MIXED MONOMOLECULAR FILM BEHAVIOR OF METHYL ISO-BRANCHED-CHAIN AND ANTEISO-BRANCHED-CHAIN AND OMEGA-CYCLOHEXYL PHOSPHATIDYLCHOLINES WITH CHOLESTEROL

This study presents the results obtained for several series of mixed monomolecular films of cholesterol with methyl iso-, methyl anteiso-, and omega-cyclohexyl phosphatidylcholines, and these are compared with previously obtained cholesterol/straight-chain phosphatidylcholine data. The amounts of cholesterol required to eliminate the phosphatidylcholine liquid expanded/liquid condensed phase transition decrease in the order straight-chain > methyl iso-branched > methyl anteiso-branched > omega-cyclohexyl alicyclic but increase with increasing chain length for both the methyl iso-branched and the methyl anteiso-branched series. In contrast there is little evidence of a chain length effect with the alicyclic omega-cyclohexyl phosphatidylcholines. Since branching and omega-cyclohexyl substitution expand the condensed state and condense the expanded state, thus reducing the extent of the liquid expanded/liquid condensed phase transition in the order straight-chain > methyl iso-branched > methyl anteiso-branched > omega-cyclohexyl alicyclic, it is clear that such substitution reduces the need for cholestrol to eliminate the transition. The increased need for cholestrol with increasing chain length for branched phosphatidylcholine phase transition elimination reflects the reduced effect of such branching with increasing chain length. It is clear that the presence of chain-branching and alicyclic chains in bacterial membranes can at least partially offset the lack of cholesterol in producing a close-packed but liquid crystalline state.