This part is mainly about methods used to measure molecular binding, especially proteināprotein interactions.
The major topics are:
BLI is a label-free optical technique used to measure molecular binding interactions.
Typical things it measures:
Such as:
This is conceptually similar to Surface Plasmon Resonance (SPR).
The core principle is optical interference of reflected light.
A biosensor tip has two reflective surfaces:
White light is sent into the sensor.
Some light reflects from the internal layer, and some reflects from the outer biolayer.
These reflected beams interfere with each other.
If molecules bind to the sensor surface, the optical thickness changes.
This changes the interference pattern / wavelength shift.
That wavelength shift is proportional to how much mass binds.
This is the key readout.
BLI does not directly āseeā the molecule identity.
It detects:
change in thickness of the molecular layer
More binding = thicker layer = bigger wavelength shift
So it is fundamentally a mass accumulation measurement at the sensor surface.
You typically get a sensorgram with phases:
Sensor in buffer only
Flat signal
Analyte is introduced
Binding begins
Signal rises
Move sensor back into buffer
Bound molecules dissociate
Signal decreases
From this you calculate:
K_D = rac{k_}{k_}
Exactly like SPR.
Yes ā generally yes, and your interpretation is correct.
The file states this clearly.
Compared with SPR, BLI often has:
Especially when detecting:
SPR is usually better.
SPR measures changes in refractive index very close to the metal surface, which can be extremely sensitive.
BLI measures optical thickness changes, which can be slightly less precise.
So your statement:
āBLI is less sensitive than SPR?ā
Yes ā this is generally true.
But important nuance:
less sensitive does NOT mean bad
BLI is still excellent for many protein interactions.
A skeptical way to think about this:
SPR = best when sensitivity matters BLI = best when robustness and throughput matter
Thatās usually how labs choose.
Yes ā and this is one of its biggest strengths.
The file explicitly mentions serum samples.
Your understanding is correct.
Because the biosensor tip is dipped directly into wells.
This is different from SPRās microfluidic flow channels.
BLI is more tolerant of:
This is one reason it is widely used in biotech.
For antibody development, many assays are done directly in serum.
For example:
BLI is often easier here than SPR.
Yes ā your statement is absolutely correct.
The file says this explicitly.
Biotināstreptavidin interaction is one of the strongest non-covalent biological interactions known.
Biotin = vitamin B7 Streptavidin = protein that binds biotin extremely tightly
K_D approx 10^{-14} - 10^{-15} M
This is almost irreversible under assay conditions.
If your protein is biotin-labeled, then it binds strongly to a streptavidin-coated sensor.
Example:
Sensor tip:
Protein:
Result:
Then another binding partner is measured.
This is extremely common in BLI.
You asked:
molecule labelled with biotin can bind to surface with streptavidin?
Yes ā exactly.
That is one of the most standard immobilization strategies.
This is also exactly right.
The file mentions NTA interaction with His-tagged proteins.
A recombinant protein is often engineered with:
6 imes His
Usually six histidines in a row.
Example: " MGHHHHHH "
This is called a His-tag.
Nickelānitrilotriacetic acid affinity
NTA = nitrilotriacetic acid
NTA chelates nickel ions:
Ni^{2+}
Histidine side chains contain an imidazole ring that coordinates nickel.
So:
His-tag ā NiĀ²āŗ-NTA
This creates selective binding.
Sensor surface:
Protein:
The histidines coordinate the nickel ion.
This immobilizes the protein.
Very selective for recombinant proteins
Widely used in:
Excellent question.
The Sartorius Octet platform is a commercial instrument system that uses BLI.
People often say:
ārun it on the Octetā
This basically means:
perform BLI measurement
Because it is high throughput.
Many samples can be run in parallel using microplates.
Typical formats:
This makes it great for:
The file refers to āoptets,ā which is almost certainly intended as Octet.
So minor correction from the file transcription:
āoptetsā ā Octet
This was not explicitly detailed in the loaded text, but it fits exactly with common BLI applications, so let me explain the theory.
This means checking whether a therapeutic protein triggers immune recognition.
Example:
monoclonal antibody drug
Can patient serum antibodies bind it?
BLI can test this by measuring serum antibody binding.
This connects to the fileās point that BLI works with serum samples.
Very important in antibody manufacturing.
Protein A affinity chromatography is used to purify antibodies.
But traces of Protein A may remain.
This is a contaminant.
Residual Protein A must be tested because it can be immunogenic.
BLI can quantify remaining Protein A.
HCP = Host Cell Proteins
These are contaminating proteins from the production cell line.
For example:
These proteins must be minimized.
BLI can be used in QC workflows to detect them.
The file briefly introduces thermophoresis.
This is conceptually different from BLI and SPR.
Molecules move in a temperature gradient.
This movement depends on:
When binding occurs, these properties change.
So the diffusion behavior changes.
That shift is measured.
Binding changes:
protein + ligand ightarrow complex
The complex has different diffusion properties.
That is what thermophoresis detects.
| Technique | Principle | Strength |
|---|---|---|
| SPR | refractive index near gold surface | highest sensitivity |
| BLI / Octet | optical thickness / interference | crude samples, high throughput |
| Thermophoresis | diffusion in temperature gradient | solution-based |
If asked:
Why choose BLI over SPR?
Best answer:
That is the central theoretical message of this section.