Quantum Biology of the PSU
It is through photosynthesis that earth's biosphere derives its energy from sunlight. Photosynthetic organisms, i.e., plants, algae and photosynthetic bacteria, have developed efficient systems to harvest the light of the sun and to use the light energy to drive their metabolic reactions, such as the reduction of carbon dioxide to sugar. The ubiquitous green color of plants is testimony to the key molecular participant in the light harvesting of plants, chlorophylls. More hidden in this respect, but no less widespread, is a second participating molecule, carotenoid. In green leaves the color of the carotenoids is masked by the much more abundant chlorophylls while in red ripe tomatoes or petals of yellow flowers, the carotenoids predominate. Chlorophyll molecules exist in slightly different chemical structures in various photosynthetic organisms, as chlorophyll a or b in plants or algae, and as bacteriochlorophyll a (BChl-a) or b in photosynthetic bacteria. Molecules such as chlorophyll and carotenoid that absorb light and impart color to living matter and other materials are called pigments.
In general, biological pigments are non-covalently bound to proteins, forming the so-called pigment-protein complexes. The pigment-protein complexes are organized as the photosynthetic unit (PSU). The bacterial PSU consists of two types of pigment-protein complexes: the photosynthetic reaction centers (RCs) and the light-harvesting complexes. The main function of the light-harvesting complexes is to gather light energy and to transfer this energy to the reaction centers for the photo-induced redox processes. In most purple bacteria, the photosynthetic membranes contain two types of light-harvesting complexes: light harvesting complex I (LH-I) and light harvesting complex II (LH-II). While LH-I is tightly bound to the photosynthetic reaction centers, LH-II is not directly associated with the reaction centers, but transfers energy to the reaction centers via LH-I.
Purple bacteria are great masters of harvesting light. Nearly all the energy gained by the absorption of a photon is transferred on to the reaction center. To illustrate how purple bacteria achieve such high efficiency, we trace the way of a photon (and its excitation energy, respectively) through the light-harvesting system. On this way we will point out the remarkable geometrical features that serve the process of harvesting light. It is the goal of our research to understand how these geometrical features translate into physical properties that ideally support the biological function. It will be shown that purple bacteria exploit elegant quantum physics, the working of which were only fully understood recently after the discovery of the structures of light-harvesting complexes and investigations into their electronic excitations.
Primary Absorption of a Photon
Light is absorbed either by bacteriochlorophylls or carotenoids in different spectral regions. Two kinds of bacteriochlorophylls absorb at slightly different energies and at different angles. The ring structure enhances absorption and generates an energy trap.
Carotenoid-Chlorophyll Excitation Transfer
In addition to photoprotecting chlorophylls, carotenoids also enhance the absorption crossection by absorbing light in the green light range. They transfer their electronic excitation within about 100 fs to the B850 BChl ring.
B800-B850 BChl Excitation Transfer
B800 BChls transfer excitation via the Foerster mechanism to the B850 BChl ring. The transfer occurs within less than one picosecond. The ring structure accelerates this transfer by enhancing the spectral overlap through exciton splitting.
Excitation Transfer between Light-Harvesting Complexes
The coplanar arrangement of BChls is ideal for excitation transfer between the BChl rings which proceeds within few picoseconds. Transfer from the LH-I ring to the RC is accelerated by bridge BChls.
Kinetics of excitation migration and trapping in the photosynthetic unit of purple bacteria. Thorsten Ritz, Sanghyun Park, and Klaus Schulten. Journal of Physical Chemistry B, 105:8259-8267, 2001.
Efficient light harvesting through carotenoids. Thorsten Ritz, Ana Damjanovic, Klaus Schulten, Jian-Ping Zhang, and Yasushi Koyama. Photosynthesis Research, 66:125-144, 2000.
Excitation transfer in the peridinin-chlorophyll-protein of Amphidinium carterae. Ana Damjanovic, Thorsten Ritz, and Klaus Schulten. Biophysical Journal, 79:1695-1705, 2000.
Excitation energy trapping by the reaction center of Rhodobacter sphaeroides. Ana Damjanovic, Thorsten Ritz, and Klaus Schulten. Int. J. Quantum Chem., 77:139-151, 2000.
Energy transfer between carotenoids and bacteriochlorophylls in a light harvesting protein. Ana Damjanovic, Thorsten Ritz, and Klaus Schulten. Physical Review E, 59:3293-3311, 1999.
Light-harvesting and photoprotection by carotenoids: Structure-based calculations for photosynthetic antenna systems. Thorsten Ritz, Ana Damjanovic, and Klaus Schulten. In G. Garab, editor, Photosynthesis: Mechanisms and Effects (Proceedings of the XIth International Congress on Photosynthesis), volume 1, pp. 487-490, Dordrecht, 1998. Kluwer Academic Publications.
Excitons and excitation transfer in the photosynthetic unit of purple bacteria. Thorsten Ritz, Xiche Hu, Ana Damjanovic, and Klaus Schulten. Journal of Luminescence, 76-77:310-321, 1998.
Pigment organization and transfer of electronic excitation in the purple bacteria. Xiche Hu, Thorsten Ritz, Ana Damjanovic, and Klaus Schulten. Journal of Physical Chemistry B, 101:3854-3871, 1997.
From simplicity to complexity and back: Function, architecture and mechanism of light harvesting systems in photosynthetic bacteria. Klaus Schulten. In H. Frauenfelder, J. Deisenhofer, and P. G. Wolynes, editors, Simplicity and Complexity in Proteins and Nucleic Acids, pp. 227-253, Berlin, 1999. Dahlem University Press. (pdf available from author upon request).
Architecture and function of the light harvesting apparatus of purple bacteria. Xiche Hu, Ana Damjanovic, Thorsten Ritz, and Klaus Schulten. Proceedings of the National Academy of Sciences, USA, 95:5935-5941, 1998.
How nature harvests sunlight. Xiche Hu and Klaus Schulten. Physics Today, 50:28-34, 1997.