Counting ions and other nucleophiles at surfaces by chemical trapping
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REVIEW
Counting ions and other nucleophiles at surfaces by chemical trapping Iolanda Midea Cuccovia 1 & Filipe da Silva Lima 2 & Hernan Chaimovich 1
Received: 16 June 2017 / Accepted: 27 July 2017 / Published online: 29 August 2017 # International Union for Pure and Applied Biophysics (IUPAB) and Springer-Verlag GmbH Germany 2017
Abstract 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 concentrationdependent properties, rather than concentration measurements. Dediazoniation, i.e. the loss of N2, 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 This article is part of a Special Issue on ‘Latin America’ edited by Pietro Ciancaglini and Rosangela Itri. * Iolanda Midea Cuccovia [email protected]
1
Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, Brazil
2
Departamento de Química Fundamental, Centro de Ciências Exatas e da Natureza, Universidade Federal de Pernambuco, Recife, Brazil
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 det
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