UC Berkeley

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.