Spectral analysis of the photodegradation of the purple protein Bacteriorhodopsin and the supporting evidence of exciton coupling as the origin of the circular dichroism signal

Date of Award


Degree Name

M.S. in Chemistry


Department of Chemistry


Advisor: Mark B. Masthay

Second Advisor

Advisor: Angela Mammana


Bacteriorhodopsin (BR) is a purple trans-membrane protein which constitutes the color and biological function of the purple membrane (PM) found in the salt-loving bacterium H. Salinarium. BR monomers consist of a covalently bound retinyl chromophore that is elliptically surrounded by seven alpha-helices. These monomers aggregate as individual trimers which arrange in a rigid, hexagonal crystalline lattice in PM. A two-photon one-monomer process allows for the photoconversion from PM to a laser-induced blue membrane (LIBM), stimulated by exposure to intense 532 nm laser pulses. Furthermore, the exposure of PM to diffuse 254 nm ultraviolet light results in a one-photon one-monomer process which dictates the PM photoconversion to an ultraviolet-induced colorless membrane (UVCM). The color changes observed in PM in the following studies are due to either the wavelength of light exposure or the concentration of divalent cations in the membrane. A highly debated issue surrounding PM is the origin of the bisignate circular dichroism (CD) signal associated with the achiral retinyl chromophore of BR. The initial purpose of this research was to address light scattering evident in PM solutions, which would provide an improvement in the analysis of the spectral behaviors of PM. A reduction in the light scattering was achieved by adding glycerol to the suspension solvent. A secondary principle of the following studies was to further investigate the influence of cation concentration on the color changes of PM before and during photo-induced degradation. Specifically, the photodegradation of calcium saturated purple membrane (CSPM) and cation-free blue membrane (CFBM) were studied upon exposure to intense 532 nm laser pulses and diffuse 254 nm ultraviolet light. Though these aspects are significant in providing new insights into the mechanism of degradation of PM, the central purpose of this research was to address the origin of the bisignate CD signal. Improvements in the light scattering achieved by adding glycerol to the suspension solvent allowed for improved spectral interpretation during these photoconversions. Conclusions on the influence of cations and the color changes of PM before and during photodegradation, and the origin of the bisignate CD signal were reached.


Bacteriorhodopsin, Photodegradation, Circular dichroism, Chemistry, Biochemistry, Biophysics, Physical Chemistry, Bacteriorhodopsin, Purple Membrane, Circular Dichroism, Exciton Coupling, Two Photon, Photodegradation, Bisignate

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Copyright © 2017, author