J. Phys. Chem. 1994, 98, 5778-5783
5778
Time Resolution of Electronic Transitions of Photosynthetic Reaction Centers in the Infrared G. C. Walker,+*tS. Maiti,* B. R. Cowen,’ C. C. Maser,* P. L. Dutton,* and R. M. Hochstrasser’*t Department of Chemistry and The Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania I9104 Received: November 12, 1993; In Final Form: March 16, 1994”
Electronic transitions of the special pair excited state, P*, and of its positive ion, P+, have been identified by transient infrared spectroscopy in the 170&2000-~m-~region. The P* transition is present immediately (ca. 300 fs) after light absorption and decays with time constant 3.4 ps. The P+transition appears with time constant 3.4 ps. The transition dipoles of these transitions are both measured to have a squared projection of 0.63 onto the direction of the ground state to Qv- (870 nm) transition. This is interpreted to imply that both P* and P+ transitions have dipoles along the line joining the centroids of charge of the two bacteriochlorophylls (BChl) composing the dimer. The P* transition is assigned as an interexciton transition brought about by mixing of exciton and charge-separated states. The P+ transition is assigned as a transition between the symmetric and antisymmetric combination of the localized hole states of the dimer. The results are compared with theoretical calculations, static FTIR, and Stark effect measurements on the reaction center. While the results are in qualitative agreement with recent theoretical calculations, better agreement requires a larger admixture of charge resonance states in the QU-state than is found in most calculations.
Introduction In bacterial photosynthesis the primary event is a photoinduced charge separation that takes place in a transmembrane pigmentprotein complex known as the reaction center (RC). The central part consists of two protein subunits L and M, related by approximate C2 symmetry, interfaced at a bacteriochlorophyll dimer (P).l Absorption of a near-IR photon (ca. 870 nm) leads to the formation of the singlet excited state P*, which donates an electron to a bacteriopheophytin (BPhL) on the L side to form the positively charged dimer state P+ with a time constant of -3.5 ps and with a quantum efficiency of >0.98.2 Despite the near C2 symmetry, there is negligible electron transfer to BPhM. Understanding the remarkable features of this fast, efficient, and unidirectional electron transfer requires an understanding of the nature of the electronic structure of the cofactors and in particular of the excited state P* and the ionic state P+. The dominant contribution to the P* state undoubtedly comes from the lower (Qy-) of the two excitonic states (Qvand Qy+) that arise from the interaction of the individual Qy states of the two monomer constituents.3 It was proposed’ll that the P* state also contains a significantadmixture of intradimer charge transfer states. Wave functions of the electronic states of P obtained from the most recent calculationsi2do show that the P* state can be reasonably well represented by a linear combination of the excitonic (Qy-and Qy+) and charge-transfer (PL+PM-and PL-PM+) states. Spectroscopic measurements of thedipole moment change associated with the formation of the P* state’s support the substantial charge-transfer character of the P* state. To verify the extent to which the zero-order excitonic and charge-transfer states are mixed requires direct probing of the low-energy transitions of P*. Unfortunately, there are many overlapping electronic states associated with the multitude of cofactors in the RC, making it difficult to obtain unambiguous information about these eigenstatesfrom the ground-statevisible/ near-infrared (VIS/NIR) spectrum at ambient temperatures. These transitions have been probed by low-temperature hole-
* To whom correspondence should be addressed. t Department of Chemistry.
The Johnson Research Foundation. Current address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260. Abstract published in Advance ACS Absfracfs,April 15, 1994. f
8
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burning experiments which have led to an assignment of the ground- to excited-state transition energies of a number of transitions of the RC (see refs 9 and 34 for review). The chargetransfer character of the lowest excited state of the RC has also been directly addressed by Stark and VIS/NIR spectros~ o p y . ’ ~ J ~ JHowever, ~ the energies and transition dipole moments of excited-state transitions, which should contain a wealth of information regarding the specificnature of the individual excited states, have not been explored. IR methods now have time resolution and sensitivity comparable to the optical measurements and permit studies not only of transient vibrational states but also of low-energy electronic transitions. We report here the first experimental observation of transitions between the excited states of P in the 5-6-Km wavelength region. It is also important to characterize the eigenstates of P+ for understanding the charge-transfer and recombination (or lack thereof) dynamics. A wide absorption band centered at 2600 cm-1 observed in the static FTIR difference spectra of RC’s has recently been assigned to an electronic transition of the P+ state.15 In our work, the time evolution of this spectral band has been 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
Transitions of Photosynthetic Reaction Centers
5779
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followed with a subpicosecond resolution and compared with an independent measurement of the electron-transfer rate in the RC. These spectral kinetics and measured anisotropy of the transition permit a direct verification of the nature of the eigenstates of P+.
