Electrophoresis

Electrophoresis. It is a fundamental analytical chemistry technique used to separate charged particles, such as DNA, RNA, proteins, or cells, within a supporting medium under the influence of an applied electric field. The separation is based on differences in the particles' charge, hydrodynamic radius, and molecular mass, allowing scientists to analyze complex biological mixtures. The method is a cornerstone in fields like molecular biology, biochemistry, and forensic science, enabling critical analyses from genetic fingerprinting to protein purification.

Principles and theory

The core principle relies on the Coulomb's law attraction between charged molecules and electrodes of opposite polarity within an electrical circuit. When a voltage is applied via a power supply across a medium, often a buffer solution, charged analytes migrate toward the electrode of opposite charge. The rate of this electrophoretic mobility is governed by the Stokes' law relationship, where the net force equals the electric field strength multiplied by the net ionic charge. Key factors influencing separation include the pH of the buffer, which affects the ionization state of molecules like amino acids, the ionic strength of the medium, and the frictional force exerted by the gel matrix. The Debye-Hückel theory helps describe the behavior of electrolytes in such solutions. For macromolecules like DNA, which carries a uniform negative charge due to its phosphate backbone, separation in a porous gel acts essentially as molecular sieving, where smaller fragments move faster.

Types and techniques

Numerous specialized forms have been developed for different applications. Gel electrophoresis is the most common, primarily using agarose for separating large nucleic acids and polyacrylamide (PAGE) for higher-resolution separation of proteins and smaller DNA fragments. SDS-PAGE, which incorporates the detergent sodium dodecyl sulfate, denatures proteins and masks their native charge, allowing separation based almost solely on molecular weight. Capillary electrophoresis (CE) performs separations inside a narrow fused silica capillary, offering high speed and automation, and is often coupled with detection methods like laser-induced fluorescence. Two-dimensional electrophoresis, combining isoelectric focusing with SDS-PAGE, provides powerful resolution for complex proteomics samples. Other variants include pulsed-field gel electrophoresis (PFGE) for separating very large chromosomal DNA, immunoelectrophoresis for detecting antigen-antibody complexes, and zymography for detecting enzyme activity.

Applications

The technique is indispensable in modern bioscience and medicine. In genetics, it is used for DNA fingerprinting in forensic science, famously applied in cases like the O.J. Simpson trial, and for analyzing PCR products. The Human Genome Project relied heavily on capillary electrophoresis for DNA sequencing. In clinical pathology, it diagnoses conditions like sickle cell disease and multiple myeloma by analyzing hemoglobin or serum protein patterns. Pharmaceutical companies use it for quality control of biologics like insulin and monoclonal antibodies. In microbiology, pulsed-field gel electrophoresis is a standard tool for bacterial typing during outbreak investigations by agencies like the Centers for Disease Control and Prevention. Research applications range from studying gene expression via Northern blotting to analyzing post-translational modifications of proteins in cancer research.

History and development

Early observations of electrophoretic phenomena date to the early 19th century with experiments by Ferdinand Frederic Reuss at Moscow State University. The foundational work was established in the 1930s by Arne Tiselius of the Uppsala University, who developed moving boundary electrophoresis in free solution to separate serum proteins, for which he received the Nobel Prize in Chemistry in 1948. The introduction of stabilizing media like filter paper and, later, starch gels by Oliver Smithies and agarose gels in the 1960s, revolutionized the technique by preventing convection currents. The development of polyacrylamide gel electrophoresis (PAGE) by teams including Leonard Ornstein and Barry J. Davis provided vastly improved resolution. The late 20th century saw the automation and miniaturization of the method with the invention of capillary electrophoresis by James W. Jorgenson, and its subsequent integration into the DNA sequencers manufactured by Applied Biosystems that enabled rapid genome sequencing.

Instrumentation and components

A standard setup consists of several key components. A high-voltage power supply (e.g., from Bio-Rad or Thermo Fisher Scientific) provides the stable direct current electric field. The separation occurs in a chamber or cell, often made of polycarbonate, which holds the gel matrix and buffer reservoir. For gel electrophoresis, a casting tray and comb are used to form sample wells in gels typically composed of agarose or polyacrylamide. Capillary electrophoresis systems employ a fused silica capillary, a autosampler, and sophisticated detection systems such as UV-Vis absorbance, laser-induced fluorescence, or mass spectrometry interfaces. Staining and visualization require dyes like ethidium bromide, SYBR Green, Coomassie Brilliant Blue, or silver stain, followed by imaging on a gel documentation system or fluorometer. Advanced systems may include temperature control units and computer software for data analysis.

Category:Laboratory techniques Category:Analytical chemistry Category:Separation processes