Variations in microviscosity values induced by different rotational behaviour of fluorescent probes in some aliphatic...

Recently there have been large developments in the indirect estimation of phospholipid bilayer and membrane microviscosities through the use of fluorescent probes with the help of paraffin oils as standard solvents. Before applying this semi-empirical method to membrane systems, it seemed necessary to test: first, a large variety of probes (this has been done by many authors) and secondly, a large variety of aliphatic oils (there is little literature on these tests). The present paper shows the variations of the rotational relaxation rates of three probes in relation to the viscosities of some aliphatic oils. When changing the oil but keeping constant the macroscopic viscosity, large differences appear in the relaxation rates of a given fluorphore (a ratio of 30/1 is observed in the extreme cases). The microviscosities of membranes deduced from the probe motion will consequently exhibit large uncertainty, as is shown with dipalmitoyl phosphatidylcholine. The cause of these different behaviours must be looked for in the properties of the oil. Particularly, the anisotropy of the solute-solvent interaction in the site where the probe is located depends in part on the internal order of the solvent which is used as a reference.