Proton NMR T1, T2, and T1 rho relaxation studies of native and reconstituted sarcoplasmic reticulum and phospholipid vesicles

The phospholipids protons of native and reconstituted sarcoplasmic reticulum (SR) membrane vesicles yield well-resolved nuclear magnetic resonance (NMR) spectra. Resonance area measurements, guided by the line shape theory of Bloom and co-workers, imply that we are observing a large fraction of the lipid intensity and that the protein does not appear to reduce the percent of the signal that is well resolved. We have measured the spin-lattice (T1) and spin-spin (T2) relaxation rates of the choline, methylene, and terminal methyl protons at 360 MHz and the spin-lattice relaxation rate in the rotating frame (T1 rho) at 100 MHz. Both the T1 and T2 relaxation rates are single exponential processes for all of the resonances if the residual water proton signal is thoroughly eliminated by selective saturation. The T1 and T2 relaxation rates increase as the protein concentration increases, and T2 rate decrease with increasing temperature. This implies that the protein is reducing both high frequency (e.g., trans-gauche methylene isomerizations) and low frequency (e.g., large amplitude, chain wagging) lipid motions, from the center of the bilayer to the surface. It is possible that spin diffusion contributes to the effect of protein on lipid T1's although some of the protein-induced T1 change is due to motional effects. The T2 relaxation times are observed to be near 1 ms for the membranes with highest protein concentration and approximately 10 ms for the lipids devoid of protein. This result, combined with the observation that the T2 rates are monophasic, suggests that at least two lipid environments exist in the presence of protein, and that the lipids are exchanging between these environments at a rate greater than 1/T2 or 10(3) s-1. The choline resonance yields single exponential T1 rho relaxation in the presence and absence of protein, whereas the other resonances measured exhibit biexponential relaxation. Protein significantly increases the single T1 rho relaxation rate of the choline peak while primarily increasing the T1 rho relaxation rate of the more slowly relaxing component of the methylene and methyl resonances