Ubiquinone (UQn with n = 2, 3, or 10 isoprenoid groups) was incorporated into small, sonicated vesicles made of dipalmitoylphosphatidylcholine (DPPC) or dimyristoylphosphatidylcholine (DMPC). (1) The accessibility of oxidized UQ in DPPC or DMPC vesicles to the reductant sodium borohydride (NaBH4), measured by UV spectroscopy, was UQ2 > UQ3 > UQ10 (DPPC) and UQ2 > UQ3 ~ UQ10 (DMPC). (2) Catalysis of the reduction of entrapped ferricyanide by exogenous NaBH4 was more effective with UQ2 than UQ10 but was slower with all quinones than reduction by added dithionite. (3) The methoxy protons of UQ2 and UQ3 in DPPC and DMPC vesicles exhibited a single NMR resonance centered at ~3.95 ppm, whereas the methoxy groups of UQ10 gave rise to two separate proton resonances, at 3.93 ppm and a more narrow resonance at 3.78 ppm. The UQ10 population characterized by the 3.78 ppm resonance was present at a higher concentration in DPPC than in DMPC vesicles and was relatively insensitive to reduction by NaBH4. (4) UQio perturbed the melting temperature (7m) of DPPC vesicles to a smaller extent (Arm = -1 °C) than did UQ2 and UQ3 (ATm = -3 to -4 °C). The combined UV and NMR data imply the following: The UQio pool characterized by the 3.78 ppm peak corresponds to a more mobile UQ10 fraction that is not reduced by NaBH4 in 2-3 min and is thought to be localized close to the center of the DPPC bilayer since it has little effect on the DPPC Tm. The population of UQ10 in DPPC vesicles corresponding to the 3.93 ppm peak would then correspond to the UQ10 pool that is reduced slowly by NaBH4 and is located in a bilayer environment magnetically similar to that of UQ2 and UQ3. UQ2 appears to be positioned near the bilayer surface because of its accessibility to NaBH4 and relatively large effect on the lipid Tm. The distribution of UQ3 according to the kinetic data is intermediate between that of UQ2 and the UQ10 pool near the bilayer center, although the magnetic environment of UQ3 appears the same as that of UQ2. Physiological activities often require quinones with long isoprenoid chains. The present data argue against transmembrane "flip-flop" of long-chain quinones. It is proposed that one essential property of the long-chain quinones for transfer of electrons and protons across the bilayer is their residence in the hydrophobic core.
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