Petrov Jordan G, Andreeva Tonya D, Kurth Dirk G, Möhwald Helmuth
Max-Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, D-14476 Golm/Potsdam, Germany.
J Phys Chem B. 2005 Jul 28;109(29):14102-11. doi: 10.1021/jp0515028.
The dipole potential, affecting the structure, functions, and interactions of biomembranes, lipid bilayers, and Langmuir monolayers, is positive toward the hydrocarbon moieties. We show that uncharged Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) exhibit large negative dipole potentials, while the nonfluorinated docosyl ethyl ether (DEE) forms films with positive dipole potentials. Comparison of the Delta V values for these ethers with those of the previously studied(37-39) monolayers of trifluoroethyl ester (TFEB) and ethyl ester of behenic acid (EB) shows that the reversal of the sign of Delta V causes the same change Delta(Delta V) = -706 +/- 16 mV due to fluorination of heads. The Delta V values of both TFEB and EB films differ by -122 +/- 16 mV from those of DFEE and DEE monolayers, respectively, with the same density. Such quantitative coincidence points to a common mechanism of reversal of the sign of the dipole potential for the ether and ester films despite the different structure of their heads. The mechanical properties and phase behaviors of these monolayers show that both fluorinated heads are less hydrated, suggesting that the change of the sign of Delta V could, at least partially, be related to different hydration water structure. The same negative contribution of the carbonyl bond in both TFEB and EB films contrasts with the generally accepted positive contribution of the C(delta+)=O(delta-) bond in condensed Langmuir monolayers of fatty acids, their alcohol esters, glycerides, and phospholipids but concurs with the theoretical analysis of Delta V of stearic acid monolayers. Both results question the literature values of the molecular dipole moments of these substances calculated via summation of bonds and atomic group contributions. Mixed monolayers of DFEE and DEE show smooth monotonic variation of Delta V from +450 to -235 mV, indicating a way for adjustment of the sign and magnitude of the dipole potential at the membrane-water boundary and regulation of such membrane behaviors as binding and translocation rate of hydrophobic ions and ion-carriers, adsorption and penetration of amphiphilic peptides, polarization of hydration water, and short-range repulsion. The interaction of the hydrophobic ions tetraphenylboron TPhB- and tetraphenylphosphonium TPhP+ with DFEE and DEE monolayers qualitatively follows the theory of binding of such ions to lipid bilayers, but the shifts Delta(Delta V) from the values obtained on water are much smaller than those for DPPC monolayers. This difference seems to be due to the solid (polycrystalline) character of the DFEE and DEE films that hampers the penetration of TPhB- and TPhP+ in the monolayers and reduces the attractive interaction with the hydrophobic moiety. This conclusion orients the future synthesis of amphiphiles with fluorinated heads to those which could form liquid-expanded Langmuir monolayers.
偶极电势对生物膜、脂质双层和朗缪尔单层膜的结构、功能及相互作用产生影响，其对烃基部分呈正值。我们发现，二十二烷基三氟乙基醚（DFEE）的不带电朗缪尔单层膜呈现出较大的负偶极电势，而未氟化的二十二烷基乙醚（DEE）形成的膜具有正偶极电势。将这些醚类的ΔV值与先前研究的（37 - 39）三氟乙酯（TFEB）和山嵛酸乙酯（EB）的单层膜的ΔV值进行比较，结果表明，由于头部的氟化作用，ΔV符号的反转导致相同的变化Δ(ΔV) = -706 ± 16 mV。在相同密度下，TFEB和EB膜的ΔV值分别与DFEE和DEE单层膜的ΔV值相差 -122 ± 16 mV。这种定量上的一致性表明，尽管醚类和酯类膜头部结构不同，但它们偶极电势符号反转的机制相同。这些单层膜的力学性质和相行为表明，两种氟化头部的水合程度较低，这表明ΔV符号的变化至少部分可能与不同的水合水结构有关。TFEB和EB膜中羰基键的相同负贡献与脂肪酸、其醇酯、甘油酯和磷脂的凝聚朗缪尔单层膜中普遍接受的C(δ+) = O(δ-)键的正贡献形成对比，但与硬脂酸单层膜的ΔV理论分析结果一致。这两个结果都对通过键和原子基团贡献求和计算得到的这些物质的分子偶极矩的文献值提出了质疑。DFEE和DEE的混合单层膜显示出ΔV从 +450到 -235 mV的平滑单调变化，这表明在膜 - 水界面处调整偶极电势的符号和大小以及调节诸如疏水离子和离子载体的结合与转运速率、两亲性肽的吸附与渗透、水合水的极化和短程排斥等膜行为的一种方式。疏水离子四苯基硼（TPhB-）和四苯基鏻（TPhP+）与DFEE和DEE单层膜的相互作用在定性上遵循此类离子与脂质双层结合的理论，但与在水上获得的值相比，Δ(ΔV)的变化要小得多，比二棕榈酰磷脂酰胆碱（DPPC）单层膜的变化小得多。这种差异似乎是由于DFEE和DEE膜的固态（多晶）特性阻碍了TPhB-和TPhP+在单层膜中的渗透，并降低了与疏水部分的吸引相互作用。这一结论为未来合成具有氟化头部的两亲分子指明了方向，即合成能够形成液体扩张朗缪尔单层膜的两亲分子。
