  Microtubules are hollow cylindrical polymers composed of the highly
negatively-charged (~23e), high dipole moment (1750 D) protein a,b-tubulin.
While the roles of microtubules in chromosomal segregation, macromolecular
transport and cell migration are relatively well-understood, studies on the
electrical properties of microtubules have only recently gained strong
interest. Here, we show that while microtubules at physiological concentrations
increase solution capacitance, free tubulin has no appreciable effect. For a
particular microtubule concentration, we were able to quantify these effects by
determining the capacitance and resistance of a single 20 um-long microtubule
to be 1.86 x 10^(-12) F and 1.07 x 10^12 Ohms respectively. Further, we
observed a decrease in electrical resistance of solution, with charge transport
peaking between 20-60 Hz in the presence of microtubules, consistent with
recent findings that microtubules exhibit electric oscillations at such low
frequencies. Our results show that in addition to macromolecular transport,
microtubules also act as charge-storage devices through counterionic
condensation across a broad frequency spectrum. We conclude with a hypothesis
of an electrically-tunable cytoskeleton where the dielectric properties of
tubulin are polymerization-state dependent.
