Primer ΔG Thresholds for Hairpin and Dimer Risk
Need to know whether a hairpin at -3.2 kcal/mol is acceptable, or whether a 3' dimer at -8 kcal/mol is likely to break your assay? This page turns abstract ΔG numbers into practical thresholds for hairpins, self-dimers, and cross-dimers so you can judge primer risk before ordering or troubleshooting a failed PCR.
Use This Page After You Have a ΔG Result
Have a sequence but no ΔG output yet? Run the Secondary Structure Predictor first, using your annealing or assay temperature. Have a hairpin, self-dimer, hetero-dimer, or 3' end dimer ΔG value already? Use this reference page to decide whether the result is safe, borderline, or a redesign trigger.
Primer Structure Risk Check
Paste a primer sequence to compare its predicted hairpin and self-dimer ΔG values with the threshold bands below.
ΔG Threshold Reference Database
Threshold bands are practical screening ranges compiled from common primer-analysis tools and thermodynamic primer-design conventions. Use them as a design screen, not as an experimental pass/fail guarantee.
| Structure Type | Safe (A) | Caution (B) | Warning (C) | Critical (D) | PCR Impact |
|---|---|---|---|---|---|
| Hairpin | > -2.0 | -2.0 to -3.0 | -3.0 to -5.0 | < -5.0 | Can reduce target binding through self-folding |
| Self-Dimer | > -5.0 | -5.0 to -6.0 | -6.0 to -8.0 | < -8.0 | Can consume available primer |
| 3' End Dimer | > -5.0 | -5.0 to -6.0 | -6.0 to -7.0 | < -7.0 | Can be extended by polymerase and create artifacts |
| Cross-Dimer | > -6.0 | -6.0 to -7.0 | -7.0 to -8.0 | < -8.0 | Can allow primer pairs to bind each other |
All ΔG values are shown in kcal/mol at 37°C, 50 mM Na⁺. Review the actual annealing temperature and 3' end involvement before deciding whether a primer needs redesign.
ΔG Risk Bands
Decision: When to Redesign vs Optimize
Optimize conditions for Grade B-C
- Add DMSO (3-5%) to destabilize structures
- Raise annealing temperature above structure Tm
- Use hot-start polymerase
- Increase primer concentration
Redesign for Grade D
- Shift binding site 2-5 nt
- Shorten primer (maintain Tm >55°C)
- Break complementary stem with mismatches
- Avoid >3 consecutive G/C at 3' end