Binding Characterization: SPR and BLI
SPR vs BLI Decision Matrix
| Factor |
Choose SPR |
Choose BLI |
| Sensitivity |
Small molecules, fragments (<500 Da) |
Large complexes, antibodies |
| Throughput |
Low-medium (serial) |
High (96-well parallel) |
| Sample purity |
Required (clogs fluidics) |
Tolerates crude lysates |
| Kinetic resolution |
Higher (better for fast kinetics) |
Lower |
| Mass transport |
More sensitive (may distort kon) |
Less sensitive |
| Maintenance |
High (fluidics system) |
Low (dip-and-read) |
| Sample consumption |
Higher (continuous flow) |
Lower |
| Cost per experiment |
Lower chip cost, higher run cost |
Higher tip cost, lower run cost |
Key differences
SPR (Surface Plasmon Resonance)
- Mechanism: Detects refractive index changes at gold surface
- Surface: Gold chip with dextran matrix (CM5, CM7, etc.)
- Flow: Continuous microfluidics
- Best for: Small molecules, high-affinity, precise kon/koff
BLI (Biolayer Interferometry)
- Mechanism: Measures optical interference pattern shift
- Surface: Fiber optic biosensor tips (SA, Ni-NTA, AHC)
- Flow: Dip-and-read (no microfluidics)
- Best for: High-throughput, crude samples, antibody screening
Troubleshooting: Why BLI works but SPR doesn't
| Cause |
Mechanism |
Solution |
| Hydrophobic CDRs |
Adsorb to SPR gold/dextran surface |
Add 0.05% Tween-20, use CM7 chip with longer dextran |
| Aggregation |
Mass transport artifacts in SPR fluidics |
Filter sample (0.22μm), reduce ligand density |
| High instability |
Degrades during continuous flow |
Shorter cycle time, add stabilizers (trehalose 5%) |
| Charge mismatch |
Nonspecific binding to charged dextran |
Adjust buffer pH ±1 from pI, add BSA 1mg/mL |
| Slow dissociation |
Long regeneration needed (damages ligand) |
Use BLI (disposable tips) |
Why SPR works but BLI doesn't
| Cause |
Mechanism |
Solution |
| Small analyte |
BLI less sensitive for <10 kDa |
Use SPR with appropriate chip |
| Weak affinity (KD >10μM) |
Fast dissociation in BLI dip |
Increase analyte concentration |
| Low expression |
Not enough signal |
Increase biosensor loading |
Mass transport considerations
Mass transport limitation occurs when analyte cannot diffuse to the surface fast enough to maintain equilibrium. This distorts kinetic parameters.
Symptoms
- Observed kon appears slower than true kon
- Linear association phase (instead of exponential)
- kon varies with ligand density
- Rmax varies with flow rate
When mass transport matters
- High-affinity interactions (kon >10^6 M^-1s^-1)
- High ligand density (>500 RU)
- Slow flow rates (<30 μL/min in SPR)
- Large analytes (slow diffusion)
Mitigation strategies
| Strategy |
SPR |
BLI |
| Reduce ligand density |
<200 RU for high-affinity |
<0.5 nm shift loading |
| Increase flow rate |
50-100 μL/min |
Increase shake speed (1000 rpm) |
| Use oriented immobilization |
His-tag capture |
Biotinylated ligand |
| Include in fitting |
Mass transport model (kt) |
Usually less critical |
Nonspecific binding mitigation
Buffer additives (ranked by effectiveness)
| Additive |
Concentration |
Mechanism |
Best For |
| BSA |
0.5-1 mg/mL |
Blocks hydrophobic sites |
General use |
| Tween-20 |
0.02-0.05% |
Prevents surface adsorption |
Hydrophobic analytes |
| Trehalose |
1-5% |
Stabilizes + blocks |
Unstable proteins |
| Sucrose |
5% |
BLI-specific blocker |
BLI tips |
| Carboxymethyl dextran |
1 mg/mL |
Competitive blocking |
SPR with charged proteins |
| NaCl |
150-500 mM |
Reduces ionic interactions |
Charged proteins |
pH optimization
- Keep buffer pH at least 1 unit away from analyte pI
- pI near 7: Use pH 6.0 or 8.0 buffer
- Acidic proteins (pI <5): Use neutral or basic buffer
- Basic proteins (pI >9): Use slightly acidic buffer
Reference subtraction
Always include:
- Blank reference channel (no ligand)
- Buffer-only injections
- Non-specific binding controls
