Primer dimer & hairpin checker
Maintained by the Talindrew team · Last updated
A primer that is complementary to itself forms dimers or hairpins instead of binding template. This check slides the oligo against its own reverse complement and reports the longest self-complementary run, the longest run anchored at the 3′ end (the dangerous kind: polymerase extends it), and any hairpin with a stem of four or more pairs.
Result
Sliding self-alignment scan (OligoCalc approach, Kibbe 2007). Runs ≥ 4 bp, especially 3′-anchored ones, risk primer-dimers; ΔG-based scoring like IDT OligoAnalyzer's is on the roadmap. Verify borderline oligos there.
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What it looks like
Formula
Slide seq vs revcomp(seq) at every offset → longest complementary run; hairpin: inverted repeat, stem ≥ 4, loop ≥ 3
Source: Sliding self-alignment as in OligoCalc (Kibbe, NAR 2007)
Worked example
Given: 8-mer GGGATCCC (BamHI site with G/C padding)
- 1.Reverse complement of GGGATCCC is GGGATCCC: the oligo is a palindrome.
- 2.Every base pairs in the self-alignment: an 8/8 self-dimer, 3′-anchored.
- 3.Two such oligos anneal to each other completely: as a primer this sequence would dimerise, not prime.
Palindrome: fully self-complementary (8 bp run)
How the calculation flows
Units & constants
| Self-dimer threshold | Runs ≥ 4 bp worth attention; ≥ 6 bp significant |
|---|---|
| 3′-anchored runs | Most dangerous: extendable by polymerase into artefacts |
| Hairpin criteria | Stem ≥ 4 bp, loop ≥ 3 nt (minimum stable loop) |
| Method | Sequence alignment scan (v1); ΔG-based NN scoring on the roadmap |
| Cross-check | IDT OligoAnalyzer hairpin/self-dimer for thermodynamic ΔG values |
| Price | Free |
Why are 3′ self-complementary ends the worst?
A dimer paired anywhere else is an equilibrium nuisance; a dimer paired at the 3′ end is a substrate. The polymerase extends it, converting two primers into a stable double-stranded artefact that amplifies exponentially and consumes primer, the classic 'primer-dimer' band at the gel front. Design rule: avoid 3 or more bases of self-complementarity within the last 5 nucleotides.
What does a hairpin do to a primer?
An intramolecular stem-loop competes with template binding, effectively lowering the primer's available concentration and its apparent Tm. Stems of ≤ 3 bp are usually harmless at annealing temperatures; stems of 4+ closing near the 3′ end can kill priming outright. Sequence-level scanning finds the candidates; a ΔG calculation (IDT OligoAnalyzer) tells you whether they are stable at your annealing temperature.
When is self-complementarity intentional?
Palindromes are how restriction sites work, GGATCC (BamHI) reads the same on both strands, and annealed-oligo cloning deliberately orders two complementary oligos. Molecular beacons rely on engineered hairpins. The check flags the physics; whether it is a bug or the design depends on what the oligo is for.
Frequently asked questions
Not automatically. A 4 bp internal run is common and usually harmless. Redesign when the run is ≥ 6 bp, sits at the 3′ end, or IDT OligoAnalyzer reports a self-dimer ΔG more negative than about −9 kcal/mol.
Version 1 is a sequence-level scan (the OligoCalc approach): fast, dependency-free and right about which stretches can pair. Thermodynamic scoring of those stretches with nearest-neighbor ΔG, what IDT does, is on the roadmap; until then, use the linked OligoAnalyzer for borderline calls.
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