I‘
0.0056
0.0034 0.0012
Materials and Methods
Figure 2 shows absorption kinetics measured at 1710 cm-1 in a spectral region with vibrational transitions assigned to P+9-keto carbonyls.l8J9 The observed kinetics are fit to a model incor-
1930
0.0100
cm-I. The kinetics are fit to an exponentialrise of 3.4ps convoluted with an instrument function of 0.4 ps.
Results
1860
Wavenumber (cm” )
Time (ps)
Figure 2. Absorption kinetics of a vibrational band of the RC at 1710
Spectroscopic Method. Transient IR spectra following sample excitation were obtained using upconversion gating of a continuous wave (CW) IR field with a variably delayed optical pulse.16 The upconversion occurs in a crystal oriented for phase matching, and the generated visible field is detected using a photomultiplier. The time resolution is limited primarily by the excitation and gate pulse durations, and the spectral resolution is limited only by the bandwidth of the C W probe. The optical pulses are generated by a mode-locked Ti:A1203 oscillator which is pumped by a C W Ar+ laser, producing IO-nJ, 100-fs, 870-nm pulses at 80 MHz. These pulses are injected into the amplified at 4.75 kHz by a Ti:A1203 regenerative amplifier which is pumped by an intracavity doubled Q-switched Nd:YLF laser. During amplification the pulses experience temporal broadening due to dispersion, and after ejection they are recompressed by a grating pair. The output ( 5 pJ, 200 fs) is split to derive the excitation and gate pulses. The probe IR is obtained from a C O laser and is focused to 100 pm in a spinning and translating sample cell with two CaF2 windows and a spacer (24-60-pm thickness) designed to provide a fresh volume of sample every 200 ps yet permit 1 s for sample recovery. The pump beam is delivered to the sample collinearly with the probe, whose transmitted field is upconverted with a gating pulse in a crystal of AgGaS2. The pump beam is chopped, and the transmitted IR intensities are sampled with lock-in detection. Anisotropies were obtained by introducing a half-wave plate into the pump beam and rotating the plate to vary the polarization of the pump pulse with respect to the probe I R beam. RC Preparation. Isolation of reaction centers from the bluegreen mutant of the photosynthetically grown Rhodobacter sphaeroides R-26 was based on the method of Clayton and Wang,I7using the detergent lauryldimethylamine oxide (LDAO) to solubilize the photosynthetic membranes. Bulk IH2O was exchanged for Z H 2 0by successive dilution in 10 mM Tris (pH 8) 2H20 buffer, and the preparation was concentrated down to about 1 mM with Centricon-30 and Microcon-10 centrifugal filters (Amicon).
1790
1720
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-2
1960cm’
6
14
30
22
Time (ps)
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10
20
30
Time (ps) Figure 3. (a, top) Absorption spectrum of the RC between 1700 and 2000 cm-l30 ps after excitation. The dashed portion is known to contain another vibrational band arising from the 10a ester C = O of P.12 (b, middle) Absorption kinetics of the RC at 1743 cm-I, at the lower-energy side of the electronic absorption band. The kinetics are fit to a 67% PL and 33% P+contribution,asdescribed in the text. (c, bottom) Absorption kineticsof the RC at 1960cm-I, at the higher-energysideofthemonitored portion of the electronic absorption band. The kinetics are fit to a 34% P* and 66% P+ contribution, as described in the text.
