# Proteomics/Protein Separations- Electrophoresis

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## Definitions

e•lec•tro•pho•re•sis (ĭ-lĕk\'trō-fə-rē\'sĭs) n.
\
1) The migration of charged colloidal particles or molecules through a
solution under the influence of an applied electric field usually
provided by immersed electrodes. Also called cataphoresis.\
2) A method of separating substances, especially proteins, and analyzing
molecular structure based on the rate of movement of each component in a
colloidal suspension while under the influence of an electric field.

\
an•a•lyte (a-nə-līt) n. \
A chemical substance that is the subject of chemical analysis.

!Separation under
electrophoresis

## Electrophoresis Theory

Separation by electrophoresis depends on differences in the migration
velocity of ions or solutes through a given medium in an electric field.
The electrophoretic migration velocity of an analyte is:

`<big>`{=html} $v_p = \mu_p E$ `</big>`{=html}

where E is the electric field strength and $\mu_p$ is the
electrophoretic mobility.

\
The electrophoretic mobility is inversely proportional to frictional
forces in the buffer, and directly proportional to the ionic charge of
the analyte. The forces of friction against an analyte are dependent on
the analyte\'s size and the viscosity (η) of the medium. Analytes with
different frictional forces or different charges will separate from one
another when they move through a buffer. At a given pH, the
electrophoretic mobility of an analyte is:

$\mu_p = \frac{z}{6\pi \eta r}$

where r is the radius of the analyte and z is the net charge of the
analyte.

\
Differences in the charge to size ratio of analytes causes differences
in electrophoretic mobility. Small, highly charged analytes have greater
mobility, whereas large, less charged analytes have lower mobility.
Anionic surfactants are often used to denature proteins before
electrophoresis. This gives all proteins approximately the same charge.
Since their charges are all equal their mobility is now a function of
only their masses. Electrophoretic mobility is an indication of an
analyte\'s migration velocity in a given medium. The net force acting on
an analyte is the balance of two forces: the electrical force acting in
favor of motion, and the frictional force acting against motion. These
two forces remain steady during electrophoresis. Therefore,
electrophoretic mobility is a constant for a given analyte under a given
set of conditions.

## Applications of Electrophoresis

Electrophoresis has a wide variety of applications in proteomics,
forensics, molecular biology, genetics, biochemistry, and microbiology.

One of the most common uses of electrophoresis is to analyze
differential expression of genes. Healthy and diseased cells can be
identified by differences in the electrophoretic patterns of their
proteins. Proteins themselves can also be characterized in this way, and
some sense of their structure can be derived from the masses of
fragments inside the gel. 

There are many different types of electrophoresis, and each can be used
for something different. Two-dimensional (2-D) electrophoresis, for
example, has the ability to discern many more proteins than most of its
contemporaries. Many of these methods will be discussed in detail
throughout this chapter.

## References

1.      The American Heritage Dictionary of the English Language, Fourth
    Edition. <http://www.bartleby.com/61/>

2.      The Merriam-Webster Online Dictionary. <http://www.m-w.com>

3.      Mans, Andreas et al. _Bioanalytical
    Chemistry._ Imperial College Press, 2004.

4.      Twyman, Richard. \"Two-dimensional polyacrylamide gel
    electrophoresis.\" <http://genome.wellcome.ac.uk/doc_wtd021045.html>

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