Hindered molecular rotation in 1,2-dipalmitoyl-L-phosphatidylcholine monohydrate by nuclear magnetic resonance spin-lattice relaxation in the rotating frame(T 1? )

Abstract

Spin-lattice relaxation times in the rotating frame (T_1ρ) have been measured at temperatures between –210 and 170°C for 1,2-dipalmitoyl-L-phosphatidylcholine. 1H₂O. T_1ρ minima corresponding to correlation frequencies of 68kHz are found, and assigned to motions of the terminal aliphatic methyl groups (–190°C), the N⁺(CH₃)₃ methyl groups (–100°C) and to low amplitude oscillatory motions of the methylene groups of the fatty acid residues (–23°C). Measurements on a per-deuterated analogue reveal a further T_1ρ minimum at 61°C which is not observed in the fully protonated sample, although there is some difference of the transition temperatures. This minimum is assigned to a further 34 kHz reorientation prior to a transition from crystal to liquid crystal. Spin diffusion processes are thought to be responsible for the absence of an analogous minimum for the protonated sample. The pre-transitional minimum is revealed by the addition of 19 % water, and is assigned to a new low frequency motion of the N⁺(CH₃)₃ protons of the head group. In the lamellar liquid crystalline phase, one T_1ρ is observed and may be dominated by motions of the head group protons. The absence of any change of T_1ρ is the lamellar-viscous isotropic phase transition confirms that the low frequency motions of the polar head groups protons remain unchanged, in spite of a change of symmetry. A hydrophilic-continuous model of the cubic phase structure is therefore preferred. The head group conformation in both the crystalline and liquid crystalline states is such that, except for the transition region, spin diffusion occurs between the hydrophilic and hydrophobic moieties. Efficient coupling may be achieved by a gauche conformation of the phosphatidylcholine residue in the presence of water.