porating the convolution of a 400-fs instrument response and a 3.4-ps exponential rise. The observed rate is thus consistent with optical (VIS/NIR) measurements of the first step in electron transfer performed by us on these samples (data not shown) and previously by othersS2 Figure 3a shows the absorption spectrum in the 170&2000cm-I region of reaction centers at 30 ps following excitation of 30% of the ground-state population induced by the 200-nJ pump pulse. Two features are apparent: a relatively narrow band, ca. 30 cm-l wide, at lower frequency that is assigned to the 9-keto carbonyls of the special pair cation, P+,18J9as mentioned before. The feature to higher energy is much broader and does not exhibit the typical line structure of vibrational transitions. The width is more typical of an electronic transition. Figure 3, b and c, shows the absorption kinetics monitored at the extremes of the displayed, broad spectral feature a t 1743 and 1960 cm-1, respectively. The 1743-cm-I kinetics are fit to a sum of two
-
TABLE 1: Relative Amplitudes and Anisotropies of the P+ and P* Components wavenumber relative amplitude anisotropy ( r ) (cm-1) P* P+ 0.5ps 30 ps
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contributions-an instantaneous rise with an exponential decay and an exponential rise with no decay, both having time constants of 3.4 ps, and amplitudes of 67 f 10% ( a , ) and 33 f 10% (a2), respectively. The first process is the signature of a species that appears immediately on absorption of the pump pulse. Although there is evidence from optical studies that indicate a distribution of electron-transfer rate constants in photoexcited RC’S,~Othese results would not predict any electron transfer on the time scale of the rise of the signals we have seen at 1720-1960 cm-I. The fast rising species is therefore associated with the excited state P*. The second feature is associated with the first step of electron transfer. The 1960-cm-1 kinetics are fit to the same model, but with amplitudes of 34 f 10% ( a l ) and 66 f 10% (az) for the instantaneous and slower processes. The measurements throughout the spectral range were therefore fitted to the response: a1 (A) exp(-t/3.4) al(X)[l - exp(-t/3.4)], convoluted with a 400-fs Gaussian instrument function. Figure 4 shows the plot of al(A) and a2(A) for the different probe frequencies. Systematically, the instantaneous component ( a l )is smaller and appears to be somewhat shifted to lower frequency. As discussed below, we assign these two components to distinct electronic transitions of the reaction centers special pair. The mean angle between pumped and probed transition dipoles is obtained by measuring probe absorbance changes measured parallel (Ill) and perpendicular (Il) to the pump polarization. Those absorbances are transformed by eq 1 into anisotropies, r ( t ) , where
+
The transformation from anisotropy to a mean square cosine of an angle is performed by eq 2. (cos2B(t)) = (1/3)(1
+ 5r(t))
(2)
Ameanangle, written belowas (O),isdefinedas (0) =cos-I((cos2 0 ) ) 1 / 2 . These experiments were performed using pump energies typically 3-5 times smaller than those used for the kinetics measurements and resulted in no more than 12% excitation of ground-state population. Pump energies were m. The difference between the dipole moment of the Experimentally, we observe a transition that onsets around Qy- and the ground states now has a component perpendicular 1700 cm-', reaches a peak extinction coefficient of t 500 to the pseudo-Cz atis. This difference vector is easily seen to lie L/(mol-cm),and is polarized in the RLMdirection. The complete nearly parallel to RLMwhen m > ca. 0.2. Thus, the results of Franck-Condon envelope of the transition is not observed in this anisotropy experiments in which P is pumped at 870 nm and work, so we cannot obtain the integrated area and hence the that probe the P* exciton transition or the hole-transfer transition transition dipole squared; however, it is clear from our measureor the change in dipole moment via an orientedfields can all be ment that pz > 1 D. The theoretical energy predictions seen to be mainly determined by the same a n g l e t h a t between presumably refer to Franck-Condon maxima so that they should amonomeryaxisandthelinejoiningthecentersof themonomers. be somewhat larger than the onset, or near to zero-zero band, Assignments. The experimental evidence for low-frequency frequency. Also shown in Table 2 is the dominant parentage of transitions based on electronically excited states of the reaction the calculated level in terms of the basis states of the CT exciton center is provided from several sources. The early linear dichroism model used to guide the discussion of this paper. The calculated measurements32 identified transitions at 805 and 870 nm as being states include not only the Qy and CT states of P but also excitonic in nature with the lower-energy component being the contributions from BChl and BPh. So we have calculated the antisymmetric state derived from Qyof the monomer. Transitions dipole directions of those transitions that are effectively polarized between these states are expected in the infrared region of the perpendicular to the approximate C2 axis. If