Resin to pen
in one building.

A campaign starts with the C-terminal residue anchored to the solid support. We load 2-chlorotrityl chloride resin at a deliberately low substitution — around 0.4 mmol/g rather than the 0.8 the supplier certifies — because a crowded resin makes the growing chains aggregate and every coupling after residue twelve gets harder. Loading is measured by Fmoc release before assembly begins, so the scale of the batch is a number we have verified rather than assumed.

Support

2-CTC / Rink amide

Target load

0.35–0.45 mmol/g

Load check

Fmoc UV @ 301 nm

Fmoc solid-phase synthesis, residue by residue, from the C-terminus back. Each cycle is a piperidine deprotection, a wash, a DIC/Oxyma coupling held until a ninhydrin or chloranil test comes back negative, and an acetic anhydride cap. The cap is the part that matters: any chain that failed to couple is terminated on the spot, so it shows up later as a short, well-separated peak instead of a deletion sequence hiding under the product. Arginine and histidine positions get a double coupling; aspartimide-prone Asp-Gly motifs are run with a backbone-protected building block and 0.1 M Oxyma in the deprotection to keep the base from doing damage.

Coupling

DIC / Oxyma, 4 eq

Deprotection

20% piperidine, 2 × 5 min

Capping

Ac₂O after every cycle

The peptide is released from the resin and stripped of its side-chain protecting groups in one step, using TFA with water, TIPS and — where cysteine or methionine is present — EDT or thioanisole to catch the reactive cations before they alkylate the product. Two to three hours at room temperature, then the filtrate is dripped into cold diethyl ether. The crude precipitates, is centrifuged, washed twice more with ether to pull the scavengers out, and dried. What comes off the frit is a white powder that is typically 60–75% the peptide we want.

Cocktail

TFA 95 / H₂O 2.5 / TIPS 2.5

Duration

2–3 h, ambient

Recovery

Cold ether, 3 washes

The crude is dissolved, filtered at 0.45 µm and run over a preparative C18 column with a shallow acetonitrile gradient in 0.1% TFA — typically half a percent of organic per minute across the window where the product elutes, because a shallow gradient is the only thing that separates a peptide from its own deletion sequences. Fractions are collected on UV threshold and every one of them is checked by analytical HPLC before anything is pooled. We take the apex and let the shoulders go to waste, which is why our yields are unremarkable and our traces are not. A second pass is run whenever the first pool sits below 98%.

Column

C18, 10 µm, 50 × 250 mm

Gradient

0.5% MeCN/min, 0.1% TFA

Pooling

Apex only, fraction-verified

Pooled fractions are diluted to drop the organic content, shell-frozen at −45 °C and dried over a 34-hour cycle: primary drying under 0.08 mbar with the shelf held below the collapse temperature, then a secondary ramp to +25 °C to pull the bound water down. Rushing this stage is how you end up with a collapsed cake and 8% residual moisture, so the cycle length is fixed for every batch regardless of how small it is. The dried powder is subdivided under nitrogen into the retain sample and the charge the pens are filled from — a pen is a presentation of a lot that has been released as powder first, not a shortcut around it.

Freeze

−45 °C shell

Primary drying

0.08 mbar, 26 h

Secondary

Ramp to +25 °C, 8 h

Six assays per lot, run on samples drawn from the finished powder rather than from the purification pool: RP-HPLC area purity, ESI-MS identity, net peptide content by nitrogen, Karl Fischer moisture, residual TFA by ion chromatography, and a kinetic chromogenic LAL endotoxin screen. Every result is entered against the lot number, reviewed by a second analyst, and either signed off or sent back. A lot that misses specification is destroyed with the reason recorded — it is never blended into a passing batch, and nothing is filled into a pen before it has been signed off here.

Assays

6, no exceptions

Review

Second-analyst sign-off

Out of spec

Destroyed, campaign repeated

The released powder is dissolved into its buffered vehicle at the concentration the pen declares, mixed until it goes optically clear, and checked for pH and appearance before anything is committed. The bulk is then filtered at 0.22 µm straight into the filling vessel, and the filter is integrity-tested after the run as well as before it — a filter that fails on the downstream test invalidates the batch, which is the only way that test tells you anything. Compounded bulk is held cold and filled the same shift; it is never carried over to the next one.

Vehicle

Buffered, pH verified

Filtration

0.22 µm, integrity-tested

Bulk hold

≤ 24 h at 2–8 °C

Cartridges are filled gravimetrically under argon, weighed at the start, middle and end of the run against the fill the barrel declares, and the run is stopped rather than corrected if a check drifts outside the window printed on the certificate. Each cartridge is stoppered, crimped and leak-checked, then assembled into the pen body, capped and closed with a tamper seal. Labels are printed from the lot record, so the sequence, the fill and the lot number on the barrel come from the same row as the numbers on the certificate, and every pen is labelled for laboratory research use only. Pens go out one to a carton against a single lot, on cold packs, so a pen and its certificate never come apart. A retain pen from every lot stays in the fridge here for the full retest period, beside the retained powder it was filled from.

Container

Type I glass cartridge

Fill check

Gravimetric, start / middle / end

Closure

Crimped, argon headspace, sealed body

Pack

One pen per carton, single lot

Cold chain

Shipped at 2–8 °C

Retain

One pen per lot, held 24 months