Regeneration conditions
SPR regeneration scouting (try in order)
| Condition |
Targets |
Caution |
| 10 mM Glycine pH 2.0-2.5 |
Most protein-protein |
May denature ligand |
| 10 mM Glycine pH 1.5 |
Strong interactions |
Harsh, limit exposure |
| 1-2 M NaCl |
Ionic interactions |
Mild, try first |
| 10 mM NaOH |
Very stable ligands |
Can hydrolyze proteins |
| 10 mM Glycine pH 9-10 |
Acid-stable proteins |
Can aggregate |
| 10 mM EDTA |
His-tag, metal-dependent |
Strips Ni-NTA |
| 4 M MgCl2 |
Hydrophobic interactions |
Check ligand stability |
Regeneration protocol
- Start with mildest condition (high salt)
- Test 30s contact time
- Verify complete dissociation (return to baseline)
- Verify retained ligand activity (repeat binding)
- Use shortest effective contact time
BLI tips
- Tips are often disposable (no regeneration needed)
- For reuse: Same conditions as SPR, but shorter exposure
- Anti-His tips: 10 mM Glycine pH 1.5, 30s
- Streptavidin tips: Generally not regenerable
Common artifacts and solutions
Biphasic binding
Symptoms: Two-rate association or dissociation
Causes:
- Sample heterogeneity (aggregates)
- Ligand heterogeneity (multiple conformations)
- Avidity effects (bivalent analyte)
Solutions:
- Filter/centrifuge sample
- Use monovalent Fab fragments
- Reduce ligand density
- Fit to heterogeneous model
Negative dissociation
Symptoms: Signal increases during dissociation phase
Causes:
- Ligand leaching from surface
- Analyte aggregation on surface
- Reference channel drift
Solutions:
- Use capture antibody instead of direct immobilization
- Increase buffer stringency
- Better reference subtraction
Hook effect
Symptoms: Signal decreases at high analyte concentrations
Causes:
- Surface saturation + rebinding suppression
- Crowding effects
Solutions:
- Reduce analyte concentration range
- Reduce ligand density
- Use smaller analyte fragments
Kinetic data quality checklist
Before analysis
Fitting quality
Red flags
- kon approaching the mass transport limit (>10^7 M^-1s^-1), where rates are unreliable
- koff too fast to sample (> 0.1 s^-1) or too slow to measure in the dissociation window (< 10^-5 s^-1)
- Rmax >> theoretical maximum (aggregation or avidity)
- Large difference between kinetic and equilibrium KD
References
Platform comparisons
SPR protocols
Troubleshooting
Regeneration
Mass transport
1---2name: binding-characterization3description: Guidance for SPR and BLI binding characterization experiments. Use when: (1) Planning binding kinetics experiments, (2) Troubleshooting poor/no binding signal, (3) Interpreting kinetic data artifacts, (4) Choosing between SPR vs BLI platforms.4license: MIT5---67# Binding Characterization: SPR and BLI89## SPR vs BLI Decision Matrix1011| Factor | Choose SPR | Choose BLI |12|--------|------------|------------|13| **Sensitivity** | Small molecules, fragments (<500 Da) | Large complexes, antibodies |14| **Throughput** | Low-medium (serial) | High (96-well parallel) |15| **Sample purity** | Required (clogs fluidics) | Tolerates crude lysates |16| **Kinetic resolution** | Higher (better for fast kinetics) | Lower |17| **Mass transport** | More sensitive (may distort kon) | Less sensitive |18| **Maintenance** | High (fluidics system) | Low (dip-and-read) |19| **Sample consumption** | Higher (continuous flow) | Lower |20| **Cost per experiment** | Lower chip cost, higher run cost | Higher tip cost, lower run cost |2122## Key differences2324### SPR (Surface Plasmon Resonance)25- **Mechanism**: Detects refractive index changes at gold surface26- **Surface**: Gold chip with dextran matrix (CM5, CM7, etc.)27- **Flow**: Continuous microfluidics28- **Best for**: Small molecules, high-affinity, precise kon/koff2930### BLI (Biolayer Interferometry)31- **Mechanism**: Measures optical interference pattern shift32- **Surface**: Fiber optic