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

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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

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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

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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

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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

1. Start with mildest condition (high salt)
2. Test 30s contact time
3. Verify complete dissociation (return to baseline)
4. Verify retained ligand activity (repeat binding)
5. 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

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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

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Kinetic data quality checklist

Before analysis

• [ ] Reference-subtracted properly
• [ ] Buffer injection shows flat baseline
• [ ] Rmax consistent across concentrations
• [ ] No systematic drift during association
• [ ] Complete regeneration (return to baseline)
• [ ] Duplicate/triplicate injections consistent

Fitting quality

• [ ] Residuals randomly distributed (no systematic deviation)
• [ ] Chi² < 10% of Rmax (or < 1 RU² for low signals)
• [ ] kon and koff errors < 20% of values
• [ ] KD from kinetics matches equilibrium KD (within 3-fold)
• [ ] Fitted Rmax reasonable (close to theoretical)

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

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References

Platform comparisons

• BLI vs SPR Comparison - Sartorius
• BLI vs SPR - Nicoya

SPR protocols

• SPR Guidelines - van der Merwe, Oxford
• SPR Experiment Guide - Duke DHVI

Troubleshooting

• 4 Ways to Reduce NSB in SPR - Nicoya
• 3 Ways to Limit Mass Transfer Effects - Nicoya
• Suppressing NSB in BLI - ACS Omega

Regeneration

• SPR Regeneration - SPRpages
• Mastering Regeneration - Nicoya

Mass transport

• Mass Transport Limitation in SPR - PMC
• Mass-Transfer Kinetics - SPRpages