NOVEL SYNTHETIC PHOSPHOLIPID PROTECTS LIPID BILAYERS AGAINST OXIDATION DAMAGE - ROLE OF HYDRATION LAYER AND BOUND WATER

The role of the membrane hydration layer in preventing membrane oxidat ive damage has been evaluated by comparing bilayers with and without a n extended hydration layer, The extended hydration layer was obtained through the use of a novel phospholipid in which polyethylene glycol o f 2000 Da molecular mass (PEG(2000)) was covalently attached to the ph osphate headgroup of a phospholipid backbone to form dihexadecylphosph atidyl s(hexadecyloxy)propoxy}(hydroxyphosphinoyl}-omega- methoxypoly( oxyethane-1,2-diyl) monosodium salt, The amount of water bound to free PEG and to the DHP-PEG(2000) was determined by differential scanning calorimetry, Small unilamellar liposomes composed of egg phosphatidylc holine and DHP-PEG(2000) were prepared, 44% of the phospholipid contai ned one polyunsaturated acyl chain, Oxidative damage to liposomes afte r exposure to three different oxidation procedures was measured by the disappearance of polyunsaturated acyl chains, as determined by GC, Ox idation procedures used were: (i) exposure to ionizing gamma-irradiati on (9200 Gy), for which the grafted PEG(2000) provided significant pro tection against oxidation, with minimal damage to the PEG(2000) determ ined by H-1 NMR and TLC, (ii) Storage for 6 months at 4 degrees C or f or 4 months at 4 degrees C followed by 4 d at 37 degrees C, for which the presence of DHP-PEG(2000)retarded acyl chain peroxidation. (iii) O xidation of the liposomes by 2,2'-azo(2-amidinopropane) dihydrochlorid e (a positively charged water-soluble peroxyl radical initiator), for which there was no protection by DHP-PEG(2000) (probably due to electr ostatic binding of the AAPH to the negatively charged membranes, there by overriding the hydration layer protection barrier).