Binding Characterization With SPR And BLI
Plain-language role: Use this skill when you need to plan or troubleshoot experimental binding measurements after design.
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 mass transport limit (>10^7 M^-1s^-1)
- koff faster than data acquisition (< 0.01 s^-1 requires faster sampling)
- Rmax >> theoretical maximum (aggregation or avidity)
- Large difference between kinetic and equilibrium KD
References
Platform comparisons
SPR protocols
Troubleshooting
Regeneration
Mass transport
Inputs
- Purified binders or crude samples plus a defined target and assay format preference.
- Expected affinity range, kinetic questions, and any throughput constraints.
- Instrument context such as available SPR chips, BLI tips, and regeneration chemistry.
Outputs
- An assay recommendation such as SPR versus BLI with a justified experimental setup.
- Suggested immobilization strategy, concentration series, and troubleshooting checks.
- Interpretation guidance for artifacts such as mass transport, rebinding, and non-specific signal.
Next Step
Use the recommended assay plan to run validation experiments, then feed the measured binders back into campaign prioritization.
1---2name: spr-bli-binding-characterization3description: SPR and BLI assay planning, kinetic interpretation, and troubleshooting guidance. 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---6
7# Binding Characterization With SPR And BLI
8
9**Plain-language role**: Use this skill when you need to plan or troubleshoot experimental binding measurements after design.
10
11## SPR vs BLI Decision Matrix
12
13| Factor | Choose SPR | Choose BLI |
14|--------|------------|------------|
15| **Sensitivity** | Small molecules, fragments (<500 Da) | Large complexes, antibodies |
16| **Throughput** | Low-medium (serial) | High (96-well parallel) |
17| **Sample purity** | Required (clogs fluidics) | Tolerates crude lysates |
18| **Kinetic resolution** | Higher (better for fast kinetics) | Lower |
19| **Mass transport** | More sensitive (may distort kon) | Less sensitive |
20| **Maintenance** | High (fluidics system) | Low (dip-and-read) |
21| **Sample consumption** | Higher (continuous flow) | Lower |
22| **Cost per experiment** | Lower chip cost, higher run cost | Higher tip cost, lower run cost |
23
24## Key differences
25
26### SPR (Surface Plasmon Resonance)
27- **Mechanism**: Detects refractive index changes at gold surface
28- **Surface**: Gold chip with dextran matrix (CM5, CM7, etc.)
29- **Flow**: Continuous microfluidics
30- **Best for**: Small molecules, high-affinity, precise kon/koff
31
32### BLI (Biolayer Interferometry)
33- **Mechanism**: Measures optical interference pattern shift
34- **Surface**: Fiber optic biosensor tips (SA, Ni-NTA, AHC)
35- **Flow**: Dip-and-read (no microfluidics)
36- **Best for**: High-throughput, crude samples, antibody screening
37
38---
39
40## Troubleshooting: Why BLI works but SPR doesn't
41
42| Cause | Mechanism | Solution |
43|-------|-----------|----------|
44| **Hydrophobic CDRs** | Adsorb to SPR gold/dextran surface | Add 0.05% Tween-20, use CM7 chip with longer dextran |
45| **Aggregation** | Mass transport artifacts in SPR fluidics | Filter sample (0.22μm), reduce ligand density |
46| **High instability** | Degrades during continuous flow | Shorter cycle time, add stabilizers (trehalose 5%) |
47| **Charge mismatch** | Nonspecific binding to charged dextran | Adjust buffer pH ±1 from pI, add BSA 1mg/mL |
48| **Slow dissociation** | Long regeneration needed (damages ligand) | Use BLI (disposable tips) |
49
50### Why SPR works but BLI doesn't
51
52| Cause | Mechanism | Solution |
53|-------|-----------|----------|
54| **Small analyte** | BLI less sensitive for <10 kDa | Use SPR with appropriate chip |
55| **Weak affinity (KD >10μM)** | Fast dissociation in BLI dip | Increase analyte concentration |
56| **Low expression** | Not enough signal | Increase biosensor loading |
57
58---
59
60## Mass transport considerations
61
62Mass transport limitation occurs when analyte cannot diffuse to the surface fast enough to maintain equilibrium. This distorts kinetic parameters.
