| Impedance spectroscopy is a technique that reveals information, such as macromolecular charges, dipole moments, and dielectric properties, about protein suspensions and other materials. We performed impedance measurements of saline solutions and alpha-beta tubulin heterodimers in buffer solutions over the frequency range 1 Hz-1 MHz. These and other polyelectrolyte suspensions show enormous dielectric responses at low frequencies, due 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 and a microscopic mechanism based on the motion of the charged particles close to the electrode to account for the electrode polarization effects. For tubulin suspensions we observe that, 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/A for the assembled microtubules. These results are potentially important for fundamental electrostatic processes in biomolecules and suggest the possibility of developing future bio-electronic applications. |
