Effect of n-Decane and n-Octadecane on the Autoperoxidation of Egg Yolk Phosphatidyl -choline in Multilamellar Liposomes: Correlation with the Lipid Bilayer Thickness andBilayer Stability

Overview

Conjugated dienes (CD) and thiobarbituric acid reactive substances (TBARS) have been estimatedduring the autoperoxidation of chromatographically pure phosphatidylcholine from hen eggs(EYPC) in multilamellar liposomes by UV-VIS spectrophotometry. During the propagation phaseof autoperoxidation reaction, n-decane (C10) and n-octadecane (C18) gradually inhibit EYPCperoxidation up to 0:2:1 and 1:1 alkane:EYPC molar ratios, respectively. At higher molar ratios, theyield of estimated autoperoxidation products increases. At the highest molar ratio studied (alkane:EYPC = 2:1), the CD and TBARS concentrations exceed their levels in control sample without alkane added. The changes in the lipid bilayer thickness estimated from the small-angle neutronscattering (SANS) curves of unilamellar dioleoylphosphatidylcholine (DOPC) liposomes have indi-cated that C10 is located in the bilayer hydrophobic region parallel to the DOPC acyl chains at lowmolar ratios (C10:DOPC £ 0.4). At higher molar ratios (0.6 £ C10:DOPC £ 1.0), the alkane changesits location into the center of the bilayer between the apposing monolayers. The alkane locationparallel to polyunsaturated lipid fatty acyl chains RH separates these chains resulting in a decreased frequency of their encounters, in decreased yields of ROO L +RH -> ROOH+R L , 2 ROOH ->RO L +ROO L +H2O and RO L +RH -> R L +ROH free radical reactions, and consequently, in decreasedautoperoxidation. Autoperoxidation returns to the control values due to alkane redistribution intothe bilayer center. 1 H decoupled 31 P NMR spectra of EYPC+C10 aqueous dispersions have shownthat the lipid bilayer transforms into nonbilayer phases at C10:EYPC > 1:1 molar ratios. Theinverted hexagonal HII phase with C10 (or C18) preferentially located in the interstitial regions ofthe HII unit cell displays higher autoperoxidation than the control bilayer sample due to greatermotional freedom of RH chains in the HII phase.

Key words:
autoperoxidation - liposome - phosphatidylcholine - n-alkane - SANS - P NMR