl 1, 4, and 7) are very parallel to the C3 symmetry axis, while the remaining ones are very much in the plane of the trimer. This very distinctive individual dipole moment structure should be critically effective in restricting the site energies of the pigments with respect to each other and relative to the characteristic absorption peaks. Figure 4 illustrates how such a restriction would work. One can clearly follow the progressive red shift of the individual LD fingerprints of the selected pigments if the site energies of the remaining six are kept at a common value (802.6 nm) but
Figure 4. Characteristic LD fingerprints of Bchl 7 and Bchl 3 in the FMO trimers. Site energies of Bchl 7s (a) and Bchl 3s (b) are shifted progressively to the red. The arrows indicate the progressive red shift of the characteristic LD fingerprint. The site energies are 813.1, 818.5, and 823.9 nm for Bchl 7s and 807.6, 818.5, and 823.9 nm for Bchl 3s. The other Bchl’s are fixed at 802.6 nm.
TABLE 2: Bchl Site Energies (Energy (eV)/Wavelength (nm))a Bchl no.
this study
F292 (ref 3)
OKT (ref 4)
PS (ref 4)
1 2 3 4 5 6 7
1.542/804.2 1.545/802.6 1.540/805.2 1.538/806.2 1.535/807.8 1.520/815.8 1.544/803.1
1.546/801.9 1.546/801.9 1.550/800.0 1.575/787.4 1.550/800.0 1.525/813.0 1.509/822.0
1.580/784.7 1.553/798.5 1.548/801.0 1.543/803.4 1.550/800.0 1.527/811.8 1.507/822.6
1.556/801.0 1.536/807.0 1.518/817.0 1.534/808.0 1.589/780.4 1.532/809.5 1.555/797.5
a
See Figure 1 for the Bchl numbering convention.
one (Bchl 7 in Figure 4a and Bchl 3 in Figure 4b) is progressively red-shifted. Bchl 7 and Bchl 3 not only are chosen as the typical representatives of the two groups of pigments but also are favored in particular as they are the red-most pigments identified in the extensive computer search based simultaneous current best ABS and CD fits (referred as the F292,3 OKT, and PS4 sets from this point on, see Table 2 for the corresponding site energy sets). OKT and its precursor F292 necessitate a closer attention as these sets are favored over the PS set for several reasons and since in both sets Bchl 7 is relatively well red-shifted (822 nm). Although it may still be possible that the negativeness of the Bchl 7 LD fingerprint will be reduced by mixing with another
17686 J. Phys. Chem., Vol. 100, No. 44, 1996
Figure 5. Simulated LD vs LD of VM corresponding to the F292, OKT, and PS sets. All are simulated in the monopole approximation using the site energy sets given in Table 2 and the parameters given in refs 3 and 4. For the F292 simulation, the best-fit absorption line widths, and for the OKT and PS simulations, the best-simultaneous-fit absorption and CD bandwidths are used. In the PS panel, the solid line is the simulation with all equal (80 cm-1) bandwidths and the dashed line is the actual PS simulation.
