Three distinct types of unfrozen water in fully hydrated phospholipid bilayers: a combined 2H- and 31P-NMR study
✍ Scribed by Chang-Huain Hsieh; Wen-guey Wu
- Publisher
- Elsevier Science
- Year
- 1995
- Tongue
- English
- Weight
- 612 KB
- Volume
- 78
- Category
- Article
- ISSN
- 0009-3084
No coin nor oath required. For personal study only.
✦ Synopsis
Combined 2H-and 3~P-nuclear magnetic resonance (NMR) studies of six D~O/phospholipid systems with different headgroup are presented to identify the molecular origin of unfrozen water detected in phospholipid membranes. When phospholipids are dispersed in excess water, NMR signals of water molecules from the interbilayer space at subzero temperatures are identifiable because their spin-lattice relaxation time (T~) are relatively short in comparison with those from bulk ice. Three types of interbilayer unfrozen water are then revealed by studying the temperature-dependent behavior of isotropic 2H-NMR unfrozen D20 signal with Tj values in the ms range for fully hydrated D20/phospholipid bilayers. The first type is the supercooled water in D20/phosphatidylethanolamine and D20/phosphatidic acid. The unfrozen water of these systems can only be detected from -20 to -35°C and will freeze upon reaching the homogeneous nucleation temperature of ice formation for D20, i.e., -35°C. The second type is the perturbed water in D20/phosphatidylcholine and DEO/sphingomyelin. The isotropic 2H-NMR signals of these systems broaden with the decreasing temperature from -20 to -70°C. The third type is the bound water in DzO/phosphatidylserine and DeO/phosphatidylinositol. The 2H-NMR signals of these systems remain unchanged in terms of their signal intensity and linewidth with decreasing temperature even at the lowest studied temperature of -70°C. The 3~p-NMR spectra obtained in all hydrated phospholipid systems at -40°C show an axially asymmetric powder pattern similar to those obtained from dry lipids at room temperature suggesting that the rotational motion of phosphorous group is frozen at -40°C. We conclude that molecular groups attached to the phosphate segment in hydrated phospholipid systems are mainly responsible for the unfrozen water detected by NMR.