Principles and Applications of Electrophoresis for Biomolecules

Course Learning Outcomes

Understand Basic Principles
- Comprehend how electric fields influence charged molecules.
- Recognize the fundamentals of electrophoretic separation.
Differentiate Techniques
- Distinguish gel electrophoresis from capillary electrophoresis (CE) concepts & applications.
Identify Separation Modes
- List different gel modes (e.g., PAGE, 2-D-GE, IEF) and capillary modes (CZE, CGE, CIEF, MEKC).

Core Definitions & Physical Basis

Electrophoresis (EP): Separation of analytes by differential migration in an electrolyte under an electric field.
- Molecules travel toward the electrode of opposite charge.
- Rate governed by electrophoretic mobility $\mu = \frac{v}{E}$ (velocity per unit field strength).
Driving forces
- Charge-to-mass ratio (or charge density).
- Molecular size and 3-D shape.
- Medium viscosity & pore size.
Two concurrent flows (esp. in CE)
- Electrophoretic mobility: intrinsic to each analyte.
- Electroosmotic flow (EOF): bulk buffer migration produced by capillary wall charges.

General Factors Affecting Separation Quality

Size / shape ➜ larger or more globular species experience greater friction in gels.
Charge-to-mass ratio ➜ higher negative (or positive) charge accelerates migration.
Field strength (voltage)
- High V increases speed but can decrease resolution & overheat samples.
Matrix characteristics
- Viscosity, % gel, pore radius, polymer type.
Buffer properties
- pH (controls analyte ionization), ionic strength, temperature stability.

Electrophoresis Media Categories

Free Solution (No Matrix)
- Classic capillary zone electrophoresis (CZE); separation primarily by charge/size.
Non-Conductive / Restrictive Matrices
- Agarose & polyacrylamide (PA) gels: introduce molecular sieving so size becomes dominant variable.

Classification Overview

Gel Electrophoresis
- PAGE (native & SDS), 2-D-GE, IEF.
Capillary Electrophoresis
- CZE (free solution), CGE, CIEF, MEKC (micellar).

Gel Electrophoresis — Universal Principles

Medium: Agarose or PA slab/tube serves as stationary phase.
Suppression of EOF: The cross-linked gel prevents bulk buffer flow, so only charged analytes migrate.
Migration direction
- DNA/RNA (always –) ➜ toward anode.
- Proteins: direction depends on pH vs isoelectric point $pI$ ; basic buffers (high pH) impart net negatives.
Running Buffer Roles
1. Conducts electricity via ions.
2. Maintains constant pH throughout run.
3. Provides counter-ions for predictable migration.
Instrumentation
- Power supply (≈200–500 V, 400 µA – 100 mA).
- Electrophoresis chamber with electrodes & reservoirs.
- Gel formats: vertical tubes, vertical slab, horizontal agarose.
- Typical slab: 1-3 mm thick; mini-gels 8 × 8 cm; large gels 40 × 20 cm.
Workflow
1. Gel casting → buffer equilibration.
2. Sample loaded into cathodic wells.
3. Apply voltage; negatives migrate to anode.
4. Stop run when desired separation reached.
5. Visualize with stains (e.g., ethidium bromide for nucleic acids; Coomassie Blue or silver for proteins).

Comparison of Gel Media

Polyacrylamide (PA)
- Synthetic; creates restrictive gel.
- Small pores (tunable 3–150 nm) → ideal for proteins & small DNA (≤ 1 kb).
Agarose
- Natural polysaccharide (algae).
- Large, heterogeneous pores (150–500 nm) → optimal for large DNA (50 bp → Mb) & high-MW proteins (>200 kDa).

Agarose Gel Electrophoresis — DNA Focus

Separation window: 50 bp to several megabases.
Key variables
1. DNA size: mobility ∝ $1/\log_{10}(\text{bp})$ .
2. Voltage: ↑V = faster but lower resolution.
3. Gel % w/v: higher % → smaller pores, resolves smaller fragments.
Applications: genotyping, restriction mapping, PCR product check, RNA integrity.

PAGE Variants

Native PAGE

Proteins kept folded; separation by combined charge, size, shape.
Maintains activity; suited for enzyme assays, oligomer studies, conformational changes, purification of active proteins.

SDS-PAGE (Denaturing)

Reagents: SDS (anionic detergent), β-mercaptoethanol or DTT (reducing), heat (≈90 °C × 5 min).
Mechanism
- SDS binds ~1.4 g SDS per g protein ⇒ uniform negative charge.
- Reducing agents cleave disulfide bonds ⇒ unfold subunits.
- Result: Proteins have identical $\frac{\text{charge}}{\text{mass}}$ so migrate purely by MW.
Gel sieving: PA % controls resolution range (e.g., 10% for 20–200 kDa).
Visualization: Coomassie, silver stain.
Molecular weight markers / ladders provide size standards; plot $\log(MW)$ vs relative migration Rf → unknown MW within 5–10 %.
Limitations
- Cannot discriminate proteins with MW differences < ~5 %.
- Fails to resolve isoforms of identical mass but different charge/sequence.

