VISUALIZATION OF LATERAL PHASES IN CHOLESTEROL AND PHOSPHATIDYLCHOLINE MONOLAYERS AT THE AIR WATER INTERFACE - A COMPARATIVE-STUDY WITH 2 DIFFERENT REPORTER MOLECULES/

This study has compared two chemically distinct NBD-lipids with regard to their partitioning properties into lateral phases of pure and mixe d cholesterol/phosphatidylcholine monolayers. Pure NBD-cholesterol 7-n itrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bi- snor-5-cholen-3-ol) wh ich has the NBD-function in the sterol side chain (at carbon 22), gave a liquid-expanded force-area isotherm on water at 22 degrees C (havin g a compressibility of 0.005 to 0.007 m/mN), although epifluorescence microscopy of the compressed NBD-cholesterol monolayer revealed that i t had a solid-like surface texture. When the compressed NBD-cholestero l monoIayer was allowed to expand, it fragmented into large flakes (te ns to hundreds of mu m in width) which eventually dissolved into a liq uid state. The force-area isotherm of pure NBD -phosphatidylcholine (1 -hexadecanoyl-2-(1 2-(7- nz-2-oxa-1,3-diazol-4-yl)amino)dodecyl-sn-gly cero- phosphocholine) was also liquid-expanded. When a compressed (30 mN/m) monolayer of NBD-phosphatidylcholine was examined by microscopy, it displayed many bright crystalline spots (about 50 mu m across) whi ch appeared to form when the monolayer was allowed to stabilize at thi s lateral surface pressure. These bright spots disappeared when the mo nolayer was expanded. When the surface texture of a pure cholesterol m onolayer was examined, both probes (at 1 mol%) partitioned very simila rly in the sterol monolayer. At low lateral surface pressures (1 and 5 mN/m) the probes appeared to be excluded from the cholesteror phase, forming very bright liquid-like areas against a uniformly black choles teroI phase. At 30 mN/m, NBD-phosghatidylcholine appeared to distribut e increasingly into the cholesteror phase, whereas NBD-cholesterol sti ll did not to mix with choIesterol. The characteristic surface texture of the liquid-expanded to liquid-condensed lateral phase transition o f pure dipalmitoyl phosphatidylcholine (DPPC) monolayers could be visu alized identically with both probes, indicating that these were simila rly excluded from the liquid-condensed solid phase of DPPC. Finally, i n mixed monorayers containing cholesterol and DPPC (molar ratio 33:67) , both probes (at 1 mol%) revealed a similar surface texture of the mo nolayers (examined at a lateral surface pressure of 0.5 mN/m), suggest ing that these partitioned similarly between the different lateral pha ses present in the mixed monolayer. In conclusion, although the two NB D-probes differed from each other in chemical and physical properties, both acted like 'impurities' when admired into pure or mixed monolaye rs, and appeared to be equally excluded from lateral phases in which t he packing density was high.