Long Oligos (100+ Bases): Chemistry and Support Changes

This article lists the specific chemistry and support changes that improve full-length yield on oligos of roughly 100 bases and longer. It assumes your cycle already runs well at shorter lengths. For the underlying reasons long syntheses are less forgiving, including moisture control and capping performance, see Synthesizing Long Oligos.

Applies to: All K&A H-series and Shasta synthesizers. DNA and RNA chemistry.

Before you change anything

Confirm the instrument is clean at short lengths. Run a 20-mer or your standard test sequence and check crude purity. A cycle losing efficiency at 20 bases will not be rescued by any change below.
Confirm moisture control. Dry acetonitrile, sealed bottles, dry inert gas, and a room below 50% RH. Moisture is the most common cause of long-oligo failure and it costs nothing to rule out first.
Record a baseline. Note the trityl trace shape and the crude full-length percentage before you change anything, so you can tell whether a change helped.

NOTICE: Changing several variables at once makes the result uninterpretable. Make one change, run the same test sequence, and compare against your baseline before making the next change.

Why small efficiency gains matter this much

Full-length product (FLP) is the stepwise coupling efficiency raised to the power of the number of couplings. At 100 bases and beyond, differences that are invisible on a 25-mer dominate the result.

Going from 99.0% to 99.5% roughly doubles the yield of a 150-mer. That is the entire argument for the changes below.

1) Switch to dG(ibu) amidite

Use isobutyryl-protected dG, written dG(ibu), rather than dimethylformamidine-protected dG, written dG(dmf), for long sequences.

The earliest bases in a long synthesis sit through every remaining detritylation cycle. A 150-mer exposes the first residue to 150 acid treatments, and depurination accumulates across all of them.
dG(ibu) is the more robust choice under that cumulative acid exposure and deprotects cleanly under standard ammonia conditions.
Check your deprotection conditions when you switch. dG(ibu) and dG(dmf) are not interchangeable in deprotection time and temperature. See Oligo Deprotection Guide (DNA and RNA).

2) Deblock with 3% DCA instead of TCA

Replace trichloroacetic acid (TCA) with 3% dichloroacetic acid (DCA) for detritylation on long syntheses. DCA is the milder acid and produces less depurination over the large number of cycles a long oligo requires.

WARNING: DCA and TCA solutions are corrosive and cause skin and eye burns. Contact with the eyes can cause permanent damage. Wear chemical splash goggles and nitrile gloves and handle in a fume hood when preparing or changing deblock reagent.

Because DCA is milder, detritylation can require more contact time to run to completion. Incomplete detritylation produces deletion impurities, so verify against the trityl trace rather than assuming the existing timing carries over.
Confirm the solvent your deblock reagent is supplied in and keep it consistent across runs.

3) Add a second coupling past base 100

DNA: add a second coupling, or additional coupling repetitions or contact time, from approximately base 100 onward.
RNA: start earlier, from approximately base 50. The 2' protecting group on RNA amidites is sterically demanding and RNA couples less efficiently than DNA from the first cycle.
Double coupling consumes roughly twice the amidite over the affected region. Confirm bottle volumes will carry the run before starting.

NOTE: [PLACEHOLDER: insert the exact control names and screen locations for setting per-base coupling repeats on the Shasta and in KA_Labs, quoted verbatim from the current software.]

4) Move to a larger pore support

Use 3000Å CPG, or larger where available, for oligos of 100 bases and above.
2000Å CPG is generally adequate below 100 bases.
Larger pore supports carry lower loading capacity and are more fragile. Expect less material per column and handle columns carefully.

5) Consider non-porous polystyrene supports

Non-porous polystyrene beads remove pore steric hindrance entirely, since all synthesis occurs on the outer surface.
The tradeoff is much lower surface area, and therefore lower yield per column, than a porous support at the same column size.
This is worth testing when a sequence fails on 3000Å CPG despite good chemistry, and when purity matters more than quantity.

6) Check scale and loading density

Long-oligo performance is scale dependent, and larger is not better.

Higher loading density crowds the growing strands and reduces effective coupling as length increases.
Smaller scales give each strand more effective reagent equivalents and often perform better at long lengths.

If long oligos fail at your normal scale, run the same sequence at a smaller scale or on a lower-loading support before changing anything else in the cycle.

Verify the change worked

Run the same test sequence you used for your baseline and compare all three of the following. A change that improves one and degrades another has not helped.

Trityl trace. Compare the slope of the trace across the run, not just the final value. A trace that falls away in the last third points to a coupling or moisture problem rather than a support problem.
Crude full-length percentage. Analyze the crude by HPLC and compare the FLP peak area against your baseline run.
Deletion profile. Check whether the n-1 family shrank. Yield can rise while purity falls if capping is weak.

NOTE: Above roughly 60 to 70 bases, RP-HPLC no longer resolves n-1 from full-length product. For oligos in the range this article covers, plan on PAGE purification, or a DMT-on strategy, when purity is critical. See Purification Methods for Oligonucleotides.

Troubleshooting

Symptom	Likely cause	Resolution
Trityl trace falls away steadily after the first third of the run	Coupling efficiency decaying, usually moisture uptake or amidite age	Replace acetonitrile and amidite solutions, verify inert gas dryness, then re-run
Yield acceptable but heavy n-1 and n-2 ladder	Incomplete capping, or incomplete detritylation after the switch to DCA	Increase capping contact time and volume, verify detritylation runs to completion
Depurination products and low recovery, worse on G-rich sequences	Cumulative acid exposure	Confirm dG(ibu) is in use and the deblock is 3% DCA, and shorten detritylation to the minimum that completes
Performance falls off only above about base 100	Steric crowding on the support	Move to 3000Å CPG and reduce loading density or scale
Results were fine, then degraded after the instrument sat idle	Moisture equilibration in lines and reagents	Purge and condition the fluid path and refresh reagents before the next long run

Do not force a single ultra-long oligo unless you need one

If the goal is a gene-length construct rather than one continuous oligo, it is usually more reliable to synthesize shorter oligos your system makes well and assemble them into longer DNA by PCR tiling, ligation, and cloning. That keeps the chemistry in the range where it performs best and gives you a sequence-verified plasmid as a stable source afterward.

See Assembling Long DNA from Shorter Oligos (PCR Tiling and Cloning).

Questions on a specific sequence or instrument, contact support@sierrabio.com.

Last reviewed: 2026-09-03. Owner: Field Operations. Applies to all K&A H-series and Shasta synthesizers.