= Apparent protein unfolding midpoint
An <apparent protein unfolding midpoint> is an operationally defined <temperature> marking the middle of an observed unfolding-associated transition. In a <thermal shift assay>, it is often written $T_m$ or $T_{m,\mathrm{app}}$. “Apparent” keeps the measured reporter transition distinct from an established reversible equilibrium melting point.<Evaluation of fluorescence-based thermal shift assays for hit identification in drug discovery by Mei-Chu Lo and others>[Abstract; unfolding-temperature shifts and fluorescence readout]{ref}
Consider a deliberately simple two-state reporter model. If $f_U$ is the unfolded fraction and the folded and unfolded signals are constant values $F_F,F_U$, then
$$
F=(1-f_U)F_F+f_UF_U.
$$
At $F=(F_F+F_U)/2$, algebra gives $f_U=1/2$, provided $F_U\ne F_F$. If the signal changes from $100$ to $800$ arbitrary units, its midpoint is $450$ units. This arithmetic identifies half-unfolding only under the stated reporter assumptions.
Real traces can have drifting baselines, irreversible changes or <protein aggregation>. A fitted midpoint or derivative peak must therefore be defined consistently when comparing samples. A higher apparent midpoint supports greater thermal stability under those conditions; it is not by itself a <dissociation constant> or proof that the <protein> retains biological activity.
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