SDS-PAGE and Gel Electrophoresis

Introduction to Gel Electrophoresis

Gel electrophoresis is a fundamental analytical technique used to separate biomolecules, such as proteins and nucleic acids, based on their size and charge. In analytical chemistry, it is widely applied for the determination of molecular weight, purity, and concentration of proteins.

• Definition: Gel electrophoresis is the migration of charged molecules through a gel matrix under the influence of an electric field.

• Applications: Determination of unknown molecular weights (MW), quantification of protein concentrations, and assessment of sample purity.

• Example: SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis) is commonly used for protein analysis.

Sample Loading and Gel Structure

Samples are introduced into wells at the top of the gel, which are formed by inserting a comb into the liquid gel before it solidifies. The gel is placed in a plastic casing and connected to an electric field.

• Sample Buffer: Contains glycerol to increase density and viscosity, ensuring samples sink into wells.

• Multiple Samples: Several samples can be run simultaneously without interference.

• Migration: Protein fragments move through the gel towards the anode.

Separation Principle of Gel Electrophoresis

The separation of analytes in gel electrophoresis is based on a molecular sieve effect, which depends on the pore size distribution of the gel matrix.

• Pore Size Effect: Smaller analytes (low MW) migrate faster than larger analytes (high MW).

• Direction: Migration occurs from the wells towards the anode under the influence of an electric field.

• Gel Matrix: Common matrices include agarose and polyacrylamide, which form three-dimensional networks with adjustable pore sizes.

SDS-PAGE: Sample Preparation and Principle

SDS-PAGE is a specialized form of gel electrophoresis for proteins, involving denaturation and derivatization to ensure uniform charge-to-mass ratio.

• Denaturation: Proteins are treated with SDS (anionic surfactant) and reducing agents (e.g., β-mercaptoethanol or dithiothreitol) at high temperature (95°C).

• SDS Binding: SDS binds to proteins, disrupting non-covalent interactions and unfolding the protein structure. Approximately 1.4 g SDS binds per g protein, resulting in a constant charge-to-mass ratio.

• Reducing Agents: Cleave disulfide bridges, converting them to thiol groups and further denaturing the protein.

• Result: All proteins acquire a similar negative charge, allowing separation based solely on molecular weight.

Equation for Charge-to-Mass Ratio:

Polyacrylamide Gel Preparation

Polyacrylamide gels are formed by radical polymerization of acrylamide monomers and crosslinkers (bisacrylamide), initiated by ammonium persulfate (APS) and catalyzed by TEMED.

• Adjustable Pore Size: Controlled by total monomer concentration (%T) and crosslinker concentration (%C).

• Formula for %T and %C:

• Advantages: Suitable for proteins up to 800 kDa, broad MW range, high resolution.

• Safety: Acrylamide is toxic; use nitrile gloves and avoid bubble formation during preparation.

Electrophoretic Mobility

The migration speed of analytes in gel electrophoresis depends on their charge, mass, and shape, as well as the viscosity and composition of the buffer.

• Key Parameters: Charge of ion, mass (molecular weight), ion radius, buffer viscosity.

• General Relationship: Increasing charge and decreasing mass increases migration speed.

Equation for Electrophoretic Mobility:

where is mobility, is charge, is viscosity, and is ion radius.

Stacking and Separation Gel

SDS-PAGE uses a stacking gel to concentrate sample bands before entering the separation gel, improving resolution through isotachophoresis.

• Stacking Gel: Low %T, large pores, low conductivity, pH ~6.8.

• Separation Gel: Higher %T, smaller pores, higher conductivity, pH ~8.8.

• Isotachophoresis: Sample ions are sandwiched between leading and trailing electrolytes, forming sharp bands.

Visualization and Quantification of Protein Bands

After electrophoresis, protein bands are visualized using staining methods such as Coomassie Brilliant Blue or silver stain.

• Coomassie Brilliant Blue: Binds to basic and hydrophobic amino acids; absorption maximum shifts from 465 nm to 595 nm upon protein binding.

• Silver Stain: Silver ions bind to proteins and are reduced to metallic silver, producing dark bands; highly sensitive but prone to artefacts.

• Densitometry: Scanning the intensity of stained bands at 595 nm allows quantification of protein concentration.

Calibration and Determination of Molecular Weight

Protein standards of known molecular weight are run alongside samples to create a calibration curve, enabling determination of unknown protein MW.

• Relative Migration: The distance migrated by each protein band is proportional to its molecular weight.

• Calibration Curve: Plotting log(MW) versus migration distance allows estimation of unknown MW.

Equation for Calibration:

Key Terms and Concepts

• Denaturation

• Electrophoretic mobility

• Disulfide linkages

• Protein structure (1°, 2°, 3°, 4°)

• SDS

• Molecular weight (MW)

• Polyacrylamide gels

• Stacking and separation gel

• Isotachophoresis

• Coomassie Brilliant Blue

• Silver stain

• Densitometry

Summary

SDS-PAGE is a major analytical tool for protein analysis, enabling assessment of purity, molecular weight, and concentration. Proper sample preparation, gel composition, and staining methods are essential for accurate results. The technique relies on the molecular sieve effect and isotachophoresis for high-resolution separation.

Additional info: Academic context and equations have been expanded for clarity and completeness.