Molecular Dynamics of Vesicles of Unsaturated Phosphatidylcholines Studied by 13C NMR Spin-Lattice Relaxation
13 C spin-lattice relaxation times, T 1Z , were measured at four different magnetic field strengths for a vesicles of a series of unsaturated 1,2-diacyl- sn -glycero-3-phosphocholines over the temperature range from 5 to 50 °C. The acyl chains of the phosphatidylcholines varied in length from 14 to 18 carbons and in the degree of unsaturation: myristoleoyl (14 : 1), palmitoleoyl (16 : 1), oleoyl (18 : 1), linoleoyl (18 : 2), linolenoyl (18 : 3). For each of the phosphatidylcholines studied the R 1Z = ( NT 1Z ) -1 values of the acyl chain methylene carbons, where N is the number of directly bonded hydrogens, decreased from the C 2 position which is situated close to the glycerol backbone towards the acyl terminal methyl group. The effect of the double bond was manifested by an increase in R 1Z and was less pronounced if it was located closer to the terminal methyl group than in the middle of the chain. A minimum in the T 1Z relaxation times was not observed, even for highest frequency of 125.8 MHz and the lowest temperature of 5 °C. In the case of the phosphocholine headgroup and glycerol backbone carbons the T 1Z times showed similar temperature and frequency dependencies as the acyl chain carbons. The results of the measurements indicate substantial differences in the dynamics of these systems in the liquid-crystalline state. Analysis of the data using various dynamical models is consistent with a distribution of motions which is not described by a single Lorentzian spectral density. This dispersion appears to be an inherent feature of the dynamics of phosphatidylcholines in the liquid-crystalline state.