Electronic Interactions in the Bacterial Reaction Center Revealed by Two-Color 2D Electronic Spectroscopy

Arkaprabha Konar, Riley Sechrist, Yin Song, Veronica R. Policht, Philip D. Laible, David F. Bocian, Dewey Holten, Christine Kirmaier, and Jennifer P. Ogilvie

The bacterial reaction center (BRC) serves as an important model system for understanding the charge separation processes in photosynthesis. Knowledge of the electronic structure of the BRC is critical for understanding its charge separation mechanism. While it is well-accepted that the “special pair” pigments are strongly coupled, the degree of coupling among other BRC pigments has been thought to be relatively weak. Here we study the W(M250)V mutant BRC by two-color two-dimensional electronic spectroscopy to correlate changes in the Qx region with excitation of the Qy transitions. The resulting Qy–Qx cross-peaks provide a sensitive measure of the electronic interactions throughout the BRC pigment network and complement one-color 2D studies in which such interactions are often obscured by energy transfer and excited-state absorption signals. Our observations should motivate the refinement of electronic structure models of the BRC to facilitate improved understanding of the charge separation mechanism.

Strongly coupled bacteriochlorin dyad studied using phase-modulated fluorescence-detected two-dimensional electronic spectroscopy

Vivek Tiwari, Yassel Acosta Matutes, Akraprabha Konar, Zhanqian Yu, Marcin Ptaszek, David F. Bocian, Dewey Holten, Christine Kirmaier and Jennifer P. Ogilvie

Fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) projects the third-order non-linear polarization in a system as an excited electronic state population which is incoherently detected as fluorescence. Multiple variants of F-2DES have been developed. Here, we report phase-modulated F-2DES measurements on a strongly coupled symmetric bacteriochlorin dyad, a relevant ‘toy’ model for photosynthetic energy and charge transfer. Coherence map analysis shows that the strongest frequency observed in the dyad is well-separated from the excited state electronic energy gap, and is consistent with a vibrational frequency readily observed in bacteriochlorin monomers. Kinetic rate maps show a picosecond relaxation timescale between the excited states of the dyad. To our knowledge this is the first demonstration of coherence and kinetic analysis using the phase-modulation approach to F-2DES.

Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy

Andrew Niedringhaus , Veronica R. Policht , Riley Sechrist , Arkaprabha Konar , Philip D. Laible , David F. Bocian, Dewey Holten , Christine Kirmaier , and Jennifer P. Ogilvie

In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an impor- tant model system for probing the structure–function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial re- action centers (BRCs) from two mutants of the purple bacterium Rho- dobacter capsulatus, spanning the Qy absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two- step sequential reaction scheme in which charge separation proceeds +− bacteriochlorophyll BA*

Two-Dimensional Electronic Stark Spectroscopy

Anton Loukianov, Andrew Niedringhaus, Brandon Berg, Jie Pan, S. Seckin Senlik, and Jennifer P. Ogilvie

Characterizing ultrafast energy and charge transfer is important for understanding a wide range of systems, from natural photosynthetic complexes to organic photovoltaics. Distinguishing the kinetic processes of energy transfer and charge separation in such systems is challenging due to the lack of clear spectral signatures of charge transfer states, which are typically nonradiative. Stark spectroscopy has proven to be a valuable method for uncovering charge transfer states. Here we extend the dimensionality of Stark spectroscopy to perform two-dimensional electronic Stark spectroscopy. We demonstrate the method on TIPS-pentacene in 3-methylpentane at 77 K. The additional frequency dimension of two-dimensional Stark spectroscopy promises to enable the identification of charge transfer states, their coupling to other charge transfer and exciton states, and their involvement in charge separation processes.