the P* absorption spectrum. Hole-burning experiments by Small and c o - ~ o r k e r s ~ ~ *we ~ ~observe is the lowest-energy transition of P*, then our results have identified zero phonon transitions to excitonic levels of P* would confirm those calculations that place a state that is a mixture from the ground state of P, which are split by ca. 1900 cm-l. of IPS)and IC,) in the region 1720 cm-l (onset). The transitions Their measurements suggest that the IP,) state more strongly that are in reasonable agreement with the observations are marked mixed with IC) states than the IPS)state. The observed amplitude by an arrow. It is clear that the spectrum of P*, in the range 5 al(X)has the proper kinetics to correspond to population of P*, to 1 pm, should contain many other transitions of great interest so our measurements suggest fixing the zero-zero band energy to testing and refining theoretical calculations. of a P* transition at ca. 1700 cm-1. The asymmetry in the charge distribution needed to explain One can utilize the published wave functions for the first few the observed spectrum of P+ was also incorporated by Breton et calculated excited states of P and estimate the properties of al.I5in their description of the hole pair states of thedimer cation. transitions between them. The result of such an estimate, based Their analysis implied that resonance interaction did not play a on the results reported in refs 9,10,12, and 35, is shown in Table large role in determining the transition energy but that the energy 2. The transition dipole moment is obtained from a weighted gap between their cationic basis states, theasymmetric sitesplitting sum of the electronic states of the individual cofactors. The of the hole states, was significant as a result of intrinsic asymmetry. direction and magnitude of these dipole moments are obtained The energy spacings of both the low-energy P* and hole pair from measurements and c a l c ~ l a t i o n s . ~These - ~ ~ are then vecstates may have very similar asymmetry contributions. However, torially added to obtain the total transition dipole moment it seemsunlikely that these IR transition energies are alldominated direction which subsequently yields (cosz 0 ) . We note here that by asymmetric side splittings,I0J4 so the observation that the la) these calculations are based on the R C from Rp. viridis, while 1s) and Ih,) Ih,) transition energies are about equal is not our experimental results were obtained from the R C of Rb. explained. these states given above, the angle measured would be zero because in this model there is a pseudo-Cz axis along which the permanent dipoles must be directed. If the PLand PMsites are asymmetric, as is expected from the structures and is widely ac~epted,~8-31J~ a better representation of the antisymmetric P* state is
-
-
-
-
sphaeroides.
A number of calculations have indicated that the two lowestenergy states of the R C are the exciton-like Qy- and Qy+ states (our IP,) and IPS))with Qy- lowest. A shoulder in the absorption of P at 850 nm was attributed by Eccles et ~ 1 . to ~ 3the upper exciton component, but this same shoulder was associated with B states by Parsons and Warshel.gJ0 In the Parsons and Warshel calculation the upper exciton component is strongly mixed with other cofactor states but should appear in the BChl spectrum at ca. 812 nm. The calculations of Zerner and co-workers on the bacteriochlorophyll dimerlz place the upper component about 1300 cm-* above Qy-, in agreement with Eccles and co-workers (- 1500 cm-1). Both these groupsgJOJ2 find Qy- and Qy+ as the two lowest-energy excited states. However, the polarization of
Conclusion An exploration of the kinetics and spectra of the IR electronic transitions of the R C undertaken in the present work permits a comparison of this previously unexplored region of the spectrum with the predictions made by theoretical calculations. While most of the calculations9.'0,'2,35 do predict electronic transitions of the special pair in the appropriate region, they vary greatly in predicting the observed polarization. The most consistent explanation for the anisotropy observed in this experiment calls for a greater mix of charge resonance states in the 'Qy-'' and UQy+nstates of the special pair than is predicted by most of the theoretical calculations. The results thus agree best with the
Transitions of Photosynthetic Reaction Centers calculation that predicts the largest such mixing, uiz., that of Fischer et al.35 Comparison of these results with Stark spectra3 in the context of the theoretical framework used in this work supports the existence of an asymmetric charge distribution in the “Qy-” state. This asymmetry is conceivably the origin of the asymmetric charge transfer in the RC, as has been proposed.g-2*-3lJ5 The polarization and kinetic behavior of the P+ band is in excellent agreement with it being a transition between symmetric and antisymmetric combinations of hole transfer states, as proposed by Breton et a1.Is A complete characterization of all the accessible IR transitions of the Qystate thus promises an accurate understanding of the relevant eigenstates of the RC. Characterization of the R C spectrum over a broader region of the IR is underway.