biosensor tips (SA, Ni-NTA, AHC)33- **Flow**: Dip-and-read (no microfluidics)34- **Best for**: High-throughput, crude samples, antibody screening3536---3738## Troubleshooting: Why BLI works but SPR doesn't3940| Cause | Mechanism | Solution |41|-------|-----------|----------|42| **Hydrophobic CDRs** | Adsorb to SPR gold/dextran surface | Add 0.05% Tween-20, use CM7 chip with longer dextran |43| **Aggregation** | Mass transport artifacts in SPR fluidics | Filter sample (0.22μm), reduce ligand density |44| **High instability** | Degrades during continuous flow | Shorter cycle time, add stabilizers (trehalose 5%) |45| **Charge mismatch** | Nonspecific binding to charged dextran | Adjust buffer pH ±1 from pI, add BSA 1mg/mL |46| **Slow dissociation** | Long regeneration needed (damages ligand) | Use BLI (disposable tips) |4748### Why SPR works but BLI doesn't4950| Cause | Mechanism | Solution |51|-------|-----------|----------|52| **Small analyte** | BLI less sensitive for <10 kDa | Use SPR with appropriate chip |53| **Weak affinity (KD >10μM)** | Fast dissociation in BLI dip | Increase analyte concentration |54| **Low expression** | Not enough signal | Increase biosensor loading |5556---5758## Mass transport considerations5960Mass transport limitation occurs when analyte cannot diffuse to the surface fast enough to maintain equilibrium. This distorts kinetic parameters.6162### Symptoms63- Observed kon appears slower than true kon64- Linear association phase (instead of exponential)65- kon varies with ligand density66- Rmax varies with flow rate6768### When mass transport matters69- **High-affinity interactions** (kon >10^6 M^-1s^-1)70- **High ligand density** (>500 RU)71- **Slow flow rates** (<30 μL/min in SPR)72- **Large analytes** (slow diffusion)7374### Mitigation strategies7576| Strategy | SPR | BLI |77|----------|-----|-----|78| Reduce ligand density | <200 RU for high-affinity | <0.5 nm shift loading |79| Increase flow rate | 50-100 μL/min | Increase shake speed (1000 rpm) |80| Use oriented immobilization | His-tag capture | Biotinylated ligand |81| Include in fitting | Mass transport model (kt) | Usually less critical |8283---8485## Nonspecific binding mitigation8687### Buffer additives (ranked by effectiveness)8889| Additive | Concentration | Mechanism | Best For |90|----------|---------------|-----------|----------|91| BSA | 0.5-1 mg/mL | Blocks hydrophobic sites | General use |92| Tween-20 | 0.02-0.05% | Prevents surface adsorption | Hydrophobic analytes |93| Trehalose | 1-5% | Stabilizes + blocks | Unstable proteins |94| Sucrose | 5% | BLI-specific blocker | BLI tips |95| Carboxymethyl dextran | 1 mg/mL | Competitive blocking | SPR with charged proteins |96| NaCl | 150-500 mM | Reduces ionic interactions | Charged proteins |9798### pH optimization99- Keep buffer pH at least 1 unit away from analyte pI100- pI near 7: Use pH 6.0 or 8.0 buffer101- Acidic proteins (pI <5): Use neutral or basic buffer102- Basic proteins (pI >9): Use slightly acidic buffer103104### Reference subtraction105**Always include**:106- Blank reference channel (no ligand)107- Buffer-only injections108- Non-specific binding controls109110---111112## Regeneration conditions113114### SPR regeneration scouting (try in order)115116| Condition | Targets | Caution |117|-----------|---------|---------|118| 10 mM Glycine pH 2.0-2.5 | Most protein-protein | May denature ligand |119| 10 mM Glycine pH 1.5 | Strong interactions | Harsh, limit exposure |120| 1-2 M NaCl | Ionic interactions | Mild, try first |121| 10 mM NaOH | Very stable ligands | Can hydrolyze proteins |122| 10 mM Glycine pH 9-10 | Acid-stable proteins | Can aggregate |123| 10 mM EDTA | His-tag, metal-dependent | Strips Ni-NTA |124| 4 M MgCl2 | Hydrophobic interactions | Check ligand stability |125126### Regeneration protocol1271. Start with mildest