Circular dichroism of the laser-induced blue state of bacteriorhodopsin, spectral analysis and new insights into the purple-->blue color change

Date of Award


Degree Name

M.S. in Chemistry


Department of Chemistry


Advisor: Mark B. Masthay


The purple membrane (PM) of the salt-loving bacterium H. Salinarium owes both its color and physiological function to the protein bacteriorhodopsin (BR). The PM is comprised of BR trimers arranged in a crystalline hexagonal lattice. PM converts to an ultraviolet-induced colorless membrane (UVCM) upon exposure to diffuse ultraviolet (UV) light through a 1-monomer 1-photon process and to a laser-induced blue membrane (LIBM) upon exposure to intense green laser pulses through a 1-monomer 2-photon process. The color changes which BR molecules undergo depend on the (1) BR aggregation state (e.g. crystalline trimeric PM and monomers) (2) wavelength of light (3) intensity of the light and (4) divalent cations, as earlier results indicate that calcium ions Ca²⁺ are removed from the PM surface during the PM-->LIBM photoconversion. The origin of the unconventional biphasic band in CD spectrum has caused much debate. The principle purpose of the research is to further elucidate the mechanisms responsible for the well known PM-->LIBM and PM-->UVCM photoconversions. Both of these processes have significant implications regarding potential photocooperative processes and exciton coupling between BR molecules within the PM. They also have important implications for the long-standing debate about the unusual bisignate" circular dichroism (CD) spectrum of PM in the visible region of the spectrum, which has been attributed to both exciton coupling between the BR molecules and BR protein heterogeneity within the trimer. To accomplish this objective, I characterized the changes in the absorption and CD spectra of five separate BR species upon irradiation with intense 532 nm laser pulse and diffuse 254nm-UV-light: Native PM, Delipidated PM, Monomeric BR, cation-free blue membrane and calcium saturated PM. Conclusions were drawn about the relative roles of inter-trimer and intra-trimer photocooperativity, exciton coupling and BR protein heterogeneity in the photochemistry and color changes of BR in PM."


Bacteriorhodopsin Spectra, Circular dichroism, Absorption spectra, Chemistry, Circular Dichroism, Bacteriorhodopsin, Photocooperativity, Exciton Coupling, Protein Heterogeneity, Cation Binding in Bacteriorhodopsin, ICP of Bacteriorhodopsin, Absorption Spectrum of Laser Induced Blue State of Bacteriorhodopsin

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