Cuccovia Iolanda Midea, da Silva Lima Filipe, Chaimovich Hernan
Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, Brazil.
Departamento de Química Fundamental, Centro de Ciências Exatas e da Natureza, Universidade Federal de Pernambuco, Recife, Brazil.
Biophys Rev. 2017 Oct;9(5):617-631. doi: 10.1007/s12551-017-0299-x. Epub 2017 Aug 29.
The interfaces of membranes and other aggregates are determined by the polarity, electrical charge, molecular volume, degrees of motional freedom and packing density of the head groups of the amphiphiles. These properties also determine the type of bound ion (ion selectivity) and its local density, i.e. concentration defined by choosing an appropriate volume element at the aggregate interface. Bulk and local ion concentrations can differ by orders of magnitude. The relationships between ion (or other compound) concentrations in the bulk solvent and in the interface are complex but, in some cases, well established. As the local ion concentration, rather than that in the bulk, controls a variety of properties of membranes, micelles, vesicles and other objects of theoretical and applied interests, measurement of local (interfacial, bound) ion concentrations is of relevance for understanding and characterizing such aggregates. Many experimental methods for estimating ion distributions between the bulk solution and the interface provide indirect estimates because they are based on concentration-dependent properties, rather than concentration measurements. Dediazoniation, i.e. the loss of N, of a substituted diazophenyl derivative provides a tool for determining the number of nucleophiles (including neutral or negatively charged ions) surrounding the diazophenyl derivative prior to the dediazoniation event. This reaction, defined as chemical trapping, and the appropriate reference points obtained in bulk solution allow direct measurements of local concentrations of a variety of nucleophiles at the surface of membranes and other aggregates. Here we review our contributions of our research group to the use, and understanding, of this method and applications of chemical trapping to the description of local concentrations of ions and other nucleophiles in micelles, reverse micelles, vesicles and solvent mixtures. Among other results, we have shown that interfacial water determines micellar shape, zwitterionic vesicle-forming amphiphiles display ion selectivity and urea does not accumulate at micellar interfaces. We have also shown that reaction products can be predicted from the composition of the initial state, even in non-ideal solvent mixtures, supporting the usefulness of chemical trapping as a method to determine local concentrations. In addition, we have analysed the mechanism of dediazoniation, both on theoretical and experimental basis, and concluded that the formation of a free phenyl cation is not a necessary part of the reaction pathway.
膜及其他聚集体的界面由两亲分子头部基团的极性、电荷、分子体积、运动自由度和堆积密度决定。这些性质还决定了结合离子的类型（离子选择性）及其局部密度，即在聚集体界面处通过选择合适的体积元定义的浓度。本体离子浓度和局部离子浓度可能相差几个数量级。本体溶剂和界面中离子（或其他化合物）浓度之间的关系很复杂，但在某些情况下已得到充分确立。由于局部离子浓度而非本体离子浓度控制着膜、胶束、囊泡以及其他理论和应用感兴趣对象的各种性质，因此测量局部（界面、结合）离子浓度对于理解和表征此类聚集体具有重要意义。许多用于估计本体溶液和界面之间离子分布的实验方法提供的是间接估计值，因为它们基于与浓度相关的性质，而非浓度测量。取代重氮苯基衍生物的去重氮化反应，即氮的损失，为确定去重氮化事件发生前围绕重氮苯基衍生物的亲核试剂（包括中性或带负电荷的离子）数量提供了一种工具。这个被定义为化学捕获的反应，以及在本体溶液中获得的合适参考点，使得能够直接测量膜和其他聚集体表面各种亲核试剂的局部浓度。在此，我们回顾我们研究小组对该方法的应用、理解以及化学捕获在描述胶束、反胶束、囊泡和溶剂混合物中离子及其他亲核试剂局部浓度方面的应用所做的贡献。在其他结果中，我们表明界面水决定胶束形状，两性离子形成囊泡的两亲分子具有离子选择性，并且尿素不会在胶束界面积累。我们还表明，即使在非理想溶剂混合物中，也可以根据初始状态的组成预测反应产物，这支持了化学捕获作为一种确定局部浓度方法的实用性。此外，我们在理论和实验基础上分析了去重氮化反应的机制，并得出结论，自由苯基阳离子的形成不是反应途径的必要组成部分。
