J . Phys. Chem. 1987, 91, 5524-5527
5524
signal as a 2-ps wide spike in the thermal grating rise which serves as a t = 0 marker. The malachite green data in Figure 3 shows a 30-ps rise followed by a slow decay due to thermal diffusion, without acoustic modulation. With the extremely narrow fringe spacings, the grating decay due to thermal diffusion is 5.4 ns in accord with the known diffusion constant of ethan01.I~ Similar decays were found for the heme proteins except the decays were significantly slower than that due to diffusion which is again evidence for a slow grating component in agreement with the narrow-angle studies. In Figure 4, the thermal grating rise time is shown in an expanded time scale. Figure 4A shows the calculated thermal grating response convolved to the excitation probe pulse shapes for rise times varying from 10 to 30 ps. In Figure 4B, the upper curve is the grating signal rise time for disodium fluorescein in ethanol. This grating formation is due to excited-state formation and gives the instrument response for an instantaneous rise time. The next lowest curve depicts the data for cyanomethemoglobin in an aqueous buffer. The grating rise time is 17 f 4 ps and is clearly discernible in comparision to the fluoroescein data. Nearly identical rise times were observed for cytochrome c and metmyoglobin. The lowest curve is the thermal grating rise time of malachite green in ethanol. The observed time is 28 f 4 ps. The slower rise time corresponds to the 30% slower speed of sound in ethanol than in water.. In water, the intrinsic rise time of the thermal grating for an instantaneous rise in the lattice temperature approximates a 22-ps exponential rise time. The observed rise times are all in this range for the heme proteins which demonstrates that the fast vibrational energy relaxation component to the protein exterior is faster than the temporal resolution of the technique. Therefore, the vibrational relaxation process from the heme porphyrin ring to the protein backbone must be faster than 20 ps. In addition, for the energy to be transferred to the aqueous bath on this time scale, the (13) Eichler, H.; Salje, G.; Stahl, H. J . Appl. Phys. 1973, 44, 5383.
vibrational energy must be extensively delocalized over the protein structure for it to occur so rapidly on the protein's exterior. These results are in excellent agreement with predictions of a recent molecular dynamics sim~1ation.I~The simulation calculated a fast 10-20-ps vibrational relaxation process from the heme porphyrin to the protein backbone with complete vibrational energy delocalization over the protein structure for both cytochrome c and deoxymyoglobin. Conclusion Vibrational energy relaxation and dispersion in heme proteins following optical excitation consists of a fast component of less than 20 ps and a much slower relaxation process occurring on a 10-ns time scale. Vibrational energy transfer from the heme group through the van der Waals contacts must occur on a time scale faster than 20 ps. The vibrational energy transferred to the protein matrix becomes extensively delocalized and is efficiently transferred to the tightly bound water layer surrounding the protein within this same time scale. The slow relaxation component is due to relaxation of a high potential energy ground electronic state. The potential energy surface within the heme pocket and protein matrix contains thousands of minima corresponding to small perturbations in nuclear coordinates. During the vibrational energy relaxation process numerous minima are sampled. Once the protein reaches a lower internal temperature the nuclear coordinates become trapped in these higher energy local potential minima and only slowly relax to the lowest energy configuration.
Acknowledgment. Lynn Richard made significant contributions to the early stages of this research. The work was supported by the Department of Energy Office of Basic Sciences (R.J.D.M.) and N I H GM33881 (G.M.). R. J. D. Miller is a recipient of an N S F Presidential Young Investigator Award. (14) Henry, E. R.; Eaton, W. A.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 1986. 83. 8982. (15) Salcedo, J. R:; Siegman, A. E.; Dlott, D. D.; Fayer, M. D. Phys. Rev. Lett. 1978, 41, 131.
Hole Burnlng Spectroscopy of a Core Antenna Complex J. K. Gillie, J. M. Hayes, G. J. Small,* Ames Laboratory-USDOE and Department of Chemistry, Iowa State University, Ames, Iowa 5001 1
and J. H. Golbeck Department of Chemistry, Portland State University, Portland, Oregon 97207 (Received: June 1 1 , 1987)
Zero-phonon, vibronic, and phononic satellite hole structure are reported for a core antenna complex (C670) of photosystem I. The electron-protein phonon coupling of C670 is compared with that for the primary electron donor state P700. Low-frequency phonons (-30 cm-') are implicated as the primary acceptor modes for excitation transport from the antenna to the reaction center. Excitation transport within the antenna at helium temperatures is markedly slower than at room temperature.