red-shifted pigment (Bchl 6 in the OKT (812 nm) and F292 (813 nm) sets), it is quite unlikely that the kind of positive (inplane) signal around 825 nm will result. The PS set has two in-plane pigmentssBchl 3 (817 nm) and Bchl 6 (809.5 nm)sas the red side pigments and therefore has the potential of giving a better-fit LD on the red edge. It has however an overall trouble of yielding a good simultaneous ABS and LD fit only if the line widths are allowed to be unreasonably broader (185-350 cm-1) than the experimentally observed values. In Figure 5 these three current best simulation sets are analyzed in terms of their LD properties and compared with the LD of VM. LD using the PS site energy values but with more realistic line widths (80 cm-1 ) is also shown. The LD simulations are carried out under the assumption that the C3 symmetry axis is exactly perpendicular to the macroscopic alignment axis. As mentioned above it is difficult to establish
Gu¨len the “exact” extend of ordering in the sample. The sensitivity of the LD on the degree of orientation is an important aspect which remains to be investigated. However, due to the very distinctive dipole moment directions of the Bchl’s in the FMO complex of Prosthecochloris aestuarii, the LD bands should maintain their fingerprint characters to a large extend even in the nonperfectly oriented samples. It should also be mentioned that the line widths in the LD which will depend on the degree of ordering should not necessarily be the same as in the other spectra. Figure 6 provides a comparison of the STAD spectra (in the point dipole approximation) of the three sets with the VM data. Bchl has no triplet states in the vicinity of the Qy region of the absorption spectrum; therefore, under the assumption that the triplet associated with the red-most singlet is also the lowest triplet state, the STAD spectra have been approximated as the difference in the Qy region absorption with and without the redmost pigment. Upon observing a considerable amount of inconsistency in both the LD and the STAD spectra of the F292, OKT, and PS sets, it was decided to make a fresh start. A Preliminary Assignment of the Red-Most Pigment. Unless at least one of the Bchl’s is red-shifted, the absorption band around 825 nm cannot be produced. It has an integrated absorption strength of 1 Bchl/monomer. The other three absorption bands can be produced reasonably well if all Bchl’s have a common site energy around 803 nm. It may therefore be quite adequate to assume that the characteristics of the 825nm band are decided predominantly by one of the Bchl’s. It then becomes plausible to ask which of the Bchl’s is likely to produce the key features of the ABS, LD, and STAD simultaneously if relatively well red-shifted from the others. The priority in selecting a possible candidate is given to the LD with its remarkable dependence on the fingerprints of the individual Bchl’s. Secondly, the STAD with features over the entire Qy band is used as a good evidence that the red-most pigment is likely to be one of the Bchl’s in good communication with almost all the others. Bchl 7 is one of such pigments as discussed in detail in connection to “the Bchl 7 must clearly be the red-most pigment hypothesis”.3 Bchl 6 might be even better/ more special than Bchl 7 in this respect as the best interconnected pigment within a subunit. In this perspective, several possibilities, each basically involving a different starting point, have been tried and Bchl 6 is tentatively assigned as the most likely candidate. III. Results and Discussion In the following simulations, the Gaussian line widths (fwhm) of all trimeric exciton transitions are fixed at 80 cm-1 and the dipole moment magnitudes for all Bchl’s are assumed equal (7.38 D). The site energies of the symmetry-related Bchl’s in the different subunits of the trimer are taken equal. With these restrictions, seven Bchl site energies remain as the only adjustable parameters. The site energy for Bchl 6 is more or less fixed around 815820 nm, a common energy for the others that would produce the key features of the ABS and LD upon searching is found to be around 805 nm. Further fine-tuning is done by manual variation of the site energies around this starting set guided somewhat by the shift tendencies of the important monomeric pairwise interactions, and the fits are judged reasonable by simple eye cognition. Simultaneous ABS, LD, and STAD results for a typically successful simulation are given in Figure 7. All the simulated trimeric spectra agree reasonably well with the experimental
FMO Complex of Prosthecochloris aestuarii
J. Phys. Chem., Vol. 100, No. 44, 1996 17687
Figure 7. ABS, LD, and STAD simulations for the FMO trimers of this study (solid lines) vs the VM data. Also shown are the ABS, LD, and STAD simulations for one of the monomeric subunits (dashed lines) for comparison. The site energy set used is given in Table 2. The transition dipole moment strength of each Bchl is equal to 7.38 D and the fwhm of the symmetric Gaussians is 80 cm-1.
