MRI as a Complement to Solid-State NMR for Lateral Lipid Diffusion

Lipid membranes are an integral part of the human body; allowing for cell structure and stability, they are extremely difficult to study due to their complex organization and size. Researchers commonly study these structures by utilizing liposomes in their place. As such, a variety of techniques exist to study these complex structures in-depth. Solid-state nuclear magnetic resonance (NMR) is the current tool most commonly used to measure lateral lipid diffusion. As more information is gathered from orthogonal techniques, more accurate and extensive simulations are produced to understand different interactions and properties of membranes, which can be applied to more complex cellular systems.\nDiffusion Magnetic Resonance Imaging (MRI) is a new technique being adapted to study the random motion of hydrogen nuclei in molecules. Fast and slow-moving particles can be differentiated according to their diffusion coefficients which correlate to the of these different sized molecules in space. In this experiment, high diffusion numbers correspond to faster molecular movement. In a complex structure consisting of various molecular species, identification of known values, such as water, allows for easier identification of interest lipids in samples. The diffusion coefficient provides information that allows scientists to calculate the rate of movement of a particle, particularly with the help of the modified Einstein relation. This technique can be used to simulate the lateral movement of lipids along the plane of the membrane, and define how they behave in more complex environments.\nUsing a homogenized mixture of phosphatidylcholine lipids from sunflowers and water, lipid movement was studied using a series of diffusion MRI experiments. The sensitivity to specific lipid diffusion was increased with each experiment to refine instrument parameters. The data was fitted to a biexponential curve which allowed for the separation of the slow and fast diffusion coefficients in 3 axes: x, y and z. Using information from the initial trials, a T2-Diffusion Correlation map was generated and allowed for more accurate results of other samples tested later on. As well, a sample was tested on a supported lipid bilayer, to produce a greater signal by limiting the planes the lipid could move in.\nThe information obtained was similar to previous measurement attempts using alternative methods and reinforces the current value of applying MRI as a novel method to study lipid dynamics in both free-floating liposomes and supported bilayers. With more testing, MRI may show promise to complement solid-state NMR and aid in better understanding these complex systems.

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