Ge M, Freed J H
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, USA.
Biophys J. 1999 Jan;76(1 Pt 1):264-80. doi: 10.1016/S0006-3495(99)77195-X.
The effect of aggregation of gramicidin A' (GA) on the phase structure of dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles was studied by cw-ESR using a chain-labeled lipid (16PC) at temperatures between 30 degrees and 45 degreesC that span the main phase transition of DPPC. Boundary lipids were observed only in dispersions with GA/DPPC molar ratios >1:15, where GA aggregates. Detailed fits by nonlinear least squares (NLLS) methods are consistent with the boundary lipid being characterized by a large negative order parameter ( approximately -0.4), indicative of a dynamic bending of the end of the acyl chain, and a substantially reduced motion, about an order of magnitude slower than that of the bulk lipid. The NLLS analysis compares favorably with a recent two-dimensional Fourier transform ESR study on DPPC/GA vesicles, which accurately discerned the bulk lipid. The detailed ESR observables are discussed in terms of the ordering effect of GA at low concentration of GA, the dissociation of the GA channel and the dynamic bending of the end chain segment of boundary lipid at high concentration of GA, and of HII phase formation induced by GA. It is suggested that these phenomena can be interpreted in terms of the combined effects of partial dehydration of the lipid headgroup by the GA and of the hydrophobic mismatch between GA and DPPC molecules. Substantial hysteresis is observed for heating versus cooling cycles, but only for a GA/DPPC molar ratio >1:15. This is consistent with the aggregation of GA molecules at high concentrations.
通过连续波电子自旋共振（cw-ESR），使用链标记脂质（16PC），在30℃至45℃的温度范围内研究了短杆菌肽A'（GA）聚集对二棕榈酰磷脂酰胆碱（DPPC）多层囊泡相结构的影响，该温度范围跨越了DPPC的主要相变。仅在GA/DPPC摩尔比>1:15的分散体中观察到边界脂质，此时GA会发生聚集。通过非线性最小二乘法（NLLS）进行的详细拟合结果表明，边界脂质的特征是具有较大的负序参数（约为-0.4），这表明酰基链末端存在动态弯曲，并且其运动显著减慢，比主体脂质慢约一个数量级。NLLS分析与最近一项关于DPPC/GA囊泡的二维傅里叶变换ESR研究结果相符，该研究准确地识别出了主体脂质。根据GA在低浓度时的有序化效应、GA通道的解离、GA高浓度时边界脂质末端链段的动态弯曲以及GA诱导的HII相形成，对详细的ESR观测结果进行了讨论。有人认为，这些现象可以通过GA对脂质头部基团的部分脱水作用以及GA与DPPC分子之间的疏水不匹配的综合效应来解释。对于加热和冷却循环，仅在GA/DPPC摩尔比>1:15时观察到明显的滞后现象。这与高浓度下GA分子的聚集现象一致。