Figure 6. Simulated STAD vs STAD of VM corresponding to the F292, OKT, and PS sets. All are simulated in the point dipole approximation using the site energy sets given in Table 2 and the parameters given in refs 3 and 4. The excitonic transitions are dressed with the symmetric Gaussians of fwhm ) 80 cm-1.
data. In Figure 7, the simulations for the monomeric subunit of seven Bchl’s are also given for comparison. Features of the main 812-nm band are considerably influenced by the inclusion of the inter-subunit interactions. Although a fit is not intended for the CD, it is encouraging to observe that the CD shown in Figure 8 is not totally off. The overall polarities of the main features around 790 (-), 805 (+), and 817 (-) nm are produced. Several characteristics of the exciton transitions are detailed in Tables 2-5. The site energy set used in the simulations of Figure 7 is given in Table 2 and the “stick” spectral information
of the 21 excitonic transitions are summarized in Table 3. For the compactness of the representation, Table 4 containing the delocalization characteristics is confined to the exciton states of the corresponding monomer. Table 5 provides a comparison of the experimental results with the hole-burning (J-S)11 and the fourth-derivative absorption spectra.14 Correlation between the calculated excitonic energies and the transitions identified experimentally is quite close: some disagreement around 807 nm with both sets and some disagreement with the hole burning for the two red-most transitions. The discrepancy around 807 nm may not be totally inconsistent with the hole-burning spectra since this transition is assigned tentatively as a transition superimposed on a broad (10 nm) antihole centered around 805 nm. Transitions 4-6 correlate well with “the weakly absorbing band midway between the 825 and 813 nm bands” assignment of J-S. The Bchl 6 energy is around 816 nm, and the site energies of the remaining pigments all lie within (3 nm (45 cm-1) of 805 nm. This is a property that is not observed in the F292,
17688 J. Phys. Chem., Vol. 100, No. 44, 1996
Gu¨len TABLE 5: Comparison of the Transition Energies with Experimental Data wavelength (nm) transition
this work
1, 2 3 4, 5
825.2 824.8 817.8
6 7, 8 9 10, 11
816.5 815.4 812.5 804.7
12 13, 14
804.1 801.6
15 16
801.5 793.3
17, 18 19, 20 21
792.4 790.8 789.6
hole burning (ref 11)
fourth derivative ABS (ref 14)
827.1 824.4
825
816.3
819
813 807.8
814 810 806
Figure 8. CD simulations for the FMO trimers of this study vs the experiment (dots). The parameters of Figure 7 are used in the simulation represented by dashed line In the simulation represented by a solid line, the line widths are taken to be broader (fwhm ) 120 cm-1). The CD was measured (ref 16) at a higher temperature (77 K) than the other spectra (4 K).
TABLE 3: Trimer Exciton Stick Spectra Characteristics strengthsa (D2) transition no.
wavelength (nm)
ABS
LD
1, 2 3 4, 5 6 7, 8 9 10, 11 12 13, 14 15 16 17, 18 19, 20 21
825.19 824.80 817.82 816.46 815.37 812.50 804.73 804.10 801.57 801.53 793.31 792.38 790.76 789.63
93.1 0.3 6.6 4.8 71.6 328.5 69.6 65.2 47.4 80.0 11.2 12.7 23.2 6.5
46.6 -0.3 3.3 -4.8 35.8 -328.5 34.8 -65.2 23.7 -80.0 -11.2 6.4 11.6 -6.5
a Strengths corresponding to only one of the degenerate states are listed.
TABLE 4: Excitation Probabilities of Each Bchl in the Monomer Exciton Transitions transition (wavelength, nm)
1
2
1 (824.99) 2 (817.13) 3 (814.43) 4 (803.84) 5 (802.33) 6 (792.61) 7 (790.63)
0.004 0.102 0.409 0.017 0.024 0.314 0.130
0.001 0.193 0.255 0.017 0.001 0.344 0.189
a
amplitude on Bchla 3 4 5 0.004 0.232 0.010 0.186 0.482 0.004 0.081
0.082 0.285 0.196 0.005 0.034 0.169 0.229
0.340 0.004 0.003 0.160 0.275 0.096 0.122
6
7
0.510 0.162 0.061 0.003 0.167 0.021 0.076
0.059 0.023 0.064 0.613 0.017 0.055 0.169
Contributions more than 10% are shown in bold.
