Tm = 81.5 + 16.6 * Log10[Na+eq] + 0.41 * (%GC) - 600/L
where [Na+eq] is the equivalent sodium molar concentration, (%GC) is the percent of G/C in the primer, and L is the primer length (Sambrook et al., 1989). This method is widely used in many computer programs, partially because it is relatively easy to implement. Despite its simplicity, Tm values calculated in this way usually have reasonable accuracy and are only a few degrees different from empirically determined Tm values. In addition, this method is considered to be the most accurate way to calculate Tm for very long DNA duplex, such as PCR amplicon.
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