Quantum Physics of Photosynthetic Light -Harvesting

216 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2001.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 Rhodobacter sphaeroides has been established recently. The photosynthetic unit (PSU) of purple bacterium 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 Rhodospirillum molischianum 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.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD