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 R 0 . The observed relaxation rates, R 2 CP (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 R 0 g 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 2 CP (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 R 2 CP (ω) does not depend on R 0 , the effective lateral tension σ, and the viscosity of the surrounding fluid η. On the other hand, R 0 , σ, η, and κ determine the frequency at which R 2 CP (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.

Cholesterol-induced suppression of membrane elastic fluctuations at the atomistic level

“…Given a simple composite membrane deformation model (Brown et al, 2002; Nevzorov et al, 1998), the R 1 Z rates are due to a broad spectrum of 3D collective bilayer excitations, with effective rotations of the lipids. Transverse 2 H NMR spin relaxation studies also provide evidence for 2D collective motions of the membrane film, albeit at lower frequencies (Althoff et al, 2002a; Althoff et al, 2002b; Bloom and Evans, 1991; Bloom et al, 1991; Molugu et al, 2012, 2013). By contrast, local trans-gauche isomerizations along the chains modulate the same static NMR coupling tensor, and do not yield such a square-law.…”

An NMR database for simulations of membrane dynamics

“…The combined dispersions represent frequency-dependent spectral density J m (ω) contributions following a three-dimensional power-law trend (ω −1/2 , d = 3) that spans nearly the full MHz regime, from 2 MHz to 938 MHz for the carbon acyl chain segments. On the other hand, a two-dimensional power law scaling (ω −1 , d = 2) of 31 P NMR phospholipid headgroup dynamics has been identified in dynamics has been identified in T 2 measurements in the kHz regime [154,155]. The difference in the power-law scalings may reflect the sensitivity of the headgroup 31 P nucleus to surface or smectic-like undulations, while measurements using 2 H or 13 C as a probe of hydrocarbon dynamics may reflect the 3D anisotropy of the interior of the hydrocarbon membrane.…”

Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. II. Second-order contributions of the fluctuating director

“…Electronic mail: g.moro@chfi.unipd.it measured for dimyristoyl-phosphatidylcholine membrane vesicles. [19][20][21] The analysis provided valuable information on the elastic properties of the vesicles including the bending rigidity and the lateral tension of the membranes. More recently, the same method of analysis has been applied to liquid crystal polymers.…”