Ultrafast energy transfer within the photosystem II core complex

J. Pan, A. Gelzinis, V. Chorosajev, M. Vengris, S. S. Senlik, J. R. Shen, L. Valkunas, D. Abramavicius and J. P. Ogilvie

We report 2D electronic spectroscopy on the photosystem II core complex (PSII CC) at 77 K under different polarization conditions. A global analysis of the high time-resolution 2D data shows rapid, sub-100 fs energy transfer within the PSII CC. It also reveals the 2D spectral signatures of slower energy equilibration processes occurring on several to hundreds of picosecond time scales that are consistent with previous work. Using a recent structure-based model of the PSII CC [Y. Shibata, S. Nishi, K. Kawakami, J. R. Shen and T. Renger, J. Am. Chem. Soc., 2013, 135, 6903], we simulate the energy transfer in the PSII CC by calculating auxiliary time-resolved fluorescence spectra. We obtain the observed sub-100 fs evolution, even though the calculated electronic energy shows almost no dynamics at early times. On the other hand, the electronic–vibrational interaction energy increases considerably over the same time period. We conclude that interactions with vibrational degrees of freedom not only induce population transfer between the excitonic states in the PSII CC, but also reshape the energy landscape of the system. We suggest that the experimentally observed ultrafast energy transfer is a signature of excitonic-polaron formation.

Using coherence to enhance function in chemical and biophysical systems

Gregory D. Scholes, Graham r. Fleming, Lin X. Chen, Alán Aspuru-Guzik, Andreas Buchleitner, David F. Coker, Gregory S. Engel, Rienk van Grondelle, Akihito Ishizaki, David M. Jonas, Jeff S. Lundeen, James K. McCusker, Shaul Mukamel, Jennifer P. Ogilvie, Alexandra Olaya-castro, Mark A. Ratner, Frank C. Spano, K. Birgitta Whaley & Xiaoyang Zhu

Coherence phenomena arise from interference, or the addition, of wave-like amplitudes with fixed phase differences. Although coherence has been shown to yield transformative ways for improving function, advances have been confined to pristine matter and coherence was considered fragile. However, recent evidence of coherence in chemical and biological systems suggests that the phenomena are robust and can survive in the face of disorder and noise. Here we survey the state of recent discoveries, present viewpoints that suggest that coherence can be used in complex chemical systems, and discuss the role of coherence as a design element in realizing function.

Two-Color Nonlinear Spectroscopy for the Rapid Acquisition of Coherent Dynamics

S. Seckin Senlik, Veronica R. Policht, and Jennifer P. Ogilvie

There has been considerable recent interest in the observation of coherent dynamics in photosynthetic systems by 2D electronic spectroscopy (2DES). In particular, coherences that persist during the “waiting time” in a 2DES experiment have been attributed to electronic, vibrational, and vibronic origins in various systems. The typical method for characterizing these coherent dynamics requires the acquisition of 2DES spectra as a function of waiting time, essentially a 3DES measurement. Such experiments require lengthy data acquisition times that degrade the signal-to-noise of the recorded coherent dynamics. We present a rapid and high signal-to-noise pulse-shaping-based approach for the characterization of coherent dynamics. Using chlorophyll a, we demonstrate that this method retains much of the information content of a 3DES measurement and provides insight into the physical origin of the coherent dynamics, distinguishing between ground and excited state coherences. It also enables high resolution determination of ground and excited state frequencies.

Pulse-shaping based two-photon FRET stoichiometry

Daniel C. Flynn, Amar R. Bhagwat, Meredith H. Brenner,  Marcos F. Nunez, Briana E. Mork, Dawen Cai, Joel A. Swanson, and Jennifer P. Ogilvie

Forster Resonance Energy Transfer (FRET) based measure- ments that calculate the stoichiometry of intermolecular interactions in living cells have recently been demonstrated, where the technique utilizes selective one-photon excitation of donor and acceptor fluorophores to isolate the pure FRET signal. Here, we present work towards extending this FRET stoichiometry method to employ two-photon excitation using a pulse-shaping methodology. In pulse-shaping, frequency-dependent phases are applied to a broadband femtosecond laser pulse to tailor the two-photon excitation conditions to preferentially excite donor and acceptor fluorophores. We have also generalized the existing stoichiometry theory to account for additional cross-talk terms that are non-vanishing under two- photon excitation conditions. Using the generalized theory we demonstrate two-photon FRET stoichiometry in live COS-7 cells expressing fluorescent proteins mAmetrine as the donor and tdTomato as the acceptor.

