Lipid chain order and dynamics at different bilayer depths in liposomes of several phosphatidylcholines studied by differential polarized phase fluorescence

The influence of acyl chain length, double bond number and position on the structural and dynamic properties of phosphatidylcholine bilayers in the liquid crystalline state was studied. The range and rate of the rotation of 1,6-diphenyl-1,3,5-hexatriene and a set of n-(9-anthroyloxy)stearates (n = 2, 7 and 12), which are located at different depths in the lipid bilayers, were measured using differential polarized phase fluorometry in multilamellar liposomes of several unsaturated synthetic phosphatidylcholines. For the anthroyloxystearate probes, two rotational modes, ‘in’ and ‘out’ of the plane of the anthroyl aromatic ring, were partially resolved by measuring at different excitation wavelengths. The data obtained here indicate that in lecithins containing the same acyl chain in sn-1 and sn-2, the introduction of double bonds towards the deep interior of the lipid bilayer decreases the order in the external region of the bilayer and the viscous resistance to the rotation of all the probes tested. On the contrary, double bonds located towards the external region of the bilayer increase order and viscous resistance to the rotation of probes located in the external region. Increasing acyl chain length has different effects depending on the probe and on the rotational mode considered. Measurements in mixed chain lecithins indicate that the influence of increasing unsaturation in sn-2 has different consequences depending on the acyl group that is present in sn-1. Similarly, increasing acyl chain length in sn-1 influences differently the bilayer properties depending on the fatty acid that is present in sn-2.