= RNA melting temperature
{c}
{title2=$T_m$}
An \b[RNA melting temperature], $T_m$, characterizes the midpoint of a specified RNA conformational transition under stated conditions. In a reversible, unimolecular <two-state RNA melting model>, it is the temperature at which folded and unfolded populations are equal. A midpoint assigned to an optical trace requires a model connecting <absorbance> to those populations.
Let unfolding free energy be $\Delta G_u=G_U-G_F$. If unfolding enthalpy and entropy are treated as temperature independent,
$$
\Delta G_u(T)=\Delta H_u-T\Delta S_u,\qquad
T_m=\frac{\Delta H_u}{\Delta S_u}.
$$
The formula assumes a finite relevant transition and uses absolute <temperature>. With original stipulated values $\Delta H_u=100\,\mathrm{kJ\,mol^{-1}}$ and $\Delta S_u=300\,\mathrm{J\,mol^{-1}\,K^{-1}}$, $T_m\approx333.3\,\mathrm K$. Holding enthalpy fixed but changing entropy to $310\,\mathrm{J\,mol^{-1}\,K^{-1}}$ gives approximately $322.6\,\mathrm K$. A melting temperature reflects both energetic and entropic contributions.
Molecularity matters. An ideal intramolecular hairpin transition need not change its midpoint with total strand concentration. A transition involving association of separate strands generally does: the equilibrium relation includes their concentrations. The <concentration dependence of RNA melting> can therefore help test which structural model is appropriate. In the 2010 tetraloop measurements, concentration-independent curves across a broad concentration range supported stem–loop formation.<Thermodynamic characterization of naturally occurring RNA tetraloops by Justin P. Sheehy and others>[Materials and Methods, repeated melts and concentration dependence.]{ref}
A raw halfway absorbance is not automatically this thermodynamic midpoint. If the folded and unfolded <RNA melting baselines> at one temperature are $0.55$ and $0.75$, an absorbance of $0.60$ implies unfolded fraction $0.25$, not $0.50$. Intermediates, temperature-dependent parameters or <RNA melting hysteresis> can further complicate midpoint assignment. Report the solution conditions and definition of the measured transition rather than treating $T_m$ as a context-free constant of an RNA sequence.
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