Effect of sterol side-chain structure on sterol-phosphatidylcholine interactions in monolayers and small unilamellar vesicles

In this study we have characterized the monolayer behavior of analogues of cholesterol having different side-chain structures and their interaction with phosphatidylcholines in mixed monolayers and small unilamellar vesicles (SUVs). Two series of side-chain analogues of cholesterol were synthesized, one with an unbranched side chain (the n-series, from 3 to 7 carbons in length), and the other with a single methyl-branched side chain (the iso-series, from 5 to 10 carbons in length). The length and conformation of the sterol side chain markedly influenced both the mean molecular area of the pure sterols and their monolayer stability (i.e., collapse pressure). Shorter side chains gave smaller mean molecular areas and decreased monolayer stability. The sterols from the n-series also had smaller mean molecular areas than the corresponding sterols in the iso-series. In mixed 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/sterol monolayers (equimolar ratio; at 22°C), all of the sterols tested decreased the monolayer stability as judged by the lower collapse pressure with sterol than without sterol. A similar trend was observed in mixed monolayers containing 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), except that sterols from the iso-series with a chain length of 8 or 10 carbon atoms actually stabilized the monolayer compared with the sterol-free SOPC monolayer. The ability of the sterols to condense the molecular packing of DPPC was similar with all sterols (3-5% condensation at 10 mN/m), irrespective of the length or structure of the side chain. 5-Androsten-3β-ol, however, which lacks the side chain, did not all condense the monolayer packing of DPPC. With SOPC mixed monolayers, all side chain containing sterols caused a 18-20% condensation (at 10 mN/m) of monolayer packing. The condensing effect of 5-androsten-3β-ol on SOPC packing was again much smaller (about 10%) compared with that of the side-chain sterols. The rate of sterol oxidation by cholesterol oxidase (at 37°C) in DPPC-containing SUVs increased as a function of increasing the side-chain length (iso-series). With sterols from the n-series, the same trend was seen, except that the n-C7 analogue was oxidized much slower than the n-C4, n-C5, and n-C6 analogues. With SOPC SUVs, a similar side-chain dependent oxidation pattern was observed. Our results support and extend previous knowledge about the importance of the sterol side chain in determining sterol-sterol and sterol-phospholipid interactions, both in mono- and bilayers.