#SPR

Protein-Protein Interaction

In SPR, analyte is:

In SPR terminology, ligand denotes macromolecule covalently immobilized to sensor chip dextran via amine coupling, while analyte is soluble binding partner in mobile phase. During association phase, analyte solution is injected at constant flow over ligand surface, allowing complex formation monitored as increase in resonance units. After injection, buffer flow induces dissociation. This orientation distinguishes immobilized state from flowing state. Detection is optical plasmon shift, not ELISA enzymatic reaction, and analyte is never immobilized or used for coating in conventional setup.

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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Protein-Protein Interaction

Which interaction is label-free and measured in real-time?

Ideal interaction analysis provides association rate, dissociation rate, and equilibrium dissociation without labeling proteins, avoiding artefacts from fluorophores or radioisotopes and enabling kinetic quantification. Surface Plasmon Resonance achieves this by optical detection of refractive index changes upon analyte-ligand binding on gold sensor chip. It operates continuously, generating sensorgrams in real-time with high sensitivity. EMSA requires labeled nucleic acid, FRET needs fluorescent fusions, BiFC relies on split fluorescent protein complementation. SPR therefore provides unbiased, label-free quantification of protein-protein binding specificity and affinity in real-time.

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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Protein-Protein Interaction

In SPR, what causes change in resonance units?

Surface Plasmon Resonance monitors binding kinetics in real-time on a gold-coated sensor chip without labels. Ligand is covalently immobilized on dextran matrix, analyte flows over. Incident polarized light excites surface plasmons; resonance angle depends on refractive index near surface. When analyte binds ligand, local mass concentration increases, altering refractive index and shifting resonance angle, recorded as resonance units versus time. Temperature fluctuations or UV are minimized and electric fields are not relevant; angle shift predominantly reflects binding-induced refractive index change.

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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Hybridization

What is the final stage in SPR interaction study?

Complete Surface Plasmon Resonance interaction cycle comprises ligand immobilization, baseline stabilization, analyte association, dissociation with buffer wash, and final regeneration to strip residual bound analyte while preserving ligand functionality. Regeneration restores baseline resonance units using mild regeneration buffers, enabling reuse of same chip for multiple concentration injections and kinetic replication. Analysis, dissociation, and calibration are intermediate steps, but regeneration represents definitive concluding stage preparing surface for next cycle. Optimization prevents ligand denaturation and ensures consistent binding capacity across experiments, critical for high-quality kinetic and affinity measurements.

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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Hybridization

In SPR, what must be immobilized first?

In Surface Plasmon Resonance workflow, ligand must be covalently or non-covalently immobilized first onto dextran or gold surface of sensor chip to establish stable baseline. Immobilization chemistry may use amine coupling, thiol, streptavidin-biotin, or antibody capture, optimizing orientation and activity. Only after ligand immobilization and blocking of residual sites does analyte injection occur, allowing measurement of binding kinetics. Probe, analyte, and substrate denote soluble interaction partners or downstream molecules, not the initially attached species. Correct order of steps is critical for reproducible kinetic evaluation and regeneration cycles.

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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Hybridization

Which process follows analyte binding in SPR?

After analyte binding reaches steady state in Surface Plasmon Resonance cycle, regeneration step is essential to remove bound analyte while preserving immobilized ligand activity for subsequent cycles. Mild acidic, basic, high salt, or detergent solutions disrupt analyte-ligand interaction without denaturing ligand coating. Successful regeneration restores baseline resonance units, enabling replicate measurements and concentration series on same chip. Immobilization occurs before binding, whereas denaturation or dissolution would destroy sensor functionality. Regeneration conditions require optimization to balance complete removal and ligand stability across multiple analytical cycles.

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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Hybridization

In SPR, the binding response is measured in:

In Surface Plasmon Resonance instruments, interaction response is quantified as Resonance Units, abbreviated RU, where 1 RU approximately corresponds to binding of 1 picogram per square millimeter on sensor surface. RU reflects shift in resonance angle due to refractive index change proportional to bound mass. Real-time sensograms plot RU versus time, displaying association, equilibrium, and dissociation phases. Units such as nanometers, volts, or generic optical units are not used for SPR quantification. Calibration of RU allows calculation of stoichiometry, kinetics, and affinity constants with high sensitivity and reproducibility.

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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Hybridization

What causes SPR signal increase?

In Surface Plasmon Resonance, signal intensity increases when soluble analyte specifically binds to ligand immobilized on the sensor surface, thereby increasing mass concentration and local refractive index within the evanescent field approximately 300 nm from gold film. This accumulation shifts resonance angle, recorded as increase in resonance units over time during association phase. Dissociation or buffer washing decreases signal. Evaporation, light absorption, or refractive index decrease would lower or destabilize signal rather than increase it. Specificity is validated using reference flow cells and concentration series for accurate kinetic characterization.

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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Hybridization

In SPR, the ligand is:

In Surface Plasmon Resonance terminology, ligand refers to the molecule permanently attached to the sensor chip dextran matrix, while analyte is the soluble partner that flows in microfluidic channels. Immobilization is typically achieved via amine, thiol, or streptavidin coupling, ensuring stable baseline and reproducible orientation. Ligand density influences response magnitude and kinetic accessibility. Analyte binding to ligand produces resonance unit increase. Distinguishing ligand and analyte is crucial for kinetic modeling, since immobilization restricts diffusion and may affect binding activity. Antibody coating or bead systems are not used in conventional SPR sensor design.

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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Hybridization

Which physical property does SPR detect?

Surface Plasmon Resonance detects minute shifts in refractive index at the sensor chip interface, which directly correlate with mass concentration variations close to the gold surface. When light excites surface plasmons, resonance occurs at a specific angle dependent on the dielectric environment. Adsorption or binding of biomolecules increases the refractive index within the evanescent field, shifting the resonance angle. This optical readout is independent of magnetic or pH changes and does not measure bulk optical density alone. Sensitivity to refractive index enables quantitative, real-time monitoring of unlabeled interaction kinetics and equilibrium binding parameters.

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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Hybridization

SPR is used to study:

Surface Plasmon Resonance is a powerful label-free technique for monitoring biomolecular interactions in real time without fluorescent or radioactive tags. One interaction partner is immobilized on a gold-coated sensor chip while the other flows over the surface. Binding changes the refractive index near the surface, altering the resonance angle proportionally to mass accumulation. This allows determination of association and dissociation rate constants, affinity, and specificity. Applications include protein-protein, protein-DNA, antibody-antigen, and small molecule screening. Traditional processes like DNA replication, transcription, and translation are studied by other molecular biology techniques.

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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Hybridization

What does SPR stand for?

SPR stands for Surface Plasmon Resonance, a physical optical phenomenon occurring at the interface of a thin noble metal film, typically gold, and a dielectric medium. Upon illumination with p-polarized light under conditions of total internal reflection, conduction electrons undergo collective oscillations called surface plasmons. The resonance angle is highly sensitive to refractive index changes near the metal surface. Biomolecular binding increases local mass and refractive index, shifting resonance. This principle enables real-time, label-free detection of biomolecular interactions and is extensively used in biosensor technology for kinetic 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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