Dynamics of amino acid side chains in membrane proteins by high field solid state deuterium nuclear magnetic resonance spectroscopy. Phenylalanine, tyrosine, and tryptophan

Abstract

We have obtained the first deuterium NMR spectra of individual types of aromatic amino acids in a defined membrane protein, bacteriorhodopsin, in the photosynthetic purple membrane of Halobacterium halobium R1. Isotopic labeling and high field (8.5 Tesla) operation permitted relatively rapid data acquisition at a variety of temperatures. At the temperature of growth (37 degrees C), we find that all 7 tryptophan residues are rigid on the time scale of the NMR experiment (approximately 10(-5) s), except for likely librational motions of approximately 10 degrees amplitude. By contrast, nearly all (9 +/- 2) of the 11 tyrosines and (13 +/- 2) 13 phenylalanines undergo rapid (greater than 10(5) s-1) 2-fold rotational flips about C gamma-C zeta, causing formation of line shapes dominated by effectively axially asymmetric (asymmetry parameter eta = 0.66) deuteron electric field gradient tensors. On cooling the phenylalanine- and tyrosine-labeled samples to approximately -30 degrees C, all such motions are frozen out, i.e. occur at rates less than 10(4) s-1, and axially symmetric (eta approximately 0.05) line shapes are observed. At T greater than 91 degrees C, phenylalanine-, tyrosine-, and tryptophan-labeled membrane spectra undergo dramatic narrowing to an isotropic line of approximately 4-9 kHz width. This transition is a reflection of the loss of tertiary structure in the membrane protein with resultant fast unrestricted motion of these aromatic side chains, and is only partly reversible. These results, in conjunction with those obtained using [gamma-2H6]valine-labeled bacteriorhodopsin (Kinsey, R. A., Kintanar, A., Tsai, M-D., Smith, R. L., Janes, N., and Oldfield, E. (1981) J. Biol. Chem., 256, 4146-4149) indicate the rather rigid nature of amino acid side chains in the H. halobium purple membrane, the principal fast lage amplitude motions being methyl group rotation and discontinuous benzene ring "flipping."