31P NMR study of head group behavior in sonicated phosphatidylcholine liposomes in the gel and in the liquid state

Abstract

We measured the 31P[ 1H] Nuclear Overhauser Effect (NOE) as a function of temperature and of 1H irradiation frequency, the linewidth Δν 1 2 as a function of temperature and the relaxation time T 1 above and below the thermal transition temperature, of the 31P-NMR signal in sonicated liposomes of 1,2-dimiristoyl-3- sn-phosphatidylcholine (DMPC), 1,2-dipalmitoyl-3- sn-phosphatidylcholine (DPPC) and 1,2-dimiristoyl-3- sn-phosphatidylcholine (DSPC). The same measurements were repeated in the presence of high molecular weight dextrans. They strongly reduce the NOE and produce longer relaxation times T 1. According to the current models, we were able to evaluate, in the different situations, the correlation time of the internal motion τ G and the distance r between interacting groups in the region of the polar head groups. While the first parameter changes abruptly through the phase transition and under the effect of dextrans, the latter does not appear modified in any case. These results are discussed in terms of a conformational change of the phosphocholine head groups.