OKT, and PS sets: in all, there is a rather broad scattering of the site energies even among the non-red-shifted Bchl’s (see Table 2). Consequently, the exciton states associated with Figure 7 are less “mini” than Pearlstein’s. Delocalization characteristics of the exciton states show an overall agreement with the J-S statement that “certain exciton states are better correlated (albeit not perfectly) than the others”. In particular, the support for the long-standing J-S argument that the redmost band is the most localized one while the others are more delocalized can be observed in Table 4. Several hole-burning characteristics of the 825- and 813-nm bands have been interpreted to provide good evidence that these two bands share the same pigments. Here, no strong correlation between the 825-nm (mostly a 6-5 state) and 813-nm (mostly a 1-2-4 state) bands is disclosed. Among all, transitions 4-6 and 1921 are particularly good exciton states (extensively delocalized).
804.8 801.3
801
793.6
793
-
-
Being quite apart from the other three bands, the 825-nm characteristics have been analyzed in by far the most detail experimentally. If indeed it can be attributed to an energetically well-isolated Bchl, it is to be expected that the characteristics of the 825-nm band are predominantly determined by the C3 symmetric trimer formed by the inter-subunit interactions of that particular Bchl. A general C3 symmetric trimer offers degenerate in-plane states and a nondegenerate out-of-plane state. If the individual dipole moments of a C3 trimer are largely out-of-plane, almost all the absorption strength is associated with the nondegenerate, out-of-plane transition. This is the kind of trimer formed by the Bchl 7’s. As also discussed above in relation to the specific out-of-plane direction of the Bchl 7, this is the main reason for the difficulties associated with the LD simulations of the F292 and OKT sets. While a C3 symmetric trimer consisting of the in-plane individual dipole moments yields the opposite, the degenerate in-plane state is strongly allowed or the nondegenerate out-of-plane state is strongly forbidden. The C3 symmetric trimer formed by the Bchl 6 interactions is a good example of this. It is therefore expected to observe a good correlation between the results of this writeup and the earlier predictions whenever they are based on an in-plane red-most trimer hypothesis. The two degenerate in-plane transitions (around 825.2 nm) have almost all the dipole strength. The prediction of VM was that the in-plane transition should have more than 95% of the total absorption strength of the red-most band. The angle that the 825-nm monomeric transition makes with the plane of the trimer is 7.2°, which was predicted to be at most 13° by VM including a misalignment uncertainty of 5°. The biggest problem of the current simulation with its strongly allowed in-plane red-most state is its failure in explaining the negative polarization of the bleaching on the red-most side of the 825-nm band which according to VM can only be understood if the red-most state is an out-of-plane state, unless there is some heterogeneity in the site energies of the symmetryequivalent pigments. On the other hand, one can go through a simple Boltzmann factor analysis: if the hole-burning assignments (824.8 and 827.1 nm) were accepted, it would be rather difficult to explain the high quantum yield of the 825-nm emission observed by