Note Added in Proof. In recent work in this laboratory we have discovered a new electronic transition of P* centered at 5900 cm-‘ that is more intense than the one reported in this paper. The details are in process of publication [Wynne, K.; Reid, G. D.; Hochstrasser, R. M. ‘New Mid-IR Electronic Transitions in the Photosynthetic Reaction Center”]. A conference report by P. 0. J. Scherer and s. F. Fischer recently has come to our attention [Scherer, P. 0. J.; Fischer, S. F. In The Photosynthetic Reaction Center;Breton, J., Vermeglio, A., Eds.; NATO AS1 Series A, Life Sciences; Plenum Press: New York, 1992; p 1931 in which transitions of P* are predicted at 1600 cm-1 (Pa -Ps) and one at 4800 cm-I, which is much more intense. Acknowledgment. This research was supported by grants from N I H and N S F to R.M.H. and N I H Grants 41048 and 48130 to P.L.D. References and Notes (1) Deisenhofer, J.; Epp, 0.;Miki, K.; Huber, R.; Michel, H. J . Mol. Biol. 1984, 180, 385. Deisenhofer, J.; Epp, 0.;Miki, K.; Huber, R.; Michel, Deisenhofer, J. EMBO J . H. Nature 1985, 318, 618. Michel, H.; Epp, 0.; 1986, 5, 2445. Deisenhofer, J.; Michel, H. The Photosynthetic Bacterial Reaction Center: Structure of Dynamics; Breton, J., Vermiglio, A., Eds.; (NATO AS1 Series 149; Plenum Press; New York, 1988; pp 1-3. Chang, C. H.;Tiede, D.;Tang, J.;Smiyh,U.;Noris, J.R.;Shigger,M. FEBSLett. 1986, 205, 82. Yates, T. 0.;Komiya, H.; Chirino, A.; Rees, D. C.; Allen, J. P.; Feher, G. Proc. Natl. Acad. Sci. U.S.A. 1988,85, 7993-7997. (2) Breton, J.; Martin, J.-L.; Petrich, J.; Migus, A.; Antonetti, A. FEBS Lett. 1986, 209. Martin, J.-L.; Breton, J.; Hoff, A. J.; Migus, A.; Antonetti, A. Proc. Natl. Acad. Sci. U.S.A. 1986,83,957-961. Fleming, G. R.; Matin, J.-L.; Breton, J. Nature 1988, 12, 190. Kirmaier, C.; Holten, D. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 3552. Breton, J.; Martin, J.-L.; Migus, A.; Antonetti,A.;Orszag,A. Proc. Natl. Acad.Sci. U.S.A. 1986,83,5121-5125. Woodbury, N. W.; Becker, M.; Middendorf, D.; Parson, W. W. Biochemistry 1985, 24, 7516-7521. Kirmaier, C.; Holten, D. FEBS Lett. 1988, 239, 21 1. Breton, J.; Martin, J.-L.; Fleming, G. R.; Lambry, J. C. Biochemistry 1988, 27, 8276-8284. Wraight, C. A,; Clayton, R. K. Biochim. Biophys. Acta 1973, 333, 246-260. (3) Vermeglio, A.; Paillotin, G. Biochim. Biophys. Acta 1982,681, 3240. (4) DeLeeuv, D.; Malley, M.; Butterman, G.; Okamura, M. Y.; Feher, G. Biophys. SOC.Abstr. 1982, 31, 11 la. Lbsche, M.; Feher, G.; Okamura,
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5783 M. Y. The Photosynthetic Bacterial Reaction Center-Structure and Dynamics; Breton, J., Vermeglio, A., Eds.; Plenum Press: New York, 1988; pp 151-164. Braun, H. P.; Michel-Beyerle, M. E.; Breton, J.; Buchanan, S.; Michel, H. FEBS Lett. 1987, 221, 221-225. ( 5 ) Lockhart, D. J.; Boxer, S. G. Biochemistry 1987,26,664-668,2958. (6) Lockhart, D. J.; Boxer,% G. Proc. Natl. Acad.Sci. U.S.A.1988,85, 1 0 7 1 1 1 (7) LGsche, M.; Feher,G.;Okamura, M. Y. Proc.Nat1. Acad.Sci. U.S.A. 