Technical Note ◆ fcRn-spr-protocol

Step-by-Step: FcRn SPR Protocol with pH 6.0 / 7.4 Dual-Buffer SwitchPUBLISHED

Target FcRn Application Fc engineering, antibody half-life, SPR Hub fcRn Last reviewed 2026-09-03 Reading ~2 min

Step-by-Step: FcRn SPR Protocol with pH 6.0 / 7.4 Dual-Buffer Switch

> DRAFT — 文献依据（待湿实验验证）。运行前请与平台科学家确认缓冲液与芯片兼容性。

Measure IgG/Fc binding to FcRn at pH 6.0 and release kinetics at pH 7.4 — the in-vitro proxy for antibody half-life.

Biotinylated FcRn (FCGRT & B2M heterodimer), QC: SEC ≥95% monomer, endotoxin ≤0.1 EU/µg
Streptavidin sensor/chip (SPR or BLI)
Running buffers (degassed, filtered 0.22 µm):
Buffer A (pH 6.0): 50 mM Na-phosphate, 150 mM NaCl, 0.05% Tween-20, pH 6.0
Buffer B (pH 7.4): 50 mM Na-phosphate, 150 mM NaCl, 0.05% Tween-20, pH 7.4
Analyte: monomeric IgG / Fc fragment (confirm by SEC)
Negative control: FcRn-binding-deficient IgG mutant (e.g., I253A/H310A/H435A)

Immobilize FcRn at low density: capture biotinylated FcRn on streptavidin surface; target Rmax 30–80 RU (SPR) / 0.3–0.8 nm (BLI). Low density prevents avidity.
Equilibrate in Buffer A (pH 6.0) until baseline stable (≥5 min).
Association: inject analyte (IgG) at 30–50 µL/min (SPR) or 100–200 rpm shake (BLI) in Buffer A; use a concentration series (e.g., 2× dilution from ~10× expected KD). Include reference channel subtraction.
Dissociation phase 1 (pH 6.0): 60–120 s in Buffer A — record k_off at pH 6.0.
pH 7.4 release step: switch running buffer to Buffer B (pH 7.4) and monitor release 120–300 s — this is the half-life correlate.
Regeneration: short pulse (10–30 s) of pH 7.4 buffer or 10 mM glycine pH 2.5–3.0 (validate ≥5 cycles activity stability before screening).
Analysis: 1:1 Langmuir global fit on pH-6.0 data (KD, k_on, k_off); report pH-7.4 dissociation half-life (t½ = ln2/[email protected]).

Higher KD@pH6 = weaker endosomal binding → shorter half-life (in most models).
Slower pH-7.4 dissociation = longer recycling → longer half-life.
Compare candidates relative to a wild-type IgG1 control run in the same session.

Symptom	Likely cause	Fix
KD looks too tight	Avidity (dense surface / aggregated analyte)	Lower density; SEC-purify analyte
No binding at pH 6.0	FcRn not intact heterodimer / buffer pH drifted	Verify lot; re-make Buffer A, check pH meter
High baseline drift	Poor regeneration / non-specific binding	Tighten regeneration; add BSA to running buffer
pH 7.4 release too fast to measure	High flow + fast off-rate	Lower flow during release; increase Rmax slightly (keep <100 RU)

Roopenian DC, Akilesh S. FcRn: the neonatal Fc receptor comes of age. *Nat Rev Immunol.* 2007;7(9):715–725.
Raghavan M, Bonagura VR, Morrison SL, Bjorkman PJ. Analysis of the pH dependence of the neonatal Fc receptor/immunoglobulin G interaction using antibody and receptor variants. *Biochemistry.* 1995;34(45):14649–14657.
Vaccaro C, Zhou J, Ober RJ, Ward ES. Engineering the Fc region of immunoglobulin G to modulate in vivo antibody levels. *Nat Biotechnol.* 2005;23(10):1283–1288.

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