Abstract
Microtubules are highly negatively charged proteins which have been shown to behave as bio-nanowires capable of conducting ionic currents. The electrical characteristics of microtubules are highly complicated and have been the subject of previous work; however, the impact of the ionic concentration of the buffer solution on microtubule electrical properties has often been overlooked. In this work we use the non-linear Poisson Boltzmann equation, modified to account for a variable permittivity and a Stern Layer, to calculate counterion concentration profiles as a function of the ionic concentration of the buffer. We find that for low-concentration buffers ([KCl] from 10 μM to 10 mM) the counterion concentration is largely independent of the buffer's ionic concentration, but for physiological-concentration buffers ([KCl] from 100 to 500 mM) the counterion concentration varies dramatically with changes in the buffer's ionic concentration. We then calculate the conductivity of microtubule-counterion complexes, which are found to be more conductive than the buffer when the buffer's ionic concentrations is less than ≈100 mM and less conductive otherwise. These results demonstrate the importance of accounting for the ionic concentration of the buffer when analyzing microtubule electrical properties both under laboratory and physiological conditions. We conclude by calculating the basic electrical parameters of microtubules over a range of ionic buffer concentrations applicable to nanodevice and medical applications.
Highlights
Microtubules (MTs) are cytoskeletal protein polymers of great interest in fundamental biological research and nanodevice design
Our results demonstrate that the ionic concentration of the buffer is a critical parameter and that conclusions of theoretical and experimental papers which use a particular buffer ionic concentration should only be extended to other buffers with care
Counterionic condensation around MTs is strongly dependent on the buffer ionic concentration, which can be divided into two distinct regimes: the low concentration regime (10 μM to 10 mM), and the physiological concentration regime (100– 500mM)
Summary
Microtubules (MTs) are cytoskeletal protein polymers of great interest in fundamental biological research and nanodevice design. MTs have been shown to demonstrate a number of interesting electrical properties, including long-distance propagation of ionic signals, signal amplification, electrical oscillations, and memristive responses (Priel et al, 2006; Priel and Tuszynski, 2008; Sataricet al., 2009; Sekulicet al., 2011; Sekulicand Sataric, 2012; Cantero et al, 2018, 2019; Tuszynski et al, 2020) These properties have been theorized to play an important role in biological processes, and may be leveraged for the fabrication of MT nanodevices in the near future (Van den Heuvel et al, 2006; Isozaki et al, 2015; Kalra et al, 2020a, 2021). This work revisits these approximations in order to arrive at more reliable parameter estimates under a range of ionic concentration conditions
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