Primer Secondary Structure Predictor: Hairpin & Primer Dimer

Check hairpins, self-dimers, hetero-dimers, and primer dimerization risk in DNA primers and oligonucleotides at your working temperature. Use ΔG values to decide whether a structure is harmless, needs PCR optimization, or should trigger primer redesign.

Enter a DNA sequence above to begin analysis

Default: 37°C (physiological temperature)

Typical range: 10-100 mM

For PCR: typically 1.5-2.5 mM

pH note: This predictor exposes temperature plus Na+/Mg2+ settings. It does not model pH-dependent protonation effects or provide a pH 5 mode; treat results as near-neutral nearest-neighbor estimates unless you validate acid-pH behavior experimentally.

Analysis Tips

  • • Hairpins with ΔG < -9 kcal/mol may interfere with PCR amplification
  • • Self-dimers reduce effective primer concentration in PCR
  • • For primer design, avoid 3' end complementarity (primer-dimer)
  • • Higher salt concentration stabilizes secondary structures

No Analysis Yet

Enter a DNA sequence and click"Analyze Structure" to predict secondary structures.

Interpret ΔG Thresholds

Use the threshold database when you need a redesign vs. optimize decision.

Troubleshoot PCR Failure

Match gel symptoms to hairpins, self-dimers, cross-dimers, and rescue steps.

Review the Full Primer

Check Tm, GC%, molecular weight, dimers, hairpins, and mismatch effects together.

What Are DNA Secondary Structures?

Use this page when you need to check primer hairpins, self-dimers, hetero-dimers, primer-dimer risk, or ΔG thresholds. Use the Primer Analyzer when you need Tm, GC%, molecular weight, and structure checks together.

Hairpins, self-dimers, and hetero-dimers can reduce the amount of usable primer, block target binding, or create short primer-dimer products during PCR. The most important review is whether a detected structure is stable near the annealing temperature and whether any paired region includes an extendable 3' end.

Structure stability is reported as Gibbs free energy (ΔG). More negative values indicate more stable structures. For PCR primer review, hairpins near or below -3 kcal/mol, dimers near or below -5 kcal/mol, and any 3' end complementarity deserve closer review before ordering.

The predictor applies nearest-neighbor thermodynamic parameters and runs the calculation in the browser. Sequences are not uploaded to a server, which makes the page suitable for early primer checks, troubleshooting, and pre-order sequence review.

How to Use the Secondary Structure Predictor

  1. Enter your DNA sequence (10-200 nucleotides) in the input field.
  2. Set the analysis conditions: temperature (default 37°C for PCR setup) and Na⁺ concentration (default 50 mM).
  3. For hairpin and self-dimer analysis: click "Analyze" with a single sequence.
  4. For hetero-dimer analysis: select "Hetero-dimer" mode and enter both forward and reverse primer sequences.
  5. Review results: the predictor shows detected structures with ΔG values, structure diagrams, and risk levels.
  6. Pay special attention to 3' end complementarity in hetero-dimers — even weak 3' overlap can cause primer-dimer artifacts.

Frequently Asked Questions

What ΔG values indicate problematic secondary structures?
For PCR primers: hairpins with ΔG below -2 kcal/mol are concerning, and below -3 kcal/mol are problematic — they can block primer extension. Self-dimers and hetero-dimers with ΔG below -5 kcal/mol will cause significant primer-dimer artifacts, and below -6 kcal/mol they will likely dominate over target amplification. Structures with ΔG above 0 kcal/mol are thermodynamically unfavorable and will not form under your reaction conditions.
Why is 3' end complementarity especially dangerous?
When two primers have complementary 3' ends, DNA polymerase can extend from the matched 3' end, creating a primer-dimer product. Even 2-3 base pairs of 3' complementarity with a moderately negative ΔG can cause problems because: (1) extension is initiated from the 3' end, and (2) once extended, the primer-dimer product becomes a high-efficiency PCR template that exponentially amplifies. This is why we flag 3' end overlap even when overall ΔG is borderline acceptable.
How does temperature affect secondary structure stability?
Higher temperature destabilizes secondary structures by providing thermal energy to disrupt hydrogen bonds. This is why PCR uses a denaturation step (95°C) followed by a lower annealing temperature. Structure ΔG becomes less negative (less stable) as temperature increases. Our predictor calculates ΔG at your specified temperature, so you can evaluate whether a structure will persist at your actual annealing temperature. A hairpin that is stable at 37°C (ΔG = -4 kcal/mol) may be completely destabilized at 65°C.
Can I use this tool for oligo pool quality control?
Yes. For oligo pools, secondary structure analysis is critical because structures can cause synthesis failure (incomplete coupling during array-based synthesis) and functional failure (reduced target binding). Use Batch Mode to analyze up to 100 sequences for hairpins. For larger pools (1,000+ sequences), open the Batch Sequence QC tool for a faster, pool-level assessment including secondary structure risk scoring.
What is the difference between self-dimer and hetero-dimer?
A self-dimer forms between two identical copies of the same oligonucleotide — for example, two copies of your forward primer binding to each other through complementary regions. A hetero-dimer forms between two different sequences — typically your forward and reverse primers. Both types consume primers and produce spurious amplification products, but hetero-dimers with 3' end overlap are generally more problematic because the extended product can serve as a PCR template.
Why can ΔG values differ between structure tools?
Most short-oligo structure tools use nearest-neighbor thermodynamic concepts, but they may differ in loop handling, dangling-end treatment, salt correction, and how many suboptimal structures are reported. Use ΔG values as a design-screening signal, then review 3' end involvement and validate critical assays experimentally.

Related Tools

Tm Calculator

Calculate melting temperature — primers with strong secondary structures may have lower effective Tm.

Primer Analyzer

All-in-one primer analysis including Tm, GC%, self-complementarity, and structure prediction.

GC Content Analyzer

High GC content correlates with secondary structure risk. Analyze GC distribution in your sequences.

Review request

Check a result or ordering detail

Send the calculation, settings, or pool submission detail that needs a second look.

Related reading

Continue with the page or tool that matches the next decision in your experiment.

ΔG Threshold Database — When to Redesign vs OptimizeSalt Correction Method ComparisonAccuracy Validation MethodsValidate PCR Primers Before OrderingPre-Order Oligo Pool QCStructure Method Reference