(756g) Self Assembly and Solubilization Dynamics Revealed by Electrophoresis of Alkylated DNA In Dilute Micellar Solutions

Precise measurement of the time scales of monomer-micelle exchange, micelle dissolution, and solubilization is a challenging enterprise.  Standard stopped-flow and/or phosphorescence experiments do not always probe the desired processes and are tedious to implement in some cases.  Here we present a completely different approach to measuring these key kinetic parameters using end-alkylated DNA oligonucleotides (aDNA) in a form of capillary electrophoresis (CE).  aDNA attaches to most surfactant micelles readily, and the high negative charge and complete monodispersity of the aDNA oligos allows for facile analysis of the peak sharpness in CE electropherograms.  Under appropriate conditions, peak sharpness reveals the exchange dynamics of nonionic and anionic surfactant monomers.

Initially developed as a rapid means of gel-free electrophoretic DNA separation, the “micelle-ELFSE” method developed in our lab aDNA introduced into a capillary pre-filled with a running buffer containing the micellar solution of interest.  On application of the electric field, aDNA migrate through the capillary, interacting with micelles that bind to the aDNA and retard its mobility.  The amount of this retardation is established by the aggregation number of the micelle, fluctuating during the run as dictated by monomer exchange kinetics.  As a result, sharp peaks will only be observed if the micelles, attached to aDNA, are given sufficient time to statistically sample all possible aggregation numbers.  Hence, by varying the total run time and measuring the resulting peak sharpness, one can obtain a measure of monomer exchange dynamics.

Here we present a moment-type analysis of CE-derived peak sharpness in the micelle-ELFSE system, accounting for all sources of peak broadening.  These include diffusion of aDNA, diffusion of aDNA-micelle complexes, injection, detection, wall adsorption, and the monomer exchange process.  We then present a means to identify run conditions that prove monomer exchange dynamics.  We apply the method to analyzing a wide range of CiEj-type nonionic surfactants and mixtures thereof, at various temperatures and in the presence of urea.  We find that the exchange timescales are less than one millisecond and are extremely sensitive to temperature and urea content of the running buffers.  The results are generalized by comparison to the cloud point and initial results using anionic surfactants will also be discussed.