Partitioning of aryl radicals in micellar systems

Abstract

Determining the association constants of radicals to biomimetic systems is not a simple task because of the inherent experimental difficulties associated to their “in‐situ” generation together with their high chemical instability, requiring the use of radical trapping agents in combination with, for example, magnetic (eg, EPR) and/or time‐resolved techniques (eg, frequency comb spectroscopy) to indentify and quantify them. Here, we have exploited the unique electrochemical properties of arenediazonium ions, ArN2+, to estimate the association constant of the electrochemically generated aryl radicals derived from benzenediazonium ions, BD, with sodium dodecyl sulfate (SDS) micelles. When ArN2+ are reduced on an Hg electrode, they acquire an electron yielding the corresponding arenediazenyl radical, ArN2•, which undergoes further decomposition to produce the aryl radical, Ar•. BD partitions between the aqueous and micellar pseudophases, and, upon increasing [SDS], peak currents decrease and the peak potentials are shifted. Results indicate that the association constant of the aryl radical to SDS micelles is 3 times higher than that of the parent substrate as a consequence of its higher hydrophobicity compared with that of the parent molecule. The method is feasible because of the large number of arenediazonium ions that can be prepared and because it does not depend on the reactivity of the electrochemically generated radical.

Aryl Radical Selectivity in Biphasic Systems

“…To evaluate the potential applicability of aryl radicals in biological systems, two general questions can be addressed: Might typical, naturally occurring and hydrophilic aryl radical scavengers such as tyrosine units in peptides be sufficiently reactive to compete with phase transfer of even lipophilic aryl radicals? And, is the phase-transfer step significantly influenced by the presence of surfactants that generate an artificial monolayer membrane? In this work, we give insights into the phase selectivity of substituted aryl radicals in biphasic mixtures.…”