Native vs SDS Summary

Feature	Native	SDS
Protein state	Folded	Denatured
Separation basis	Charge + size + shape	Size only
Activity retained	Yes	No
Resolution for size	Lower	Higher
Typical uses	Enzyme activity, complexes	MW, purity checks

Isoelectric Focusing (IEF)

Principle: Proteins migrate in a stable pH gradient until $pH=pI$ (net charge 0) ⇒ sharp stationary bands.
pH gradient: Anode (low pH) → cathode (high pH); can resolve 0.002 pI units.
Charge behaviour
- pH < pI → protein positive → moves to cathode (–).
- pH > pI → protein negative → moves to anode (+).
Applications
- Precise $pI$ determination; micro-heterogeneity mapping; first dimension of 2-D gels; mutation & PTM detection.

Two-Dimensional Gel Electrophoresis (2D-GE)

Concept: Combine orthogonal separations → IEF (charge) then SDS-PAGE (size).
Process
1. IEF on immobilized pH strip (pH 3–10) → equilibrate strip with SDS.
2. Lay strip atop vertical PA gel.
3. Run SDS-PAGE; spots separate downwards by MW.
4. Stain & scan; generate proteomic fingerprint (≤10,000 spots).
Importance: Comparative proteomics, isoform mapping, preparative isolation for MS.

Capillary Electrophoresis (CE) — Fundamentals

Setup: Fused-silica capillary (25–100 µm I.D., 25–100 cm long) filled with buffer.
Voltage: 5–30 kV DC.
Advantages
- Extremely high efficiency (100k–200k plates) versus HPLC (5k–20k).
- Nanoliter sample; rapid (< minutes); automation.
Sample handling
- Hydrodynamic or electrokinetic injections (0.1–10 nL).
Detection
- UV (200–214 nm proteins; 260 nm DNA), fluorescence, electrochemical, MS coupling.
Electroosmotic Flow (EOF)
- Silanol groups on silica deprotonate at pH>2 ⇒ negative wall.
- Cationic layer pulled toward cathode → drags solvent; flat velocity profile.
- Ordering of migration: Cations fast (same direction as EOF), neutrals ≈ EOF, anions slow (oppose EOF).

CE Modes

Capillary Zone Electrophoresis (CZE)

Free solution; separates small ions, drugs, peptides primarily by charge/size.

Capillary Gel Electrophoresis (CGE)

Capillary packed with polymer gel (linear polyacrylamide, dextran).
Provides molecular sieving → size-based separation for
- ssDNA/dsDNA, RNA, proteins, PCR fragments, Sanger sequencing.
Performance: Smaller analytes migrate faster through pores; resolution surpasses slab gels.
Pros / Cons
- + Automation, no hand casting, high resolution, no staining needed.
- – Single-channel (no parallel lanes), no 2-D capability, higher instrumentation cost.

Micellar Electrokinetic Chromatography (MEKC)

(Brief mention) Uses charged surfactant micelles to separate neutrals by partitioning.

Capillary Isoelectric Focusing (CIEF)

Miniaturized IEF inside capillary with ampholytes.
Workflow
1. Establish pH gradient between anolyte (low pH) & catholyte (high pH).
2. Focus proteins at $pI$ in minutes.
3. Mobilize zones past on-capillary detector.
Benefits: Automation, nanoliter sample, reproducible quantification.
Detection order: Basic proteins (higher pI) emerge first when mobilizing toward detector, followed by neutrals and acids.

Key Equations & Relationships

Electrophoretic velocity: $v = \mu E$ .
Relative mobility in gels: $Rf = \frac{\text{migration distance of band}}{\text{migration distance of dye front}}$ .
SDS-PAGE MW determination: Linear regression of $\log(MW)$ vs $Rf$ for standards; unknown MW from calibration.

Practical / Real-World Relevance

Clinical diagnostics: Monitor hemoglobin variants (IEF), serum proteins, DNA sizing for genetic disorders.
Biotechnology: Purity checks for recombinant proteins (SDS-PAGE), titer of PCR products.
Forensic science & paternity: STR analysis on capillary gels.
Proteomics & drug discovery: 2D-GE maps differential expression; CE–MS enables trace biomarker quantification.
Quality control: Verify vaccine protein profiles, monoclonal antibody charge heterogeneity (CIEF).

Ethical & Safety Considerations

Ethidium bromide = potent mutagen → adopt safer stains (GelRed, SYBR Safe); proper waste disposal.
High-voltage equipment requires insulated leads, interlock covers.
Reducing agents (β-ME) release toxic fumes; perform heating in fume hood.
UV light sources demand eye/skin protection.

Summary of Strengths & Limitations Across Techniques

Gel methods
- + Visual, inexpensive, parallel lanes, easy to set up.
- – Manual labor, longer run times, limited quantitation, diffusion band broadening.
Capillary methods
- + Speed, efficiency, automation, on-line detection, coupling to MS.
- – Single sample at a time, specialized hardware, smaller load capacity.
Native vs denaturing formats
- Choice dictated by whether biological activity/complexes must be preserved.
Single vs 2-D dimensions
- 1-D sufficient for simple mixtures; 2-D critical for complex proteomes.

Quick Reference Cheat-Sheet

DNA/RNA ➜ use agarose (≥50 bp), visualize with EtBr/SYBR Green.
Proteins (MW only) ➜ SDS-PAGE.
Protein charge isoforms ➜ IEF or CIEF.
Protein complexes, enzymatic assays ➜ Native PAGE.
Whole-proteome snapshot ➜ 2D-GE.
Fast, high-resolution small-volume analysis ➜ CE (CZE/CGE).