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Oligonucleotide Synthesis Methods
Modern oligonucleotide synthesis builds DNA sequences one nucleotide at a time on a solid support. Understanding the chemistry and platforms helps you interpret quality metrics, choose vendors, and design synthesis-compatible sequences.
Phosphoramidite Chemistry: The Foundation
Nearly all commercial oligonucleotide synthesis uses phosphoramidite chemistry, developed by Marvin Caruthers in the early 1980s. The process proceeds in a repeating cycle of four chemical steps, building the oligo from 3' to 5' (opposite to biological DNA synthesis).
1. Detritylation (Deprotection)
Remove the DMT (dimethoxytrityl) protecting group from the 5'-hydroxyl of the last added nucleotide, exposing it for the next coupling. Uses trichloroacetic acid (TCA) in dichloromethane. The orange DMT cation released is measured photometrically to monitor coupling efficiency.
2. Coupling
The next phosphoramidite monomer (A, T, G, or C) is activated with tetrazole and reacts with the free 5'-hydroxyl. This is the critical step — coupling efficiency is typically 98.5-99.5% per step. The unreacted fraction becomes truncation products.
3. Capping
Unreacted 5'-hydroxyl groups (those that failed to couple) are acetylated with acetic anhydride. This prevents them from coupling in subsequent cycles, which would create deletion sequences. Capped sequences are eventually washed away.
4. Oxidation
The phosphite triester linkage from coupling is oxidized to a stable phosphotriester (or phosphorothioate for modified oligos) using iodine/water/pyridine. This step creates the final phosphodiester backbone.
Why coupling efficiency matters: At 99% coupling efficiency per step, a 20-mer has 0.99²⁰ = 82% full-length product. A 100-mer drops to 0.99¹⁰⁰ = 37%. At 99.5%, the 100-mer improves to 61%. This exponential relationship is why longer oligos are harder to synthesize and require higher purification grades.
Column-Based vs Array-Based Synthesis
There are two major platforms for oligonucleotide synthesis, each with different trade-offs between quality, scale, and cost.
| Parameter | Column-Based | Array-Based |
|---|---|---|
| Coupling efficiency | 99-99.5% | 98-99% |
| Max practical length | 100-150 nt | 200-300 nt |
| Oligos per run | 1-96 (per plate) | 10,000-1,000,000 |
| Yield per oligo | nmol to µmol | fmol per sequence |
| Cost per oligo | $0.10-0.50/base | $0.001-0.01/base |
| Delivery format | Individual tubes/plates | Pooled mixture |
| Sequence error rate | ~1 in 500 bases | ~1 in 200 bases |
| Modifications | Full catalog available | Limited |
| Typical vendors | IDT, Eurofins, Sigma | Twist, Agilent, CustomArray |
| Best for | PCR primers, probes, individual oligos | Oligo pools, CRISPR libraries, gene synthesis |
When to Use Column Synthesis
- You need individual oligos (PCR primers, probes)
- You need modifications (biotin, fluorophores, phosphorothioates)
- Quantity matters (µmol scale for in vivo experiments)
- Maximum sequence accuracy is critical
When to Use Array Synthesis
- You need thousands of different sequences (CRISPR libraries)
- Cost per sequence is the primary concern
- Pooled delivery is acceptable (library screening)
- You can tolerate higher error rates (screen-based assays)
Designing Synthesis-Compatible Sequences
Knowledge of synthesis limitations helps you design oligos that synthesize cleanly:
- Avoid long homopolymer runs: Stretches of 4+ identical bases (especially poly-G and poly-C) reduce coupling efficiency and can form secondary structures on the solid support.
- Keep GC content moderate: 30-65% GC is optimal for synthesis. GC-extreme sequences have lower yield and higher error rates.
- Watch for palindromes: Internal palindromic sequences can form hairpins during synthesis, leading to truncation products.
- Consider sequence length: For column synthesis, stay under 100 nt when possible. For array synthesis, maximum recommended lengths are typically 150-200 nt.
- Request appropriate purification: Desalting is fine for PCR primers. HPLC or PAGE purification is recommended for sequences >60 nt, cloning inserts, or probes where purity matters.