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  • Title: Raman linear intensity difference of membrane-bound peptides: indole ring orientations of tryptophans 11 and 13 in the gramicidin A transmembrane channel.
    Author: Maruyama T, Takeuchi H.
    Journal: Biospectroscopy; 1998; 4(3):171-84. PubMed ID: 9639108.
    Abstract:
    A Raman linear intensity difference (RLID) method has been developed to determine orientations of chromophores in membrane-bound peptides and proteins. The method involves orientation of the peptide or protein in lipid bilayer membranes and measurement of intensity differences between Raman spectra excited with two orthogonal laser polarizations. Analysis of the RLID spectrum is simplified when the chromophore exhibits a vibrational mode for which the Raman band is enhanced through resonance with a single molecular electronic transition. To examine the indole ring orientations of Trp residues in the gramicidin A transmembrane channel, we have prepared analogues of gramicidin A, in which one of four Trp residues is replaced by deuterated Trp (Trp-2,4,5,6,7-d5). Two vibrational Raman bands omega(d)3 and omega(d)2 of deuterated Trp have been shown to gain intensity predominantly through resonance with the Bb and La electronic transitions, respectively, when excited at 244 and 257 nm. By examining the RLID spectra of the omega(d)3 and omega(d)2 bands of gramicidin A channels oriented in phospholipid bilayer membranes, we have determined the inclination angles of the Bb and La transition moments with respect to the channel axis in the absence and presence of Na+. The orientations of the Trp-11 and Trp-13 indole rings in the gramicidin channel structure have been derived from the inclination angles of the transition moments. The indole rings of Trp-11 and Trp-13, which are known to shift along the channel axis upon binding of Na+, do not reorient during their positional shifts.
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