Nuclear magnetic resonance studies of amino acids and proteins. Deuterium nuclear magnetic resonance relaxation of deuteriomethyl-labeled amino acids in crystals and in Halobacterium halobium and Escherichia coli cell membranes

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Abstract
2H Fourier transform NMR spectra were obtained for zwitterionic L-[.beta.-2H3]alanine, DL-[.gamma.-2H6]valine, DL-[.beta.,.gamma.-2H4]threonine, L-[.delta.-2H3]leucine and L-[.alpha.,.beta.,.gamma.,.gamma.'',.delta.-2H10]isoleucine in the crystalline solid state. The deuteriomethyl group spin-lattice relaxation rates were determined as a function of temperature. The results yield the Arrhenius activation energies .**GRAPHIC**. for methyl rotation and, through use of a suitable mathematical model, rotational correlation times, .tau.c. For alanine, valine, threonine, leucine and isoleucine at 37.degree. C, .tau.c and .**GRAPHIC**. values are 780, 100, 40, 38 and 18 ps and 22, 14.0, 17.6, 15.5 and 8.6 kJ, respectively. For L-[.beta.-2H3]alanine in the zwitterionic lattice, a spin-lattice relaxation time (T1) minimum of 2.1 .+-. 0.3 ms was observed (at 0.degree. C), in excellent agreement with the 1.92-ms prediction of the mathematical model. Similar .tau.c and .**GRAPHIC**. measurements are reported for bacteriorhodopsin in the purple membrane of H. halobium R1 and for E. coli cell membranes. A great similarity exists between the dynamics in amino acid crystals and in membrane proteins. Threonine exhibits a nonlinear Arrhenius behavior in bacteriohodopsin, and in the valine-, leucine-, and isoleucine-labeled membrane samples at higher temperatures (.gtorsim. 37.degree. C), there is evidence of an additional slow side-chain motion. The lipid phase state in E. coli does not appear to influence the dynamics of the valine side chains. Thus, the sensitivity of the 2H NMR technique is now adequate to study in moderate detail the dynamics of most types of amino acids in a membrane protein.