Resonance raman spectroscopy of chemically modified and isotopically labelled purple membranes. I. A critical examination of the carbon-nitrogen vibrational modes |
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Authors: | Benjamin Ehrenberg Ann T. Lemley Aaron Lewis Mark Von Zastrow Henry L. Crespi |
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Affiliation: | a School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, U.S.A. b Chemistry Division, Argonne National Laboratories, Argonne, IL 60435, U.S.A. |
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Abstract: | Resonance Raman spectra of bacteriorhodopsin are compared to the spectra of this protein modified in the following ways: (1) selective deuteration at the C-15 carbon atom of retinal, (2) full deuteration of the retinal, (3) the addition of a conjugated double bond in the β-ionone ring (3-dehydroretinal), (4) full deuteration of the protein and lipid components, (5) 15N enrichment of the entire membrane and (6) deuteration of the entire membrane (including the retinal). A detailed comparison of the 15N-enriched membrane and naturally occurring purple membrane from 800 cm?1 to 1700 cm?1 reveals that 15N enrichment affects the frequency of only two vibrational modes. These occur at 1642 cm?1 and 1620 cm?1 in naturally occurring purple membrane and at 1628 cm?1 and 1615 cm?1 in the 15N-enriched samples. Therefore, this pair of bands reflects the states of protonation of the Schiff base. However, our data also indicate that neither of these modes are simple, localized or C=N stretching vibrations. In the case of the 1642 cm?1 band motions of the retinal chain beyond C-15 are not significantly involved. On the other hand, in the 1620 cm?1 band atomic motions in the isoprenoid chain beyond C-15 are involved. |
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Keywords: | Resonance Raman spectroscopy Purple membrane Bacteriorhodopsin Chemical modification Retinal |
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