Transverse Nuclear Spin Relaxation Studies of Viscoelastic Properties of Membrane Vesicles. II. Experimental Results

Abstract

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. The observed relaxation rates, (ω), exhibit a linear dependence on the inverse pulse frequency over a wide frequency range in the kHz regime and then level off to a constant “plateau” value independent of ω. 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 ≥ 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 (ω), induced by vesicle fluctuations, depends linearly on ω-1 over a wide frequency range in the kHz regime. Notably, within this linear dispersion regime, the bending elastic modulus κ is the only relevant parameter because the magnitude of (ω) does not depend on R0, the effective lateral tension σ, and the viscosity of the surrounding fluid η. On the other hand, R0, σ, η, and κ determine the frequency at which (ω) levels off to a constant plateau value. Thus, analysis of the linear dispersion regime is a direct way to determine the bending rigidity κ. For the studied DMPC and DMPC/cholesterol vesicles, the κ values vary from (1.5 ± 0.1) × 10-20 J to (8.3 ± 0.1) × 10-20 J. From the plateau in the experimental dispersion profiles, values for the effective lateral tension of σ = 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.