We have obtained deuterium (2H) Fourier transform nuclear magnetic resonance (NMR) spectra of zwitterionic L-[β-2H3]alanine, dl-[γ-2H6] valine, DL-β,-y-2H4]threonine, L-[5-2H3]leucine, and L-[α,β,γ,γ’,5-2H10]iso-leucine in the crystalline solid state and have determined the deuteriomethyl group spin-lattice relaxation rates as a function of temperature. The results yield the Arrhenius activation energies (δE*) for methyl rotation, and through use of a suitable mathematical model, rotational correlation times, tc. For alanine, valine, threonine, leucine, and isoleucine at 37 °C, τc and δE values are 780, 100, 40, 38, and 18 ps and 22, 14.0, 17.6, 15.5, and 8.6 kJ, respectively. For L-[ß- H3]alanine in the zwitterionic lattice, a spin-lattice relaxation time (T1) minimum of 2.1 ± 0.3 ms is observed (at 0 °C), in excellent agreement with the 1.92-ms prediction of the mathematical model. Similar τc and δE† measurements are reported for bacteriorhodopsin in the purple membrane of Halobacterium halobium R1 and for Escherichia coli cell membranes. Overall, our results demonstrate a great similarity between the dynamics in amino acid crystals and in membrane proteins. However, threonine exhibits a nonlinear Arrhenius behavior in bacteriorhodopsin, and in the valine-, leucine-, and iso-leucine-labeled membrane samples at higher temperatures (>37 °C), there is evidence of an additional slow side-chain motion. The lipid phase state in E. coli does not appear to influence, on the average, the dynamics of the valine side chains. These results indicate that the sensitivity of the deuterium NMR technique is now adequate to study in moderate detail the dynamics of most types of amino acids in a membrane protein and that adequate sensitivity, in some instances, should be available for the study of individual amino acids in suitably labeled membrane proteins.
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