Photochemical Studies of Rhodopsin, Purple Membrane, and Their Analogues

Mao, Boryeu

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Description

Title

Photochemical Studies of Rhodopsin, Purple Membrane, and Their Analogues

Author(s)

Mao, Boryeu

Issue Date

1980

Department of Study

Physiology and Biophysics

Discipline

Biophysics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Biophysics, General

Language

eng

Abstract

• Rhodopsin and analogues. The absorption spectrum of the bathoproduct of rhodopsin, which contains 11-cis retinal as chromophore, is obtained at 77(DEGREES) K under the condition that on the average less than one photon can be absorbed by each rhodopsin molecule. The spectrum, with (lamda)(,max) at 542 nm, is found to be almost identical to the spectrum obtained by Yoshizawa and Wald (1963) from measurements on photostationary state mixtures. In addition, absorption spectra are also measured for the bathoproducts of two rhodopsin analogues the chromophores of which are two different isomers of retinal, isorhodopsin I with 9-cis retinal and isorhodopsin II with 9,13-dicis retinal. The bathoproduct of isorhodopsin I is identical to that of rhodopsin within experimental errors, while the spectrum for the bathoproduct of isorhodopsin II is different. The photochemistry of the dicis pigment appears to be different from rhodopsin and isorhodopsin I.
• The direct conversion from rhodopsin to isorhodopsin I after photon absorption is not detected either by absorption measurements or by chromophore extraction. This indicates that the quantum yield of the formation of isorhodopsin I from rhodopsin is negligible. This and the bathoproduct experiment above prove the following scheme to be appropriate for the description of the photointerconversions between rhodopsin, bathorhodopsin and isorhodopsin I at 77(DEGREES) K as first proposed by Yoshizawa and Wald: Rhodopsin (DBLARR) Bathorhodopsin (DBLARR) Isorhodopsin I.
• Purple membrane and analogues. The exciton theory of trimer interactions is described in detail. The results of our previous theoretical and experimental analyses on the purple membrane chromophore orientation (Ebrey et al., 1977) are discussed in the light of some new experimental evidence.
• The K intermediate, the first in the photo-induced cycle of purple membrane protein and stable at 77(DEGREES) K, is found to decay near 90(DEGREES) K. The decay product is L, the next spectrally observable intermediate in the cycle. The barrier height for the K to L transformation is estimated to be 1.22 Kcal/mole from the temperature dependence of the decay rate.
• Methylated retinal analogues are used to study the binding of chromophore in the apo-membrane and the photochemical properties of the pigment in membrane. The results indicate that the binding site of the bacterio-opsin in somewhat restricted in dimension near the Schiff base linkage.
• Two pigment analogues with 5,6-dihydro chromophore are also studied. The double bond between C(,5) and C(,6) of retinal is not required for the primary photochemistry of these pigments and therefore most likely is not involved in the photochemical reaction of native purple membrane as well.
• A spin-labeled retinal analogue is also found to form a pigment analogue with bleached purple membrane. This pigment is potentially valuable for determining the chromophore-chromophore separation in the membrane and the accessibility of the chromophore from the aqueous phase. The unstability of the spin label with respect to hydrolysis has presented difficulties for obtaining such information.

Graduation Semester

1980

Type of Resource

text

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