The importance of the electron-phonon coupling, associated with low-frequency protein modes, for electron-transfer (ET) and electronic excitation transport (EET) in the photosynthetic unit poses an interesting problem. Recently, such coupling (strong) has been argued, on theoretical grounds, to be very important for certain optical excitation and E T processes of photosynthetic reaction centers (RC). Bixon and Jortner conclude, for example, that in the RC of photosynthetic bacteria the quinone reduction and its back charge recombination with the radical cation of the primary electron donor (PED) are governed (from a nuclear tunneling point of view) by protein modes of 100 an-'in energy.l
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(1) Bixon, M.;Jortner, J. J . Phys. Chem. 1986, 90, 3795.
0022-3654/87/2091-5524$01.50/0
The role of the intramolecular modes of the donor and acceptor was judged to be unimportant.' Hayes, Gillie, and co-workersZ4 have demonstrated that recent hole burning data for the PED states P870, P960, and P700 of R. sphaeroide~,~-~ R. viridis>* and (2) Hayes, J. M.; Small, G. J. J . Phys. Chem. 1986, 90, 4928. (3) Gillie, J. K.; Fearey, B. L.; Hayes, J. M.; Small, G. J.; Golbeck, J. H. Chem. Phys. Lett. 1987, 134, 316. (4) Hayes, J. M.; Gillie, J. K.; Tang, D.; Small, G. J. Biochim. Biophys. Acta, submitted for publication. (5) Boxer, S. G.; Middendorf, T. R.; Lockhart, D. J. Chem. Phys. Lett. 1986, 123,476. (6) Meech, S . R.; Hoff, A. J.; Wiersma, D. A. Chem. Phys. Lett. 1985, 121, 287. (7) Boxer, S. G.; Middendorf, T. R.; Lockhart, D. J. FEBS Lett. 1986, 200, 231.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5525
Letters TABLE I: Electron-Phonon Coupling Parameters"
C670
r,. cm-' S w,, cm-I
r, cm-I
P700 -300 5-6 30 30
-200 C0.9
-
30 40
P870 -350 4-5 80
80
50
40
I
i
P960 -150 4-5
"Data for C670 from this work. Other data from ref 4. r,, site inhomogeneous line broadening contribution to absorption profile at helium temperatures; w,, mean phonon frequency; r, full-width halfmaximum of the one-phonon profile.
photosystem I,3 respectively, can be quantitatively understood in terms of strong linear electron-phonon coupling and inhomogeneous line broadening (rI)from pigment site inhomogeneity. That is, the absorption profiles of P870, P960, and P700 are dictated by a large Huang-Rhys factor S (long phonon progression) and rr.An alternative interpretations*6v8is that the broad (-400 cm-I) holes observed for P870 and P960 result from an ultra-fast (-25 fs) charge separation process within the special pair (a BChl dimer) prior to electron transfer to the bacteriopheophytin. The optically excited PED state is viewed as a neutral pair excitonic state and electron-phonon coupling is neglected. This model, however, cannot account for the dependence of the hole profiles on the laser burn frequency (wB)and the thermal broadening data associated with the PED ~ t a t e . ~In? ~the model of Hayes and Small2 phononic (by analogy with vibronic) transitions determine its absorption profile. The key parameters of the theory for hole burning in the presence of arbitrarily strong electron-phonon coupling are S,rI, the mean phonon frequency w,, and the width of the one-phonon profile l'.2-4 The values of these parameters for the three aforementioned PED states are given in Table I since they are germane to this paper. The Stokes shift and Franck-Condon factor for the zero-phonon transition associated with the PED states are, to a reasonable approximation, given by 2Sw, and exp(-S), respectively. It was that the large value of S for the PED states means that the optically excited PED state possesses substantial charge-transfer ~ h a r a c t e r . ~ The same conclusion was more recently reached by Lockhart and Boxerlo based on Stark measurements on P870. In this paper we report the first application of hole burning to an antenna (core) protein complex, C670 of photosystem I. These data are used to determine the importance of protein phonons for EET from the antenna complex to the PED state of the RC. The importance of site inhomogeneity to EET within the antenna complex at low temperatures is also considered. A vibronic (and phononic) satellite hole spectrum, which represents the first high-resolution optical spectrum of an antenna complex, is presented and discussed. Finally, new high-resolution photochemical hole burning data are given for the P700 absorption region. Enriched (-35:l Chl a:P700) photosystem I particles from spinach chloroplast were isolated following the procedure of Golbeck" and dissolved in a buffered (pH 8.3) glycero1:water glass-forming solvent containing 1 mM ascorbic acid. A typical visible absorption spectrum showing a distinct P700 shoulder at 1.6 K is given in ref 3. For hole burning, the optical density (OD) at the burn frequency wB was adjusted to