Vibronic coherence in oxygenic photosynthesis

Franklin D. Fuller, Jie Pan, Andrius Gelzinis, Vytautas Butkus, S. Seckin Senlik, Daniel E. Wilcox, Charles F. Yocum, Leonas Valkunas, Darius Abramavicius & Jennifer P. Ogilvie

Photosynthesis powers life on our planet. The basic photosynthetic architecture consists of antenna complexes that harvest solar energy and reaction centres that convert the energy into stable separated charge. In oxygenic photosynthesis, the initial charge separation occurs in the photosystem II reaction centre, the only known natural enzyme that uses solar energy to split water. Both energy transfer and charge separation in photosynthesis are rapid events with high quantum efficiencies. In recent nonlinear spectroscopic experiments, long-lived coherences have been observed in photosynthetic antenna complexes, and theoretical work suggests that they reflect underlying electronic–vibrational resonances, which may play a functional role in enhancing energy transfer. Here, we report the observation of coherent dynamics persisting on a picosecond timescale at 77 K in the photosystem II reaction centre using two-dimensional electronic spectroscopy. Supporting simulations suggest that the coherences are of a mixed electronic–vibrational (vibronic) nature and may enhance the rate of charge separation in oxygenic photosynthesis.

Fast second-harmonic generation frequency-resolved optical gating using only a pulse shaper

Daniel E. Wilcox, Franklin D. Fuller, and Jennifer P. Ogilvie

In many ultrafast contexts, a collinear pulse-shaping frequency-resolved optical gating (FROG) technique is desired. Some applicable techniques already exist, but they suffer from one of two issues: either they require many time points to allow for Fourier filtering, or they do not yield a traditional FROG trace. To overcome these issues, we propose and demonstrate a fast new phase-cycled FROG technique using a pulse shaper.

Tight-binding model of the photosystem II reaction center: application to two-dimensional electronic spectroscopy

Andrius Gelzinis, Leonas Valkunas, Franklin D Fuller, Jennifer P Ogilvie, Shaul Mukamel and Darius Abramavicius

We propose an optimized tight-binding electron–hole model of the photosystem II (PSII) reaction center (RC). Our model incorporates two charge separation pathways and spatial correlations of both static disorder and fast fluctuations of energy levels. It captures the main experimental features observed in time-resolved two-dimensional (2D) optical spectra at 77K: peak pattern, lineshapes and time traces. Analysis of 2D spectra kinetics reveals that specific regions of the 2D spectra of the PSII RC are sensitive to the charge transfer states. We find that the energy disorder of two peripheral chlorophylls is four times larger than the other RC pigments.

Two-color two-dimensional Fourier transform electronic spectroscopy with a pulse-shaper

Jeffrey A. Myers, Kristin L. M. Lewis, Patrick F. Tekavec and Jennifer P. Ogilvie

We report two-color two-dimensional Fourier transform electronic spectroscopy obtained using an acousto-optic pulse-shaper in a pump-probe geometry. The two-color setup will facilitate the study of energy transfer between electronic transitions that are widely separated in energy. We demonstrate the method at visible wavelengths on the laser dye LDS750 in acetonitrile. We discuss phase-cycling and polarization schemes to optimize the signal-to-noise ratio in the pump-probe geometry. We also demonstrate that phase-cycling can be used to separate rephasing and nonrephasing signal components.

Two-dimensional electronic spectroscopy with a continuum probe

Patrick F. Tekavec, Jeffrey A. Myers, Kristin L. M. Lewis, and Jennifer P. Ogilvie*

We report 2D Fourier transform electronic spectroscopy obtained in the pump–probe geometry using a con- tinuum probe. An acousto-optic pulse shaper placed in the pump arm of a standard pump-continuum probe experiment permits 2D spectroscopy that probes a broad spectral range. We demonstrate the method on a simple dye system exhibiting vibrational wavepacket dynamics that modulate the peak shapes of the 2D spectra. The broad spectral range of the continuum probe allows us to observe vibronic cross peaks in the 2D spectra.