Electron spin resonance study of the interactions of retinoids with a phospholipid model membrane

The effects of up to 20 mol% incorporation of all-trans-retinol (vitamin A), retinal (vitamin A aldehyde) and retinoic acid (vitamin A acid) on acyl chain order and dynamics in liquid crystalline dipalmitoylphosphatidylcholine membranes at pH 7.5 were studied by electron spin resonance (ESR) of 5-, 7-, 10-, 12- and 16-doxyl spin-labelled stearic acids intercalated into the membrane. Order parameters S and correlation times tau c determined from the ESR spectra demonstrate that the influence of retinoic acid differs from retinol or retinal. Whereas the latter two retinoids have negligible effect (less than 1%) on acyl chain order towards the membrane surface (5 position), retinoic acid reduces the order parameter by as much as 8% at 20 mol% incorporation. All three retinoids restrict acyl chain motion to a similar extent approaching the center of membrane (10, 12 and 16 positions), where up to 22% increases in order parameter and correlation time were observed. Complementary osmotic swelling and carboxyfluorescein release measurements show that the enhancement in permeability of egg phosphatidylcholine membranes to erythritol and carboxyfluorescein is greater with all-trans-retinoic acid than all-trans-retinol or retinal.