The effect of bacteriorhodopsin (BR) on the percolation properties of dimyristoylphosphatidylcholine/distearoylphosphatidylcholine bilayers was examined by studying the quenching of a lipid-bound fluorophore by a lipid-bound quencher, and by spin-spin interactions of a nitroxide-labeled lipid using electron spin resonance (ESR). At the low concentrations of BR used, differential scanning calorimetry showed that although the transition enthalpy was reduced in a concentration-dependent manner by incorporation of BR, the solidus and fluidus phase boundaries and overall shape of the heat capacity profiles were essentially unchanged. However, fluorescence quenching and spin-label ESR data showed that the domain topology, as reflected in the percolation properties, is strongly affected by the protein. In contrast to our previous fluorescence data for the pure lipid mixtures, quenching in the coexistence region is independent of the fluid phase fraction when BR is present. In addition, the percolation threshold estimated by spin-label ESR is shifted in the presence of BR to a higher gel phase fraction at a given lipid composition. Both the fluorescence quenching and spin-label ESR data, together with the results of earlier simulations, strongly suggest that the fluid phase domains are substantially larger and/or less ramified in the presence of BR than in its absence. We have previously reported a similar effect of a transmembrane peptide, pOmpA (Escherichia coli outer membrane protein A signal peptide), on fluid domain connectivity in binary phosphatidylcholine mixtures.

译文

通过研究脂质结合的猝灭剂对脂质结合的荧光团的猝灭以及通过自旋-自旋相互作用,研究了细菌视紫红质 (BR) 对二肉豆蔻酰磷脂酰胆碱/二硬脂酰磷脂酰胆碱双层的渗透特性的影响。使用电子自旋共振 (ESR)。在使用低浓度的BR时,差示扫描量热法表明,尽管通过掺入BR以浓度依赖的方式降低了过渡焓,但固相线和流相界以及热容分布的整体形状基本上没有变化。然而,荧光猝灭和自旋标记ESR数据表明,渗透特性所反映的结构域拓扑受到蛋白质的强烈影响。与我们先前的纯脂质混合物的荧光数据相反,存在BR时,共存区域的猝灭与液相分数无关。此外,在给定的脂质组成下,在BR的存在下,由自旋标记ESR估计的渗滤阈值被转移到更高的凝胶相分数。荧光猝灭和自旋标记ESR数据以及较早的模拟结果都强烈表明,在存在BR的情况下,流体相域比不存在BR的情况下要大得多和/或分支更少。我们以前曾报道过跨膜肽pOmpA (大肠杆菌外膜蛋白a信号肽) 对二元磷脂酰胆碱混合物中流体结构域连通性的类似作用。

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