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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2009)

Photosynthesis begins with absorbing the sun light by the light harvesting complexes. The solar energy is then funneled into the reaction center (RC) via the energy transfer between the light harvesting complexes at ultrafast rates (~1/100fs ) with extremely high quantum efficiency (~100 %). Most of the complexes are composed of pigments and protein matrices that tightly bind them. The pigments are responsible for absorbing and transferring the energy. The roles of the protein environment of photosynthetic pigment-protein complexes have been suggested, but the detailed mechanisms are still not fully understood.

In this dissertation, non-linear spectroscopic methods using ultrashort pulses (~ 40-fs FWHM), three-pulse photon echo studies are presented to investigate the roles of protein environment of the photosynthetic bacterial RC. The technique characterizes the protein dynamics around the pigments (a bacteriochlorophyll a, B and a bacteriopheophytin a, H) in the RC. In particular, two-color three-pulse electronic coherence photon echo technique is used to observe the quantum coherence between the excited states of coupled H and B, whose life time is sensitive to the protein dynamics. I found a long-lasting quantum coherence suggesting that the protein actively preserves the quantum coherence. A scenario in which the long-lasting coherence can accelerate the rate of energy trap is described with a simple Bloch model simulation.

In addition, one- and two-color three-pulse photon echo peak shift (1C- and 2C3PEPS) techniques are used to measure the coupling strength between H and B in the wild type RC. The coupling strength is facilitated from the geometry between the pigments governed by the protein environment. The simulation based on the standard response function formalism is used to obtain the coupling strength. 2C3PEPS signal from H and B of the oxidized RC is reproduced to extract the coupling constant between them by quantum-master equation which efficiently incorporates pulse overlap effect and bath memory effect. The values will enable the molecular level of studies on the photosynthetic energy and electron transfer.

    Cover page: Two-color Three-pulse Photon Echo Studies on the Photosynthetic Bacterial Reaction Center
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2009)
    The broad linewidths in absorption spectra of photosynthetic complexes obscure information related to their structure and function. Photon echo techniques represent a powerful class of time-resolved electronic spectroscopy that allow researchers to probe the interactions normally hidden under broad linewidths with sufficient time resolution to follow the fastest energy transfer events in light harvesting. Here, we outline the technical approach and applications of two types of photon echo experiments: the photon echo peak shift and two-dimensional (2D) Fourier transform photon echo spectroscopy. We review several extensions of these techniques to photosynthetic complexes. Photon echo peak shift spectroscopy can be used to determine the strength of coupling between a pigment and its surrounding environment including neighboring pigments and to quantify timescales of energy transfer. Two-dimensional spectroscopy yields a frequency-resolved map of absorption and emission processes, allowing coupling interactions and energy transfer pathways to be viewed directly. Furthermore, 2D spectroscopy reveals structural information such as the relative orientations of coupled transitions. Both classes of experiments can be used to probe the quantum mechanical nature of photosynthetic light-harvesting: peak shift experiments allow quantification of correlated energetic fluctuations between pigments, while 2D techniques measure quantum beating directly, both of which indicate the extent of quantum coherence over multiple pigment sites in the protein complex. The mechanistic and structural information obtained by these techniques reveals valuable insights into the design principles of photosynthetic light-harvesting complexes, and a multitude of variations on the methods outlined here.
      Cover page: Photon echo studies of photosynthetic light harvesting
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