How is this different from the Wallace-rule tool?

The Wallace rule (2×AT + 4×GC) ignores sequence context, salt and concentration. This model uses experimentally derived nearest-neighbour parameters and corrects for salt and oligo concentration, so it is more accurate — particularly for longer primers.

Why does primer concentration affect Tm?

Tm is the temperature at which half the strands are hybridised, which depends on how many strands are present. The C_T term in the equation captures that concentration dependence.

Nearest-neighbour melting temperature

A more accurate primer Tm than the Wallace rule, using SantaLucia 1998 nearest-neighbour thermodynamics with salt and primer-concentration corrections.

How it works

Formula

Tm = ΔH° ÷ (ΔS° + R·ln(C_T/x)) − 273.15, where ΔH° and ΔS° are summed over nearest-neighbour dinucleotides plus initiation (and a symmetry term for palindromes). A salt correction ΔS += 0.368·(N−1)·ln[Na⁺] is applied; R = 1.987 cal/mol·K, C_T is primer concentration, x = 4 (or 1 if self-complementary).

Worked example

For ATCGATCG at 50 mM Na⁺ and 250 nM primer: ΔH° = −57.8 kcal/mol and (salt-corrected) ΔS° = −168.2 cal/mol·K give Tm ≈ 14.2 °C — a realistic, low value for an 8-mer.

When to use it

For designing PCR and qPCR primers, especially longer oligos (≳14 nt) or when matching primer pairs, where the Wallace 2+4 rule is too rough. The Wallace tool remains useful for a quick desk estimate on very short oligos.

Sensible defaults

Defaults use 50 mM monovalent salt and 250 nM primer — typical PCR conditions. Adjust to your reaction’s salt and primer concentrations, which both shift Tm.

Source

Curated, not exhaustive. Nearest-neighbour parameters: SantaLucia (1998) PNAS 95:1460–1465 unified set; salt correction per the same work. Values are reproduced from the standard published table — verify against the primary source for critical design.

FAQ

How is this different from the Wallace-rule tool?: The Wallace rule (2×AT + 4×GC) ignores sequence context, salt and concentration. This model uses experimentally derived nearest-neighbour parameters and corrects for salt and oligo concentration, so it is more accurate — particularly for longer primers.
Why does primer concentration affect Tm?: Tm is the temperature at which half the strands are hybridised, which depends on how many strands are present. The C_T term in the equation captures that concentration dependence.