Impedance spectroscopy of α-β tubulin heterodimer suspensions

Abstract

Impedance spectroscopy is a technique that reveals information, such as macromolecular charges and related properties about protein suspensions and other materials. Here we report on impedance measurements over the frequency range of 1 Hz to 1 MHz of α-β tubulin heterodimers suspended in a buffer. These and other polyelectrolyte suspensions show enormous dielectric responses at low frequencies, due both to the motion of charges suspended in the medium and to an electrical double layer that forms at each electrode-medium interface. We propose an equivalent circuit model to minimize electrode polarization effects and extract the intrinsic response of the bulk medium. At megaHertz frequencies, the conductivity increases with concentration below the critical concentration of ∼1 mg/ml for microtubule polymerization, above which the conductivity decreases. This suggests that such measurements can be used to monitor the dynamics of microtubule polymerization. Finally, we obtain the net charge number per tubulin dimer of |Z| = 306 in the saline buffer, which, if maintained as the dimers polymerized, would yield a linear charge density of 3.8 e/Å for the assembled microtubules. These results are potentially important for fundamental electrostatic processes in biomolecules and suggest the possibility of developing future bioelectronic applications. © 2006 by the Biophysical Society.