This study has compared two chemically distinct NBD-lipids with regard to their partitioning properties into lateral phases of pure and mixed cholesterol/phosphatidylcholine monolayers. Pure NBD-cholesterol (22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bi- snor-5-cholen-3-ol) which 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 (having a compressibility of 0.005 to 0.007 m/mN), although epifluorescence microscopy of the compressed NBD-cholesterol monolayer revealed that it had a solid-like surface texture. When the compressed NBD-cholesterol monoIayer was allowed to expand, it fragmented into large flakes (tens to hundreds of mu m in width) which eventually dissolved into a liquid state. The force-area isotherm of pure NBD -phosphatidylcholine (1-hexadecanoyl-2-(1 2-(7- nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecyl-sn-glycero- 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) which appeared to form when the monolayer was allowed to stabilize at this lateral surface pressure. These bright spots disappeared when the monolayer was expanded. When the surface texture of a pure cholesterol monolayer was examined, both probes (at 1 mol%) partitioned very similarly 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 cholesteroI phase. At 30 mN/m, NBD-phosghatidylcholine appeared to distribute increasingly into the cholesteror phase, whereas NBD-cholesterol still did not to mix with choIesterol. The characteristic surface texture of the liquid-expanded to liquid-condensed lateral phase transition of pure dipalmitoyl phosphatidylcholine (DPPC) monolayers could be visualized identically with both probes, indicating that these were similarly excluded from the liquid-condensed solid phase of DPPC. Finally, in mixed monorayers containing cholesterol and DPPC (molar ratio 33:67), both probes (at 1 mol%) revealed a similar surface texture of the monolayers (examined at a lateral surface pressure of 0.5 mN/m), suggesting that these partitioned similarly between the different lateral phases present in the mixed monolayer. In conclusion, although the two NBD-probes differed from each other in chemical and physical properties, both acted like 'impurities' when admired into pure or mixed monolayers, and appeared to be equally excluded from lateral phases in which the packing density was high.
|Number of pages||8|
|Journal||BIOCHIMICA ET BIOPHYSICA ACTA-LIPIDS AND LIPID METABOLISM|
|Publication status||Published - 1995|
|MoE publication type||A1 Journal article-refereed|
- EPIFLUORESCENCE MICROSCOPY
- LATERAL PHASE