Ionization, partitioning, and dynamics of tryptophan octyl ester: Implications for membrane-bound tryptophan residues
Ionization, partitioning, and dynamics of tryptophan octyl ester: Implications for membrane-bound tryptophan residues
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DOI：
10.1016/s0006-3495(97)78116-5
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发表时间：
1997-08-01
影响因子：
3.4
通讯作者：
Sudha, S
中科院分区：
文献类型：
--
作者：
Chattopadhyay, A;Mukherjee, S;Sudha, S
色氨酸残基作为固有的荧光团存在于大多数蛋白质中，使它们成为这类蛋白质荧光光谱分析的明显选择。据报道，膜蛋白的色氨酸含量明显高于可溶性蛋白。色氨酸残基在膜蛋白结构和功能中的作用引起了人们的广泛关注。膜蛋白和多肽中的色氨酸残基被认为是向界面区域不对称分布的。色氨酸辛基酯（TOE）是膜结合色氨酸残基的重要模型。我们已经描述了这个分子作为荧光膜探针的电离，分配和运动特性的单层囊泡二酰基磷脂酰胆碱。利用其ph依赖性荧光特性研究了该分子在模型膜中的电离特性。对ph依赖性荧光强度和发射最大值的分析表明，在膜中，TOE的α -氨基发生去质子化，表观pK(a)接近7.5。膜结合TOE的荧光寿命也表现出pH依赖性。利用色氨酸荧光衰减的旋转体模型解释了TOE的荧光寿命。测定了质子化和去质子化形态下TOE的膜/水分配系数。其配分行为与电离无明显差异。分析了TOE的荧光极化随pH值的变化，结果表明，随着pH值的增加，极化减小，这意味着去质子化的旋转自由度增大。这进一步得到了ph依赖的红边激发位移和膜结合TOE的表观旋转相关时间的支持。TOE在监测色氨酸残基融入膜的组织和动力学方面应该是有用的。
The presence of tryptophan residues as intrinsic fluorophores in most proteins makes them an obvious choice for fluorescence spectroscopic analyses of such proteins. Membrane proteins have been reported to have a significantly higher tryptophan content than soluble proteins. The role of tryptophan residues in the structure and function of membrane proteins has attracted a lot of attention. Tryptophan residues in membrane proteins and peptides are believed to be distributed asymmetrically toward the interfacial region. Tryptophan octyl ester (TOE) is an important model for membrane-bound tryptophan residues. We have characterized this molecule as a fluorescent membrane probe in terms of its ionization, partitioning, and motional characteristics in unilamellar vesicles of dioleoylphosphatidylcholine. The ionization property of this molecule in model membranes has been studied by utilizing its pH-dependent fluorescence characteristics. Analysis of pH-dependent fluorescence intensity and emission maximum shows that deprotonation of the alpha-amino group of TOE occurs with an apparent pK(a) of similar to 7.5 in the membrane. The fluorescence lifetime of membrane-bound TOE also shows pH dependence. The fluorescence lifetimes of TOE have been interpreted by using the rotamer model for the fluorescence decay of tryptophan. Membrane/water partition coefficients of TOE were measured in both its protonated and deprotonated forms. No appreciable difference was found in its partitioning behavior with ionization. Analysis of fluorescence polarization of TOE as a function of pH showed that there is a decrease in polarization with increasing pH, implying more rotational freedom on deprotonation. This is further supported by pH-dependent red edge excitation shift and the apparent rotational correlation time of membrane-bound TOE. TOE should prove useful in monitoring the organization and dynamics of tryptophan residues incorporated into membranes.