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#protein detection

14 public questions tagged with this topic.

Immunological Applications

ELISA is commonly used for:

Enzyme-linked immunosorbent assay revolutionized immunodiagnostics by converting antigen antibody binding into measurable enzymatic colorimetric signal without radioactivity, originally described by Eva Engvall and Peter Perlmann 1971. Polystyrene microtiter plate with high binding capacity via hydrophobic interaction adsorbs capture antibody coated overnight at alkaline pH 9.6 carbonate buffer 1 to 10 microgram per ml. Non-specific sites blocked with protein bovine serum albumin five percent, casein, or nonfat milk to prevent background adsorption through hydrophobic pockets. Sample containing antigen such as viral capsid HIV p24, cytokine IL-6, or hormone insulin incubated allowing specific capture, unbound material removed by repeated washes with phosphate buffered saline containing 0.05 percent Tween-20 disrupting weak interactions. Detection uses secondary antibody conjugated covalently to horseradish peroxidase or alkaline phosphatase via periodate oxidation forming Schiff base. Addition of chromogenic substrate tetramethylbenzidine TMB with hydrogen peroxide yields oxidation to blue diimine measured after sulfuric acid stopping as yellow product absorbance 450 nm proportional to analyte concentration via four-parameter logistic standard curve with dynamic range three logs.

Ref: Engvall & Perlman Immunochemistry 1971 ELISA; Alberts MBoC ELISA HRP TMB sandwich principle diagnostic.

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Electrophoresis

What does ninhydrin detect on electrophoresis paper?

Ninhydrin, or 2,2-dihydroxy-1,3-indandione, is classical colorimetric reagent for detection of alpha-amino acids on paper chromatography or electrophoretograms. Primary amines and free amino acids undergo oxidative deamination and condensation with ninhydrin to produce deep purple chromophore known as Ruhemann's purple, absorbing at 570 nanometers, enabling sensitive visualization of separated amino acid bands. Secondary amines like proline yield yellow product. It does not specifically detect lipids, sugars, or intact proteins under similar conditions, though N-terminal residues may react weakly. Lipids require Sudan dyes, sugars require anisaldehyde or orcinol reagents.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Electrophoresis

Which electrophoresis is best for detecting isomers of same protein?

Proteins encoded by single gene often exist as multiple isoforms created by alternative splicing, phosphorylation, acetylation, deamidation or limited proteolysis, producing subtle variations in charge and mass indistinguishable in one-dimensional systems. SDS-PAGE masks charge differences and poorly resolves small mass changes, while isoelectric focusing alone cannot separate mass variants. Two-dimensional electrophoresis combining isoelectric focusing for pI separation and SDS-PAGE for molecular mass provides orthogonal resolving power, displaying isoforms as distinct spots across pH and size axes. This reveals charge variants and post-translational modifications co-migrating in single dimension, essential for proteomic isoform analysis.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Electrophoresis

What does Western blotting detect?

Blotting techniques are classified by target macromolecule: Southern blotting for DNA fragments, Northern for RNA transcripts, and Western for proteins, also called immunoblotting. Western blotting involves electrophoretic transfer of resolved proteins from polyacrylamide gel onto nitrocellulose or PVDF membrane, followed by blocking to prevent nonspecific binding and incubation with highly specific primary antibodies and enzyme-linked secondary antibodies for chemiluminescent or colorimetric detection. This identifies molecular weight, abundance and modifications of specific proteins. Lipids, DNA and sugars require thin-layer chromatography, Southern blotting or periodic acid-Schiff staining, not immunoblotting.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Electrophoresis

What is the advantage of silver staining?

Detection sensitivity is crucial for low-abundance proteomic analysis. Coomassie Brilliant Blue staining, while simple, reversible and quantitative, shows limited sensitivity around 30 to 100 nanograms per band, inadequate for trace proteins. Silver staining involves impregnation with silver nitrate binding to nucleophilic side chains, then reduction by formaldehyde to metallic silver, depositing dense grains. This yields 50 to 100 times greater sensitivity down to 0.1 to 2 nanograms, enabling visualization of minor spots and single-copy proteins. It is not faster than Coomassie, not specific for nucleic acids, representing high sensitivity rather than low sensitivity method.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Electrophoresis

What does Coomassie Brilliant Blue bind to?

