Theory of the optical spectra of bacterial photosynthetic antenna complexes
Abstract
This research aims to investigate the details of the function of one particular kind of antenna complex--the water soluble bacteriochlorophyll a-protein complex from Prosthecochloris aestuarii This is done by analyzing the interactions among the molecular and constituents of the complex, using the molecular orbital and exciton interaction theories, to predict the energy transitions of the bacteriochlorophyll a molecules in the complex. A series of different physical models and corresponding computer simulation programs have been developed for (1) low-temperature spectral simulations with standard transition dipole orientations, (2) comparative room-temperature simulations of UV-visible absorption spectra of bacteriochlorophyll in ether versus in FMO protein, (3) low-temperature simulations with adjustable dipole orientations and strengths. The fitting parameters include transition energies, transition widths and squared dipole strength. The results show: (1) There are 14 distinct transition energies ranging from 776 nm to 828 nm, 7 different transition widths ranging from 6 nm to 21 nm and a squared dipole strength around 52 D$\sp2$. (2) Four-orbital theory does not produce good simulation spectra in the short wavelength region. There are at least three electronic transitions in the short wavelength region for absorption spectrum of bacteriochlorophyll a in ether and four electronic transitions for absorption spectrum of bacteriochlorophyll a in FMO complex. (3) The two electronic transitions of lowest energy, Qx and Qy, are each rotated about 60$\sp\circ$ from their nominal axis while maintaining their 90 relative orientation. In addition to the Prosthecochloris aestuarii, strain 2K, the spectra of Chl. limicola f. thiosulfatophilum, Tassajara were also simulated. The results are similar to the Prosthecochloris aestuarii. The computation methods we have used here give us a powerful tool to understand more about bacteriochlorophyll proteins and other photosynthetic antenna systems.
Degree
Ph.D.
Advisors
Pearlstein, Purdue University.
Subject Area
Biophysics|Molecules|Computer science
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