63
64### Symptoms
65- Observed kon appears slower than true kon
66- Linear association phase (instead of exponential)
67- kon varies with ligand density
68- Rmax varies with flow rate
69
70### When mass transport matters
71- **High-affinity interactions** (kon >10^6 M^-1s^-1)
72- **High ligand density** (>500 RU)
73- **Slow flow rates** (<30 μL/min in SPR)
74- **Large analytes** (slow diffusion)
75
76### Mitigation strategies
77
78| Strategy | SPR | BLI |
79|----------|-----|-----|
80| Reduce ligand density | <200 RU for high-affinity | <0.5 nm shift loading |
81| Increase flow rate | 50-100 μL/min | Increase shake speed (1000 rpm) |
82| Use oriented immobilization | His-tag capture | Biotinylated ligand |
83| Include in fitting | Mass transport model (kt) | Usually less critical |
84
85---
86
87## Nonspecific binding mitigation
88
89### Buffer additives (ranked by effectiveness)
90
91| Additive | Concentration | Mechanism | Best For |
92|----------|---------------|-----------|----------|
93| BSA | 0.5-1 mg/mL | Blocks hydrophobic sites | General use |
94| Tween-20 | 0.02-0.05% | Prevents surface adsorption | Hydrophobic analytes |
95| Trehalose | 1-5% | Stabilizes + blocks | Unstable proteins |
96| Sucrose | 5% | BLI-specific blocker | BLI tips |
97| Carboxymethyl dextran | 1 mg/mL | Competitive blocking | SPR with charged proteins |
98| NaCl | 150-500 mM | Reduces ionic interactions | Charged proteins |
99
100### pH optimization
101- Keep buffer pH at least 1 unit away from analyte pI
102- pI near 7: Use pH 6.0 or 8.0 buffer
103- Acidic proteins (pI <5): Use neutral or basic buffer
104- Basic proteins (pI >9): Use slightly acidic buffer
105
106### Reference subtraction
107**Always include**:
108- Blank reference channel (no ligand)
109- Buffer-only injections
110- Non-specific binding controls
111
112---
113
114## Regeneration conditions
115
116### SPR regeneration scouting (try in order)
117
118| Condition | Targets | Caution |
119|-----------|---------|---------|
120| 10 mM Glycine pH 2.0-2.5 | Most protein-protein | May denature ligand |
121| 10 mM Glycine pH 1.5 | Strong interactions | Harsh, limit exposure |
122| 1-2 M NaCl | Ionic interactions | Mild, try first |
123| 10 mM NaOH | Very stable ligands | Can hydrolyze proteins |
124| 10 mM Glycine pH 9-10 | Acid-stable proteins | Can aggregate |
125| 10 mM EDTA | His-tag, metal-dependent | Strips Ni-NTA |
126| 4 M MgCl2 | Hydrophobic interactions | Check ligand stability |
127
128### Regeneration protocol
1291. Start with mildest condition (high salt)
1302. Test 30s contact time
1313. Verify complete dissociation (return to baseline)
1324. Verify retained ligand activity (repeat binding)
1335. Use shortest effective contact time
134
135### BLI tips
136- Tips are often disposable (no regeneration needed)
137- For reuse: Same conditions as SPR, but shorter exposure
138- Anti-His tips: 10 mM Glycine pH 1.5, 30s
139- Streptavidin tips: Generally not regenerable
140
141---
142
143## Common artifacts and solutions
144
145### Biphasic binding
146**Symptoms**: Two-rate association or dissociation
147**Causes**:
148- Sample heterogeneity (aggregates)
149- Ligand heterogeneity (multiple conformations)
150- Avidity effects (bivalent analyte)
151
152**Solutions**:
153- Filter/centrifuge sample
154- Use monovalent Fab fragments
155- Reduce ligand density
156- Fit to heterogeneous model
157
158### Negative dissociation
159**Symptoms**: Signal increases during dissociation phase
160**Causes**:
161- Ligand leaching from surface
162- Analyte aggregation on surface