Coomassie Brilliant Blue G-250 and R-250 are triphenylmethane dyes that bind non-covalently to proteins through combined electrostatic attraction to protonated amine groups of lysine, arginine and histidine and hydrophobic interactions with aromatic residues phenylalanine, tyrosine and tryptophan. Binding shifts absorption maximum from 465 to 595 nanometers, producing intense blue color proportional to protein amount, forming basis for Bradford assay. It does not efficiently intercalate DNA base pairs, bind RNA or stain polysaccharides, unlike ethidium bromide, SYBR Green or periodic acid-Schiff reagents. This specificity underlies both gel staining and solution quantification.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Electrophoresis

Which dye is more sensitive than Coomassie for protein detection?

Sensitivity differences among protein dyes determine detection limits in gels. Coomassie Brilliant Blue R-250 detects roughly 50 to 100 nanograms per band by binding electrostatically to basic and aromatic residues, suitable for routine quantification. Silver staining involves impregnation of gel with silver ions that bind to carboxyl, sulfhydryl and amino side chains, followed by reduction to metallic silver by formaldehyde, achieving 50 to 100-fold greater sensitivity down to 1 to 5 nanograms. Bromophenol blue merely tracks migration front, while ethidium bromide and SYBR Green intercalate nucleic acids, not proteins, limiting their utility.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Blotting Techniques

GFP is used in blotting because:

Green fluorescent protein from jellyfish Aequorea victoria contains internal tripeptide Ser65-Tyr66-Gly67 that undergoes autocatalytic cyclization and oxidation to form p-hydroxybenzylidene-imidazolinone chromophore requiring no exogenous cofactor. When fused to protein of interest, chimera exhibits intrinsic fluorescence excitable at 395 and 475 nm, emitting at 509 nm, allowing direct visualization on gel documentation systems or after blotting using anti-GFP antibodies. Utility does not derive from heat emission, blocking ability, or binding inhibition. Self-sufficient fluorophore formation makes GFP valuable reporter for localization, expression, and fusion protein detection.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Blotting Techniques

To detect expression of a protein in mitochondria after protease digestion, you use:

Proving internal mitochondrial localization requires protease protection assay combined with protein detection that provides molecular weight information. Isolated mitochondria are treated with proteinase K; outer membrane proteins degrade while matrix and inner membrane proteins protected by intact membranes remain. After lysis, samples resolved by SDS-PAGE and probed immunologically confirm persistence of target. Reverse transcription PCR measures RNA absent after protease treatment assessment, Southern detects DNA, and ELISA quantifies without size verification, unable to distinguish truncated degradation products. Therefore immunoblotting furnishes essential evidence of import and protected full-length protein inside organelle.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Blotting Techniques

What is the role of stripping and reprobing?

Stripping and reprobing constitutes essential procedure to reuse same membrane for detecting multiple different proteins. After initial detection, membrane is incubated in buffers containing SDS, beta-mercaptoethanol or dithiothreitol, and glycine at low pH or elevated temperature to dissociate and remove bound primary and secondary antibody complexes, while target proteins remain immobilized due to strong hydrophobic interactions with PVDF or nitrocellulose. Subsequent reprobing with antibody against different antigen, such as housekeeping protein beta-actin or phosphorylated isoform, allows normalization, conserves limited clinical or fractionated samples, and reduces inter-gel variability.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Blotting Techniques

Blocking agents in Western blotting are used to:

Membranes used for Western blotting, whether nitrocellulose or PVDF, possess high protein binding capacity and inevitably expose vacant hydrophobic surfaces after antigen transfer. Without intervention, primary antibodies and enzyme-linked secondary antibodies adsorb directly to these empty sites, producing high background and obscuring specific bands. Blocking step saturates unoccupied area with inert proteins such as non-fat dry milk, bovine serum albumin, casein, or irrelevant immunoglobulins, often supplemented with non-ionic detergent Tween-20. This prevents nonspecific antibody capture, enhances signal-to-noise ratio, improves specificity, and enables detection of low-abundance antigens with quantitative reliability.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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