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In comparing the electric field in gel electrophoresis to that of a parallel plate capacitor (PPC), what would be the difference between them? What I mean is, PPCs are known for their constant fields. Gel electrophoresis is accomplished by placing two electrodes into a buffer that contains a gel, which contains usually DNA.

Does the presence of the buffer medium complete the circuit in a "normal" sense, acting as a resistor of sorts. Or does it act as a medium between two plates, making it a weird PPC? The reason I ask is because I am having a hard time visualizing the path of the current through the liquid. If it really is the former, would the current take chaotic paths? The paths of particles in the medium are of course changing.

Electrophoresis occurs because of electrophoretic velocity differences between molecules. I.e. $v=\mu E$, where $\mu$ is the Henry equation. I'm currently looking into the differences between using PPCs to generate a constant electric field, and current electrophoresis methods.

I am mostly concerned about the strength of the field. I realize the field lines will probably be chaotic. Would the strength still be a simple calculation of $E = \frac{V}{d}$, with $d$ being the distance between the two electrodes?

The dielectric molecules would probably operate very similarly in both cases, right? I think they would align with both fields regardless. That is, assuming the electrophoresis method does not cause the field to be absolutely chaotic.

I apologize if my terminology is not the greatest. This is a very new area for me. Let me know if you have questions about any of my descriptions.

A video in case anyone is unfamiliar with what electrophoresis is. It's really cool!

abyssmu
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You would be correct to consider the setup for gel electrophoresis as a capacitor, with the gel itself acting as a dielectric in the traditional sense. As you said, capacitors create an electric field between the two electrodes. This electric field causes a force on the DNA, as DNA has some charge corresponding to the number of base pairs. This electric force is larger for molecules with more charge (or more base pairs for DNA). When the DNA starts to move in the gel it experiences the viscous force, which acts in the opposite direction of motion, slowing it down. The strength of the viscous force is larger on molecules with more base pairs, because their size makes it harder for them to move through the pores in the gel.

So DNA molecules with more base pairs have a larger force pulling them forward through the gel (from the electric force) and a larger force pulling them backward (from the viscous force). So the question becomes, which one of these affects the DNA more? It turns out to be the viscous force. If the number of base pairs is $N$, the viscous force is proportional to $N^2$, while the electric force is proportional to $N$. This means that the larger molecules move slower through the gel, while smaller molecules move faster, and therefore farther.

The electric field strength itself would be calculated as you would a capacitor with a dielectric. Overall, if this is related to coursework, I would assume you could treat the electrophoresis device as a parallel plate capacitor with a dielectric.

To be clear, the current through a fully charged capacitor is zero.

Source: https://en.wikipedia.org/wiki/Electrophoresis

Maarten de Haan
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