163- Reference channel drift
164
165**Solutions**:
166- Use capture antibody instead of direct immobilization
167- Increase buffer stringency
168- Better reference subtraction
169
170### Hook effect
171**Symptoms**: Signal decreases at high analyte concentrations
172**Causes**:
173- Surface saturation + rebinding suppression
174- Crowding effects
175
176**Solutions**:
177- Reduce analyte concentration range
178- Reduce ligand density
179- Use smaller analyte fragments
180
181---
182
183## Kinetic data quality checklist
184
185### Before analysis
186- [ ] Reference-subtracted properly
187- [ ] Buffer injection shows flat baseline
188- [ ] Rmax consistent across concentrations
189- [ ] No systematic drift during association
190- [ ] Complete regeneration (return to baseline)
191- [ ] Duplicate/triplicate injections consistent
192
193### Fitting quality
194- [ ] Residuals randomly distributed (no systematic deviation)
195- [ ] Chi² < 10% of Rmax (or < 1 RU² for low signals)
196- [ ] kon and koff errors < 20% of values
197- [ ] KD from kinetics matches equilibrium KD (within 3-fold)
198- [ ] Fitted Rmax reasonable (close to theoretical)
199
200### Red flags
201- kon approaching mass transport limit (>10^7 M^-1s^-1)
202- koff faster than data acquisition (< 0.01 s^-1 requires faster sampling)
203- Rmax >> theoretical maximum (aggregation or avidity)
204- Large difference between kinetic and equilibrium KD
205
206---
207
208## References
209
210### Platform comparisons
211- [BLI vs SPR Comparison - Sartorius](https://www.sartorius.hr/en/news/blog/bli-vs-spr-choosing-the-ideal-method-for-analyzing-biomolecular-interactions/)
212- [BLI vs SPR - Nicoya](https://nicoyalife.com/blog/biolayer-interferometry-vs-surface-plasmon-resonance/)
213
214### SPR protocols
215- [SPR Guidelines - van der Merwe, Oxford](https://www.path.ox.ac.uk/wp-content/uploads/2023/09/SPR-guidelines-1.pdf)
216- [SPR Experiment Guide - Duke DHVI](https://dhvi.duke.edu/sites/default/files/2022-08/SPR%20Experiment%20Guide%20v1.3.pdf)
217
218### Troubleshooting
219- [4 Ways to Reduce NSB in SPR - Nicoya](https://nicoyalife.com/blog/4-ways-reduce-non-specific-binding-spr/)
220- [3 Ways to Limit Mass Transfer Effects - Nicoya](https://nicoyalife.com/blog/3-ways-to-limit-mass-transfer-effects/)
221- [Suppressing NSB in BLI - ACS Omega](https://pubs.acs.org/doi/10.1021/acsomega.1c05659)
222
223### Regeneration
224- [SPR Regeneration - SPRpages](https://www.sprpages.nl/kinetics/regeneration)
225- [Mastering Regeneration - Nicoya](https://nicoyalife.com/blog/regeneration-buffer-spr-experiment/)
226
227### Mass transport
228- [Mass Transport Limitation in SPR - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4134667/)
229- [Mass-Transfer Kinetics - SPRpages](https://www.sprpages.nl/data-fitting/kinetic-models/mass-transfer)
230
231## Inputs
232
233- Purified binders or crude samples plus a defined target and assay format preference.
234- Expected affinity range, kinetic questions, and any throughput constraints.
235- Instrument context such as available SPR chips, BLI tips, and regeneration chemistry.
236
237## Outputs
238
239- An assay recommendation such as SPR versus BLI with a justified experimental setup.
240- Suggested immobilization strategy, concentration series, and troubleshooting checks.
241- Interpretation guidance for artifacts such as mass transport, rebinding, and non-specific signal.
242
243## Next Step
244
245Use the recommended assay plan to run validation experiments, then feed the measured binders back into campaign prioritization.
gabrielmoreira