Measurements of the bending elastic modulus in unilamellar vesicles membranes by fast field cycling NMR relaxometry

Abstract

The elastic properties of lipid membranes can be conveniently characterized through the bending elastic modulus κ. Elasticity directly affects the deformability of a membrane, morphological and shape transitions, fusion, lipid-protein interactions, etc. It is also a critical property for the formulation of ultradeformable liposomes, and of interest for the design of theranostic liposomes for efficient drug delivery systems and/or different imaging contrast agents. Measurements of κ in liposome membranes have been made using the fast field cycling nuclear magnetic relaxometry technique. We analyze the capability of the technique to provide a consistent value of the measured quantity under certain limiting conditions. Relaxation dispersions were measured acquiring a minimal quantity of points, within a reduced Larmor frequency range and, under inferior experimental conditions (in the presence of magnetic field in-homogeneity and lower power supply stability). A simplified model is discussed, showing practical advantages when fitting the data within the reduced frequency range. Experiments are contrasted with standard measurements performed in a state-of-the-art relaxometer. The methodology was tested in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine with different percentiles of cholesterol. We observe a tendency to a decrease in κ with increasing temperature, and a tendency to increase with the cholesterol percentile.