IMAC Purification of His6-Tagged Recombinant Protein from E. coli BL21(DE3) Soluble Lysate

Three independent purifications (biological replicates) from separate 1 L expression cultures. Each lysate is split for matched +/- IPTG induction controls. Imidazole is applied as a step gradient (lysis/wash/elution = 10/40/250 mM) with a 4-fraction elution series (2 mL each) to localize the protein. Samples are drawn at every stage (whole-cell, soluble, insoluble, flow-through, wash, each elution) for a 9-lane SDS-PAGE balance sheet. All steps are performed at 4 °C to limit proteolysis. Yield is quantified by A280 (NanoDrop) using the computed extinction coefficient.

BSL-1 (E. coli BL21(DE3), non-pathogenic K-12-derived). Hazards: PMSF is acutely toxic and a serine-protease inhibitor — weigh in a fume hood, wear nitrile gloves; imidazole is an irritant; nickel salts are sensitizers/possible carcinogens — avoid skin contact and aerosols. Sonication requires hearing protection and ice. PPE: lab coat, nitrile gloves, safety glasses. Decontaminate all bacterial waste by autoclaving or 10% bleach (30 min) before disposal; collect nickel-containing buffers as chemical waste.

Negative control: matched uninduced (no IPTG) culture lysate processed in parallel — should show no enriched target band in elution. Process control: insoluble pellet fraction run on SDS-PAGE to confirm the protein partitions to the soluble fraction rather than inclusion bodies. Resin-specificity control: a mock purification of untransformed BL21(DE3) lysate to identify host proteins that bind Ni-NTA nonspecifically (common ~His-rich contaminants: SlyD, ArnA). Buffer blank for A280 baseline.

A dominant Coomassie band at the predicted MW with >90% densitometric purity in pooled elution fractions; the bulk of target eluting in elution fractions 1-2. Typical yield 2-10 mg per liter for a well-behaved soluble protein. A280-derived concentration in the 1-5 mg/mL range post-concentration. Flow-through and wash lanes should show minimal target; uninduced control elution should be essentially blank at the target MW.

• E. coli BL21(DE3) transformed with pET-28a(+) encoding the N-His6 target ORF
• LB-Kanamycin (50 µg/mL) medium; IPTG (isopropyl-β-D-thiogalactopyranoside), 1 M stock, final 0.5 mM
• Lysis buffer: 50 mM NaH2PO4 pH 8.0, 300 mM NaCl, 10 mM imidazole, 1 mM PMSF, 1 mg/mL lysozyme, 1× EDTA-free protease inhibitor cocktail, 5 U/mL benzonase
• Wash buffer: as lysis but 40 mM imidazole, no lysozyme/PMSF
• Elution buffer: as lysis but 250 mM imidazole
• Ni-NTA agarose (e.g. Qiagen 30210), 1 mL bed volume per 50 mL lysate
• 4-20% Tris-glycine precast SDS-PAGE gels; Laemmli 4× sample buffer with 50 mM DTT; PageRuler Plus prestained ladder
• Coomassie R-250 stain; 30 kDa MWCO centrifugal concentrator (Amicon Ultra-15)

To isolate a single recombinant protein bearing an N-terminal His6 tag from the soluble fraction of an E. coli BL21(DE3) lysate using immobilized metal affinity chromatography (IMAC) on Ni-NTA resin. The objective is to obtain electrophoretically pure (>90% by SDS-PAGE densitometry), endotoxin-tolerant, concentration-defined protein suitable for kinetics, SPR, DSF, and SEC. Purity, identity, and yield are the explicit deliverables of this protocol.

1. Inoculate 1 L LB-Kan with a 1:100 overnight culture; grow at 37 °C, 200 rpm to OD600 0.6-0.8.
2. Induce with 0.5 mM IPTG; shift to 18 °C and express 16-18 h at 200 rpm.
3. Harvest by centrifugation 6,000 × g, 15 min, 4 °C. Resuspend pellet in 30 mL ice-cold lysis buffer.
4. Lyse by sonication on ice (10 × 15 s pulses, 50% amplitude, 45 s rest) or two passes through a microfluidizer at 18,000 psi.
5. Clarify lysate at 30,000 × g, 40 min, 4 °C. Retain soluble supernatant; save a pellet aliquot.
6. Equilibrate 1 mL Ni-NTA with 10 column volumes (CV) lysis buffer. Add clarified lysate; batch-bind by end-over-end rotation 1 h at 4 °C.
7. Pack the slurry into a gravity column; collect flow-through. Wash with 20 CV wash buffer (40 mM imidazole).
8. Elute in 4 × 2 mL fractions of elution buffer (250 mM imidazole); collect each separately on ice.
9. Read A280 of each fraction; pool peak fractions. Buffer-exchange/concentrate into storage buffer (25 mM HEPES pH 7.5, 150 mM NaCl, 1 mM TCEP) using the Amicon concentrator.
10. Run 5 µg per lane on SDS-PAGE; stain with Coomassie; image and quantify purity by densitometry.

Independent: imidazole concentration at bind/wash/elute steps; induction state (+/- IPTG); expression temperature. Dependent: bound vs. flow-through protein, elution peak position, final purity (% by densitometry), and yield (mg/L). Controlled: NaCl (300 mM throughout binding/wash to suppress ionic nonspecific binding), pH 8.0, temperature (4 °C), resin bed volume, lysate-to-resin ratio.

A correctly folded His6-tagged protein expressed in the soluble fraction will bind Ni-NTA resin via its polyhistidine tag at low imidazole concentration (10-20 mM) and elute as a discrete peak at 200-250 mM imidazole, yielding a dominant band at the predicted molecular weight on SDS-PAGE with minimal co-eluting contaminants.

Concentration computed from A280 and the ProtParam-derived molar extinction coefficient (ε280) and MW: c (mg/mL) = A280 × MW / ε280. SDS-PAGE gels imaged on a documentation system; lane densitometry in ImageJ (gel analysis tool) to compute target band area as a percentage of total lane signal = purity. A mass-balance table tracks total protein (Bradford or A280) across lysate/flow-through/wash/elution to compute % recovery.

1. No target in elution, all in flow-through: tag inaccessible or resin overloaded — reduce wash imidazole to 20 mM, increase resin, confirm tag orientation. 2. Protein in insoluble pellet only: inclusion bodies — lower induction temp to 16 °C, reduce IPTG to 0.1 mM, or refold. 3. Multiple contaminant bands co-elute: raise wash imidazole to 40-50 mM and NaCl to 500 mM; add a SEC polishing step. 4. Low A280 yield despite visible band: verify ε280 (low-Trp proteins read poorly) — use BCA assay. 5. Smearing/degradation bands: add fresh PMSF + EDTA-free inhibitors, keep everything at 4 °C, minimize processing time.

Yield (mg/L) reported as mean ± SD across n=3 independent purifications. Purity values (% target) reported as mean ± SD. Where induction conditions are compared, a two-tailed unpaired t-test (α=0.05) on yield is used; with >2 conditions, one-way ANOVA with Tukey HSD correction. n=3 biological replicates gives ~80% power to detect a 2-fold yield difference at typical CV ~25%.