Final answer:
Binding affinity is quantified by measuring the equilibrium dissociation constant (Kd), where a lower Kd represents a stronger interaction.
Step-by-step explanation:
We quantify binding strength/affinity, such as between a sensor RNA and the antibiotic tetracycline, by determining the equilibrium dissociation constant (Kd). The dissociation constant is inversely related to the binding strength; a lower Kd value indicates a higher affinity and thus a stronger interaction.
To measure the binding affinity, one of the reactants (like tetracycline) is kept at a consistent low concentration, while the other reactant's (like sensor RNA) concentration is varied. The measurement is often done using methods such as fluorescence where the binding of molecules can be correlated with changes in fluorescence intensity, like fluorescence quenching seen in albumin binding with levofloxacin.
The binding affinity is calculated by plotting the fraction of the ligand that is in a complex against the concentration of the other molecule, with the dissociation constant being the point at which 50% of the ligand is bound.