(713e) AC-Polarization and Conformational Transition of Single Weak Polyelectrolyte in Uniform AC-Electric Fields

Despite the emerging interest in employing AC-electrokinetics to effectively manipulate and assemble supramolecular aggregates, such as AC-field induced protein crystallization and DNA hybridization, the detailed AC-polarization mechanism of biomacromolecules remains little understood. In this work, we examine the conformational dynamics of synthetic polyelectrolytes under AC-electric fields of varied frequencies (10-1000 kHz) and amplitudes (1-20 V) by using fluorescence correlation spectroscopy (FCS) at a single molecular level. We focus on poly(vinyl pyridine) (PVP) as a model weak polyelectrolyte whose coil-to-globule conformation in the bulk solution can be tuned by adjusting the pH and ionic strength. To avoid the conformational change due to the imposed dielectrophoretic (DEP) force on PVP in a nonuniform AC-field, PVP is placed between two parallel quartz plates coated with ITO to achieve a uniform AC-field, where the PVP concentration is kept constant without polymer assembly at varied AC-frequencies across two plates. Surprisingly, we observe the abrupt collapse of a PVP coil to form a globule when AC-frequency is applied below 200 kHz; furthermore, the critical AC-amplitude to induce the coil-to-globule transition shows a strong correlation with critical AC-frequency and solution pH. A picture that the counterion redistribution near a PVP chain results in a cyclic compression force on the polyelectrolyte coil is accounted for the AC-induced conformational transition and further examined with varied pH and medium conductivity.