Melting Temperature Calculator

The melting temperature of an oligonucleotide is the temperature at which half of the duplex population has dissociated into single strands. Three levels of model are in common use. The Wallace rule, Tm = 2(A+T) + 4(G+C), assigns a flat 2 degrees C per AT pair and 4 degrees C per GC pair and is only defensible for probes under about 14 bases in 1 M sodium. The salt-adjusted GC formula, Tm = 64.9 + 41(G + C - 16.4)/N, scales with composition and length but still ignores sequence order. Nearest-neighbor thermodynamics does not: it sums stacking enthalpies and entropies over the ten unique dinucleotide steps, so that a CG step contributes -10.6 kcal/mol of enthalpy while an AA step contributes only -7.9, then solves Tm = dH x 1000 / (dS + R ln(Ct/4)) - 273.15 with R = 1.987 cal/mol/K. The factor Ct/4 applies to non-self-complementary duplexes where one strand is in excess. Because entropy depends strongly on counterion screening of the phosphate backbone, a salt correction is applied on top. The nearest-neighbor parameters are referenced to 1 M sodium, and the Owczarzy 2004 correction used here adjusts the reciprocal of Tm in kelvin rather than Tm itself, adding a term in ln[Na+] scaled by GC fraction and a second term in (ln[Na+]) squared. Applying those coefficients directly to a Celsius temperature is a common implementation error and produces predictions far below any real primer. Practitioners design PCR primers of 18 to 25 bases with 40 to 60 percent GC and Tm between 55 and 65 degrees C, keep the two primers of a pair within about 5 degrees C of each other, and set annealing roughly 3 to 5 degrees C below the lower Tm. The main caveats are that the model assumes clean two-state melting with no mismatches, hairpins or primer dimers, and that a monovalent-only correction ignores the magnesium and dNTPs present in real PCR buffers, which raise effective Tm.

Quantitative DNA melting begins with Julius Marmur and Paul Doty, who reported in 1962 that the midpoint of the hyperchromic absorbance transition at 260 nm rises linearly with guanine-plus-cytosine content, giving the first usable relation between composition and duplex stability. That work was aimed at whole genomic DNA in solution rather than short synthetic probes. When chemically synthesized oligonucleotides became routine hybridization tools in the 1970s, a simpler rule of thumb was needed, and R. Bruce Wallace and colleagues published the 2 degrees C per AT, 4 degrees C per GC approximation in Nucleic Acids Research in 1979 from experiments hybridizing short probes to phage phi X174 DNA. The rule of thumb broke down as probes grew longer and applications demanded precision. Kenneth Breslauer, Ronald Frank, Helmut Blocker and Luis Marky published the first widely used nearest-neighbor stacking parameter set in PNAS in 1986, treating stability as a sum over adjacent base pairs rather than a base count. Competing parameter sets accumulated through the 1990s until John SantaLucia Jr reconciled them in a 1998 PNAS paper, whose unified values remain the standard reference and are the ones used here. Richard Owczarzy and colleagues at Integrated DNA Technologies then published improved sodium-dependence corrections in Biochemistry in 2004 and a magnesium treatment in 2008, completing the buffer-aware form of the calculation now built into most primer design software.