Origin and Behavior of Deuteron Spin Echoes in Selectively Labeled Amino Acids, Myoglobin Microcrystals, and Purple Membaanes

We have obtained deuterium NMR spin-echo spectra of crystalline dl-[-γ-²H₆] valine, [S-methyl-²H₃]-methiomne, cyanoferrimyoglobin from sperm whale (Physeter catodon), containing deuteriomethyl groups at methionine-55 and methionine-131, and [-γ-²H₆]valine-labeled bacteriorhodopsin in the purple membrane of Halobacterium halobium R]. By using 90-τ-β90o (XY) and 90-τ-β0. (XX) pulse sequences and observing the dependence of the spin-echo amplitude upon the interpulse spacing τ, we have 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 two 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 τdependence 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 narrow central component of the spectrum clearly indicated the very “fluid” or “mobile” nature of a series of residues that are shown elsewhere [Keniry, M., Gutowsky, H. S., & Oldfield, E. (1984) Nature (London) 307, 383-386] to arise from the membrane surface. Our results thus suggest that such NMR methods may yield useful information on side-chain dynamics complementary to that of line-shape and spin-lattice relaxation time analyses.