Dansyl lipid haptens with three different length (short, intermediate and long) spacers have been incorporated into DMPC or DPPC liposomes. Anti-DNS-IgG bound most efficiently to these liposomes containing lipid haptens with an intermediate length spacer, although the binding efficiency became more increased when liposomes were made of a mixture of phospholipid (DMPC or DPPC) and cholesterol. To explain these results we have measured the accessibility of dansyl lipid haptens in liposomal membranes by the fluorescence quenching method. It was found that the dansyl haptens located on the surfaces of DMPC (or DPPC) membranes and fluorescence quenchers (iodide ions) possessed almost similar accessibility for the dansyl haptens with different length spacers. However, in the DMPC (or DPPC) membranes with 50% cholesterol, a part of the dansyl haptens became buried into the interior of the liposomal membranes depending on the length of the spacer and another part removed into the aqueous solution with greater affinity for antibody. These results were explained well by our recent model for antibody binding to the membrane-bound lipid haptens.
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