Davenport L, Targowski P
Department of Chemistry, Brooklyn College of the City University of New York 11210, USA.
Biophys J. 1996 Oct;71(4):1837-52. doi: 10.1016/S0006-3495(96)79384-0.
The use of the long-lived fluorescence probe coronene (mean value of tau(FL) approximately 200 ns) is described for investigating submicrosecond lipid dynamics in DPPC model bilayer systems occurring below the lipid phase transition. Time-resolved fluorescence emission anisotropy decay profiles, measures as a function of increasing temperature toward the lipid-phase transition temperature (T(C)), for coronene-labeled DPPC small unilamellar vesicles (SUVs), are best described in most cases by three rotational decay components (phi(i = 3)). We have interpreted these data using two dynamic lipid bilayer models. In the first, a compartmental model, the long correlation time (phi(N)) is assigned to immobilized coronene molecules located in "gel-like" or highly ordered lipid phases (S-->1) of the bilayer, whereas a second fast rotational time (phi(F) approximately 2-5 ns) is associated with probes residing in more "fluid-like" regions (with corresponding lower ordering, S-->0). Interests here have focused on the origins of an intermediate correlation time (50-100 ns), the associated amplitude (beta(G)) of which increases with increasing temperature. Such behavior suggests a changing rotational environment surrounding the coronene molecules, arising from fluidization of gel lipid. The observed effective correlation time (phi(EFF)) thus reflects a discrete gel-fluid lipid exchange rate (k(FG)). A refinement of the compartmental model invokes a distribution of gel-fluid exchange rates (d(S,T)) corresponding to a distribution of lipid order parameters and is based on an adapted Landau expression for describing "gated" packing fluctuations. A total of seven parameters (five thermodynamic quantities, defined by the free energy versus temperature expansion; one gating parameter (gamma) defining a cooperative "melting" requirement; one limiting diffusion rate (or frequency factor: d(infinity))) suffice to predict complete anisotropy decay curves measured for coronene at several temperatures below the phospholipid T(C). The thermodynamic quantities are associated with the particular lipid of interest (in this case DPPC) and have been determined previously from ultrasound studies, thus representing fixed constants. Hence resolved variables are r(O), temperature-dependent gate parameters (gamma), and limiting diffusion rates (d(infinity)). This alternative distribution model is attractive because it provides a general probe-independent expression for distributed lipid fluctuation-induced probe rotational rates occurring within bilayer membranes below the phospholipid phase transition on the submicrosecond time scale.
本文描述了使用长寿命荧光探针蒄（τ(FL) 的平均值约为200纳秒）来研究二棕榈酰磷脂酰胆碱（DPPC）模型双层系统中低于脂质相变温度时亚微秒级的脂质动力学。对于蒄标记的DPPC小单层囊泡（SUV），随着温度升高至脂质相变温度（T(C)），时间分辨荧光发射各向异性衰减曲线在大多数情况下最好用三个旋转衰减分量（i = 3时的φ）来描述。我们使用两种动态脂质双层模型来解释这些数据。在第一种模型，即区室模型中，长相关时间（φ(N)）被赋予位于双层“凝胶状”或高度有序脂质相（S→1）中的固定化蒄分子，而第二个快速旋转时间（φ(F) 约为2 - 5纳秒）与位于更“流体状”区域（相应有序度较低，S→0）的探针相关。这里的研究重点是中间相关时间（50 - 100纳秒）的起源，其相关振幅（β(G)）随温度升高而增加。这种行为表明围绕蒄分子的旋转环境发生变化，这是由凝胶脂质的流化引起的。因此，观察到的有效相关时间（φ(EFF)）反映了离散的凝胶 - 流体脂质交换速率（k(FG)）。区室模型的改进引入了对应脂质有序参数分布的凝胶 - 流体交换速率分布（d(S,T)），并且基于用于描述“门控”堆积涨落的适配朗道表达式。总共七个参数（五个热力学量，由自由能与温度展开式定义；一个定义协同“熔化”要求的门控参数（γ）；一个极限扩散速率（或频率因子：d(∞)））足以预测在磷脂T(C) 以下几个温度下测量的蒄的完整各向异性衰减曲线。这些热力学量与所关注的特定脂质（在这种情况下为DPPC）相关，并且先前已通过超声研究确定，因此代表固定常数。因此，可解析变量为r(O)、温度依赖性门控参数（γ）和极限扩散速率（d(∞)）。这种替代分布模型很有吸引力，因为它为磷脂相变以下双层膜内亚微秒时间尺度上由分布的脂质涨落引起的探针旋转速率提供了一个与探针无关的通用表达式。