FMO Complex of Prosthecochloris aestuarii Rijgersberg22 within a strongly forbidden red-most state model while the assignments of this writeup are devoid of such a problem. IV. Concluding Remarks The results presented provide further support to the original Pearlstein statement that the excited-state structure of the FMO complex from P. aestuarii will be understood to a large extend in terms of the excitonic interactions among the Bchl’s of the trimeric form observed in the crystalline state. The preliminary assignment of Bchl 6 as the lowest energy pigment yields a red-most state that is localized mainly on the Bchl 6 (51%) and Bchl 5 (34%), the next non-negligible contribution is from Bchl 4 (8.2%). The question of interest is the following: are some or all of these molecules strategically located to serve as the “exit” molecules for subsequent energy transfer in the aggregates of trimers? It is very promising to find out that two of the pigments contributing to the lowest energy state, Bchl 5 and Bchl 4, have already been proposed to be the most probable “exit molecule” candidates based on the geometrical considerations, if it is assumed that the FMO trimers in solution aggregate through their flat β-sheet portions based on the well-known coupling tendency of such surfaces.9 It is therefore very tempting to extend the model calculations in this direction as well. Though the rationalization of the site energies in terms of the conformational properties of the Bchl’s and in terms of their interactions with the immediate protein environment does not seem highly feasible at the current level of understanding, it may still be worth asking if there is something extraordinarily different about the dominantly red-shifted pigment if this is Bchl 6. Acknowledgment. This work has been initialized at the Vrije Universiteit, Amsterdam, The Netherlands, when the author was
J. Phys. Chem., Vol. 100, No. 44, 1996 17689 visiting Prof. R. van Grondelle’s laboratory under the one-month ESF grant (Biophysics of Photosynthesis) in August-September 1995. The author wants to thank Prof. R. S. Knox for his valuable comments on the manuscript. References and Notes (1) Fenna, R. E.; Matthews, B. W. Nature 1975, 258, 573. (2) Tronrud, D. E.; Schmid, M. F.; Matthews, B. W. J. Mol. Biol. 1986, 188, 443. (3) Pearlstein, R. M. Photosynth. Res. 1992, 31, 213. (4) Lu, X.; Pearlstein, R. M. Photochem. Photobiol. 1993, 57, 86. (5) Gudowska-Nowak, E.; Newton, M. D.; Fajer, J. J. Phys. Chem. 1990 , 94, 5795. (6) Pearlstein, R. M.; Hemenger, R. P. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 4920. (7) Meister, A. Studia Biophys. 1986, 113, 171. (8) Pearlstein, R. M.; Zuber, H. In Antennas and Reaction Centers of Photosynthetic Bacteria--Structure, Interactions and Dynamics; MichelBeyerle, Ed.; Springer-Verlag: Berlin, 1986; p 555. (9) Pearlstein, R. M. In Chlorophylls; Scheer, H., Ed.; CRC Press: Boca Raton, FL, 1991; p 1074. (10) van Grondelle, R.; Hunter, C. N.; Bakker, J. G. C.; Kramer, H. J. M. Biochim. Biophys. Acta 1983, 723, 30. (11) Johnson, S. G.; Small, G. J. J. Phys. Chem. 1991, 95, 471. (12) van Mourik, F.; Verwijst, R. R.; Mulder, J. M.; van Grondelle, R. J. Phys. Chem. 1994, 98, 10307. (13) Savikhin, S.; Struve, W. S. Biochemistry 1994, 33, 11200. (14) Whitten, W. B.; Olson, J. M.; Pearlstein, R. M. Biochim. Biophys. Acta 1980, 591, 203. (15) Gottfried, D. S.; Stocker, J. W.; Boxer, S. G. Biochim. Biophys. Acta 1991, 1059, 63. (16) Olson, J. M.; Ke, B.; Thompson, K. H. Biochim. Biophys. Acta 1976, 430, 524. (17) Philipson, K. D.; Sauer, K. Biochemistry 1972, 11, 1880. (18) Weiss, C. J. Mol. Spectrosc. 1972, 44, 37. (19) Whitten, B. W.; Pearlstein, R. M.; Phares, E. F.; Geacintov, N. E. Biochim. Biophys. Acta 1978, 503, 491. (20) Swarthoff, T.; de Groot, B. G.; Meiburg, R. F.; Rijgersberg, C. P.; Amezs, J. Biochim. Biophys. Acta 1980, 593, 51. (21) van Amerongen, H.; Kwa, S. L. S.; van Bolhuis, B. M.; van Grondelle, R. Biophys. J. 1994, 67, 837. (22) Rijgersberg, C. P. Ph.D. Thesis, Leiden, 1980.
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