Transverse nuclear spin relaxation studies of viscoelastic properties of membrane vescicles. II. Experimental results

Transverse nuclear spin relaxation measurements employing Carr-Purcell (CP) pulse sequences have been used to determine the viscoelastic properties of quasi-spherical membrane vesicles with controlled radii R0. CP The observed relaxation rates, R_2infinity^CP (omega), exhibit a linear dependence on the inverse pulse frequency over a wide frequency range in the kHz regime znd then level off to a constant "plateau" value independent of omega. Within the linear dispersion regime, the same relaxation rates are detected for unilamellar and oligolamellar vesicles, indicating that the interbilayer coupling is weak and has no effect on the measured relaxation curves. Analysis of the experimental dispersion profiles is performed using a slow-motional model in which two different relaxation processes are considered (i.e., vesicle shape fluctuations and molecular translational diffusion). It is shown that for vesicle radii R0 greater than or equal to 200 nm lateral diffusion across the vesicle shell is too slow to contribute significantly to transverse spin relaxation in the kHz range. Rather, vesicle shape fluctuations constitute the dominant transverse relaxation process. Model calculations reveal that R_2infinity^CP(omega), induced by vesicle fluctuations, depends linearly on omega^-1 over a wide frequency range in the kHz regime. Notably, within this linear dispersion regime, the bending elastic modulus is the only relevant parameter because the magnitude of R_2infinity^CP(w) does not depend on R0, the effective lateral tension, and the viscosity of the surrounding fluid. On the other hand, all these parameters determine the frequency at which R_2 (w) levels off to a constant plateau value. Thus, analysis of the linear dispersion regime is a direct way to determine the bending rigidity. For the studied DMPC and DMPC/cholesterol vesicles, the K values vary from (1.5 +/-0.1) X 10^-20 J to (8.3 +/- 0.1) x 10^-20 J. From the plateau in the experimental dispersion profiles, values for the effective lateral tension of 3+/-1 and 4+/-1 have been extracted. It appears that transverse NMR relaxation involving CP sequences represents a powerful tool for the study of the viscoelastic properties of membrane vesicles.