condition (high salt)1282. Test 30s contact time1293. Verify complete dissociation (return to baseline)1304. Verify retained ligand activity (repeat binding)1315. Use shortest effective contact time132133### BLI tips134- Tips are often disposable (no regeneration needed)135- For reuse: Same conditions as SPR, but shorter exposure136- Anti-His tips: 10 mM Glycine pH 1.5, 30s137- Streptavidin tips: Generally not regenerable138139---140141## Common artifacts and solutions142143### Biphasic binding144**Symptoms**: Two-rate association or dissociation145**Causes**:146- Sample heterogeneity (aggregates)147- Ligand heterogeneity (multiple conformations)148- Avidity effects (bivalent analyte)149150**Solutions**:151- Filter/centrifuge sample152- Use monovalent Fab fragments153- Reduce ligand density154- Fit to heterogeneous model155156### Negative dissociation157**Symptoms**: Signal increases during dissociation phase158**Causes**:159- Ligand leaching from surface160- Analyte aggregation on surface161- Reference channel drift162163**Solutions**:164- Use capture antibody instead of direct immobilization165- Increase buffer stringency166- Better reference subtraction167168### Hook effect169**Symptoms**: Signal decreases at high analyte concentrations170**Causes**:171- Surface saturation + rebinding suppression172- Crowding effects173174**Solutions**:175- Reduce analyte concentration range176- Reduce ligand density177- Use smaller analyte fragments178179---180181## Kinetic data quality checklist182183### Before analysis184- [ ] Reference-subtracted properly185- [ ] Buffer injection shows flat baseline186- [ ] Rmax consistent across concentrations187- [ ] No systematic drift during association188- [ ] Complete regeneration (return to baseline)189- [ ] Duplicate/triplicate injections consistent190191### Fitting quality192- [ ] Residuals randomly distributed (no systematic deviation)193- [ ] Chi² < 10% of Rmax (or < 1 RU² for low signals)194- [ ] kon and koff errors < 20% of values195- [ ] KD from kinetics matches equilibrium KD (within 3-fold)196- [ ] Fitted Rmax reasonable (close to theoretical)197198### Red flags199- kon approaching the mass transport limit (>10^7 M^-1s^-1), where rates are unreliable200- koff too fast to sample (> 0.1 s^-1) or too slow to measure in the dissociation window (< 10^-5 s^-1)201- Rmax >> theoretical maximum (aggregation or avidity)202- Large difference between kinetic and equilibrium KD203204---205206## References207208### Platform comparisons209- [BLI vs SPR Comparison - Sartorius](https://www.sartorius.hr/en/news/blog/bli-vs-spr-choosing-the-ideal-method-for-analyzing-biomolecular-interactions/)210- [BLI vs SPR - Nicoya](https://nicoyalife.com/blog/biolayer-interferometry-vs-surface-plasmon-resonance/)211212### SPR protocols213- [SPR Guidelines - van der Merwe, Oxford](https://www.path.ox.ac.uk/wp-content/uploads/2023/09/SPR-guidelines-1.pdf)214- [SPR Experiment Guide - Duke DHVI](https://dhvi.duke.edu/sites/default/files/2022-08/SPR%20Experiment%20Guide%20v1.3.pdf)215216### Troubleshooting217- [4 Ways to Reduce NSB in SPR - Nicoya](https://nicoyalife.com/blog/4-ways-reduce-non-specific-binding-spr/)218- [3 Ways to Limit Mass Transfer Effects - Nicoya](https://nicoyalife.com/blog/3-ways-to-limit-mass-transfer-effects/)219- [Suppressing NSB in BLI - ACS Omega](https://pubs.acs.org/doi/10.1021/acsomega.1c05659)220221### Regeneration222- [SPR Regeneration - SPRpages](https://www.sprpages.nl/kinetics/regeneration)223- [Mastering Regeneration - Nicoya](https://nicoyalife.com/blog/regeneration-buffer-spr-experiment/)224225### Mass transport226- [Mass Transport Limitation in SPR - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4134667/)227- [Mass-Transfer Kinetics - SPRpages](https://www.sprpages.nl/data-fitting/kinetic-models/mass-transfer)
FridrichMethod