1987, 84, 7537. (8) Boxer, S. G.; Goldstein, R. A.; Lockhart, D. J.; Middendorf, T. R.; Takiff, L. J. Phys. Chem. 1989, 93, 8280-8294. (9) Warshel, A,; Parson, W. W. J . Am. Chem. SOC.1987, 109, 61436152. (10) Parson, W. W.; Warshel, A. J . Am. Chem. SOC.1987, 109, 61526163. (11) Scherer, P. 0. J.; Fischer, S. F. Chem. Phys. Lett. 1986, 131, 153159. (12) Thompson, M. A.; Zerner, M. C.; Fajer, J. J . Phys. Chem. 1991,95, 5693-5700. (13) Middendorf, T. R.; Mazzola, L. T.; Lao, K.; Steffen, M. A.; Boxer, S. G. Biochim. Bioohvs. Acta 1993. 1143. 223-234. (14) McDowell,rL: M.; Kirmaier; C.; H i t e n , D. Biochim. Biophys. Acta 1990, 1020, 239-246. (15) Breton, J.; Nabedryk, E.; Parson, W. W. Biochemistry 1992, 31, 7503-75 10. (16) Locke. B.: Diller, R.: Hochstrasser. R. M. In Biomolecular Soectroscopy, Part B; Wiley: New York, 1993; p 10 ff. Diller, R.; Iannone; M.;
Cowen, B. R.; Maiti, S.; Bogomolni, R. A.; Hochstrasser, R. M. Biochemistry 1992,31, 5567-5572. (17) Clayton, R. K.; Wang, R. T. Methods Enzymol. 1971,23,696-704. (18) Mintele, W.; Wollenweber, A.; Nabedryk, E.; Breton, J. Proc. Natl. Acad. Sci. U.S.A. 1988,85,8468-8472. Maiti, S.; Cowen, B. R.; Diller, R.;
Iannone, M.; Moser, C. C.; Dutton, P. L.; Hochstrasser, R. M. Proc. NafI. Acad. Sci. U.S.A. 1993, 90, 5247-5251. (19) Maiti, S.;Cowen, B. R.; Diller, R.; Iannone, M.; M o w , C. C.; Dutton, P. L.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 52475251. (20) Kirmaier, C.; Holten, D. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 3552. (21) Hansen,P. A.;Moore, J. N.; Hochstrasser, R. M. Chem.Phys. 1989, 131, 49-62. (22) Lim-Vien, D.; Colthup, N. B.; Fately, W. G.; Grasselli, J. G. The Handbook of Infrared and Ramon Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991. (23) Eccles, J.; Honig, B.; Schulten, K. Biophys. J . 1983, 53, 137-144. (24) Breton, J. Biochim. Biophys. Acta 1985,810,235-245. Lyle, P. A.; Kolaczkowski, S. V.; Small, G. J. J . Phys. Chem. 1993, 97, 6924-6933. (25) Michl, J.; BonaEiC-Kouteckq, V. Electronic Ascpects of Organic Photochemistry; Wiley: New York, 1990. (26) Philpott, S.R.; Rice, M. J.; Bishop, A. R.; Campbell, D. K. Phys. Rev. B 1987, 36, 1735-1745. (27) Thompson, M. A.; Zerner, M. C. J . Am. Chem. SOC.1991, 113, 8210-8215. (28) Freisner, R. A.; Won, Y. Biochim. Biophys. Acta 1989,977,99-122. (29) Plato, M.; Mobius, K.; Michel-Beyerle, M. E.; Dixon, M.; Jortner, J. M. J . Am. Chem. SOC.1988, 110, 7279-7285. Gehlen, J. N.; Chandler, D.; Marchi, M. J . Am. Chem. SOC.1993, 115, 4178-4190 and references
therein. (30) Warshel, A,; Creighton, S.;Parson, W. W. J . Phys. Chem. 1988,92, 2692-2701. (31) Won, Y.; Freisner, R. A. Biochim. Biophys. Acta 1989,935,3240. (32) Breton, J. Biochim. Biophys. Acta 1985, 810, 235-245. (33) Small, G. J.; et al. Science 1993, 260, 68. (34) Reddy, N. R. S.; Lyle, P. A.; Small, G. J. Photosynth. Res. 1992, 31, 167-194. (35) Scherer, P. 0.J.; Fischer, S. F. J. Phys. Chem. 1989,93,1633-1637. Scherer, P. 0. J.; Fischer, S. F. Chem. Phys. 1989, 131, 115-127.
