Quantum physics of photosynthetic light-harvesting
Damjanović, Ana
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Permalink
https://hdl.handle.net/2142/31311
Description
Title
Quantum physics of photosynthetic light-harvesting
Author(s)
Damjanović, Ana
Issue Date
2001
Director of Research (if dissertation) or Advisor (if thesis)
Schulten, Klaus J.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
quantum physics
light-harvesting
photosynthesis
Rbodobacter spbaeroides
Language
en
Abstract
Absorption of light by light harvesting complexes and transfer of electronic excitation to the photosynthetic
reaction center (RC) constitutes the primary step of photosynthesis, i.e., the light
harvesting process. A model for an atomic level structure of a so-called photosynthetic unit of the
photosynthetic bacterium Rbodobacter spbaeroides has been established recently. The photosynthetic
unit (PSU) of purple bacteria combines a nanometric assembly of three protein complexes:
(i) the photosynthetic reaction center, (ii) a ring-shaped light harvesting complex LH-I, and (iii)
multiple copies of a similar complex, LH-II. The model describes in detail the organization of
pigments involved in primary light absorption and excitation transfer: a hierarchy of ring-shaped
chlorophyll-carotenoid aggregates which surround four centrally located chlorophylls of the photosynthetic
reaction center. This thesis presents a quantum-mechanical description of the light harvesting
process in the PSU, based on the atomic level model. Excitation transfer rates for various
excitation transfer steps have been determined through Fermi's golden rule. To describe electronic
excitations of the strongly coupled chlorophyll aggregate in LH-II, an effective Hamiltonian has
been established. This Hamiltonian has further been extended to describe also the LH-II -> LH-II
-> LH-I -> RC cascade of excitation transfer. The results suggest that, in the absence of disorder,
the electronic excitations in LH-II are coherently delocalizaed over the ring, and that such excitonic
states speed up the light-harvesting process. Influence of thermal disorder on exciton coherence
has been studied by means of a combined molecular dynamics/quantum chemistry approach. The
results indicate a significant loss of coherence due to thermal effects. Excitation transfer between
carotenoids and chlorophylls has been investigated in two light-harvesting complexes; LH-II of the
purple bacterium Rbodospirillum moliscbianum and peridinin-chlorophyll protein of the dinoflagellate
Amphidinium carterae. The electronic excitations of carotenoids and BChls have been described
by means of semi-empirical self-consistent-field configuration interaction calculations. The
electronic coupling between the various electronic excitations has been determined for all orders of
multipoles (Coulomb mechanism) and includes the electron exchange (Dexter mechanism) term.
We identified the mechanisms and pathways of singlet excitation transfer between carotenoids and
chlorophylls. The role of the symmetry breaking in achieving efficient energy transfer through the
optically forbidden carotenoid 2Ag state has been investigated.
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