The Nearest-Neighbor Thermodynamic Model
The nearest-neighbor (NN) model treats DNA duplex stability as the sum of stacking interactions between adjacent base pairs. Each of the 10 unique dinucleotide combinations has experimentally measured enthalpy (ΔH) and entropy (ΔS) values.
The melting temperature is then: Tm = ΔH / (ΔS + R × ln(Ct/4)) − 273.15, where R is the gas constant and Ct is the total strand concentration.
This model was unified by SantaLucia in 1998, reconciling parameters from seven independent labs. It remains the standard method used by virtually all primer design software.
Salt Concentration Effects on Tm
Cations stabilize DNA duplexes by neutralizing the negative charges on phosphate groups. Higher salt concentration raises Tm. The effect is sequence-dependent — GC-rich sequences are less affected than AT-rich ones.
The Owczarzy (2004) correction accounts for both the overall salt effect and the GC-dependent component: 1/Tm(salt) = 1/Tm(1M) + (4.29·f(GC) − 3.95)×10−5·ln[Na⁺] + 9.40×10−6·(ln[Na⁺])²
Mg²⁺ in PCR buffers has a larger stabilizing effect than Na⁺. Most PCR buffers contain 1.5-2.5 mM MgCl₂, which can raise Tm by 5-10°C compared to Na⁺-only predictions.
Primer Design Guidelines
Length: 18-25 nucleotides is optimal. Shorter primers lack specificity; longer primers anneal more slowly.
GC content: 40-60% provides balanced stability. Extreme GC content makes primers either too weak (low GC) or prone to secondary structures (high GC).
GC clamp: End the 3′ end with 1-2 G/C bases for stable priming.
Tm matching: For a primer pair, keep Tm within 2°C of each other. An annealing temperature 3-5°C below the lower Tm typically works well.
Avoid: Runs of 4+ identical bases, palindromes >4 bp, 3′ complementarity between primers (causes primer-dimers).
