Origin and behavior of deuteron spin echoes in selectively labeled amino acids, myoglobin microcrystals, and purple membranes

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Abstract
2H NMR spin-echo spectra were obtained of crystalline DL-[.gamma.-3H6]valine, [S-methyl-2H3]methionine, cyanoferrimyoglobin from sperm whale (Physeter catodon), containing deuteriomethyl groups at methionine-55 and methionine-131, and [.gamma.-2H6]valine-labeled bacteriorhodopsin in the purple membrane of Halobacterium halobium R1. By using 90-.tau.-.beta.90.degree. (XY) and 90-.tau.-.beta.0.degree. (XX) pulse sequences and observing the dependence of the spin-echo amplitude on the interpulse spacing .tau., it was determined that the so-called quadrupole echoes obtained in these typical selectively deuterated condensed-phase biological systems are in fact strongly modulated by proton-deuteron and deuteron-deuteron dipolar interactions. The 2 amino acids and the protein crystals behaved as typical organic solids, with no evidence of liquid-like behavior, even in the presence of excess water (in the case of the ferrimyoglobin crystals). However, with the valine-labeled bacteriorhodopsin, the .tau.-dependence of XY echoes as a function of temperature emphasized the solid-like behavior of the membrane matrix, while the basic nature of the spin-echo response for the marrow central component of the spectrum clearly indicated the very fluid or mobile nature of a series of residues that are shown elsewhere to arise from the membrane surface. Such NMR methods may yield useful information on side-chain dynamics complementary to that of line-shape and spin-lattice relaxation time analyses.

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