論文の公開元へSite-Switchable Dynamic Nuclear Polarization NMR Measurement on Dynamics of Liposome-Environment Water
概要
The phenomena of life are based on a complex of chemical processes, wherein water plays a variety of important roles, as indicated by the fact that it makes up to 70% of human body. For instance, water is essential in maintaining the isothermal nonequilibrium state of the living system and serves as a medium for transporting nutrients and excreting wastes. Furthermore, it contributes to the construction of the living system by promoting hydrophobic interaction between organic molecules: biological membranes were formed due to the existence of water. On a more microscopic level, water of hydration also aids in the folding of proteins and genes, their molecular recognitions, and the chemical reactions involved in their functions. [1–4] Thus, water is indispensable to life at all levels, macroscopic to microscopic.[5–9]
To advance the molecular-level understanding of various biological phenomena, many scientists have studied the properties and rheology of water of hydration around biological complexes, such as proteins and membranes.[10, 11]Various methods such as vibrational (infrared) spectroscopy, neutron scattering, and nuclear magnetic resonance (NMR) were employed to quantify the dynamics of water of hydration. Vibrational spectroscopy detects not only the bond oscillation of the water of hydration but also the intermolecular hydrogen bond within the water molecules.[12, 13] Neutron scattering can provide information on fluctuations in the network of water molecules in the hydrated layer.[14–15] On the other hand, NMR relaxometry[6, 12, 16] is a method that can determine the molecular motion in the correlation time region of 10 ps to 10 ns. Relaxometry is currently applied in a common method for tracking water, known as magnetic resonance imaging (MRI) in medical diagnosis.[17, 18] However, in the investigation of water dynamics in biological system at the microscopic level, it remains challenging to determine whether the measurements sense bulk water or hydrated water. Furthermore, the low sensitivity of NMR is a problem with NMR. Alternatively, dynamic nuclear polarization NMR (DNP NMR), which overcomes the sensitivity restrictions posed by general NMR for investigating water of hydration, has attracted attention in recent years. The DNP NMR is a nuclear spin polarization method via Overhasuer effect that utilizes the saturation and relaxation of electron spins.
In this study, the author aimed at understanding the dynamics of water incorporated into and surrounding lipid bilayers (Figure 1-1), especially the vesicular assembly which has a hollow spherical structure, using the DNP-NMR method. As representing by the cell membrane and thylakoid membrane, liposomal assembly plays an important role in the living system: it distinguishes the inside and the outside of a cell and an organelle, constructs the structure of the living system, and provides a medium for the transportation of materials and information. Inside the liposomal membrane, the phospholipids are aligned with their polar groups toward the surface and their aliphatic chain toward the center of the membrane. Classically, it was considered that the surface of the vesicle is surrounded by stern layer and the shear plane is around it: the plane is located 2 Å away from the surface of the vesicle. But this classical consideration was developed from the viewpoint that the vesicle was a rigid body. In addition, vesicular membrane was considered to be impervious to water because of its hydrophobic property. However, with advancements in the understanding of the vesicular membrane by considering the thermal motion of molecules, it has been suggested that the membrane is permeable to water.[19– 21] Thus, from the dynamics perspective of molecules, the understanding of the properties of liposomal vesicles, which play a fundamental role in vivo, has changed dramatically in these last few decades.
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