Light Induced Color Changes in the Purple Protein Bacteriorhodopsin


Light Induced Color Changes in the Purple Protein Bacteriorhodopsin



Carlie J McGrath



This research provides new insights into the mechanisms associated with light-induced color changes in the bacteriorhodopsin (BR)-containing halobacterial purple membrane (PM). BR is a purple, 26kDa transmembrane protein which is structurally similar to the human visual proteins rhodopsin and iodopsin. This research focuses on the influences of light scattering and spectral distortions induced by the solvent in which PM is suspended. Varying volume ratios of water and glycerol were used as the solvent for PM samples. A ratio of 60% glycerol:40% water produced spectra with the least scattering, as demonstrated by a low absorbance region (300-420 nm) in the UV-visible absorption spectra and equal intensities of the positive and negative bands in the circular dichroism (CD) spectra. Suspensions of native PM, calcium saturated purple membrane (CSPM) and cation free blue membrane (CFBM) were irradiated with intense 532 nanometer (nm) laser pulses or the 254 nm line from a mercury lamp. During irradiation the absorption and CD spectra were monitored; the spectral changes observed in pure water and glycerol:water suspensions were similar, but faster in pure water. PM, CSPM, and CFBM each produced a UV product with λmax ~360 nm; each species experienced unique changes in the CD spectra as well. With both the laser and the lamp, the absorption spectra of both PM and CSPM “crossed over” (i.e., the long wavelength edge of the main absorption band shifted bathochromically) during the early stages of irradiation; this “crossing over” was absent in CFBM. PM also underwent a later bathochromic shift of its absorption λmax with a concomitant purple-to-blue color change—but only with the laser. With both light sources CSPM and CFBM underwent a color loss but no color change. The structural and mechanistic implications of these spectral changes are detailed below.

Publication Date


Project Designation

Graduate Research - Graduate

Primary Advisor

Mark B. Masthay

Primary Advisor's Department



Stander Symposium project

Light Induced Color Changes in the Purple Protein Bacteriorhodopsin