Why might secondary reactions be used when measuring binding interactions?

Using secondary reactions when measuring binding interactions can be particularly advantageous when the substrate-product pairs lack spectroscopic properties. In many biochemical assays, a direct measurement of product formation or substrate depletion is crucial. However, if the compounds involved do not possess inherent spectroscopic characteristics—such as fluorescence, absorbance, or any detectable signals—it becomes challenging to monitor the reaction. In such cases, a secondary reaction can be designed that generates a product with detectable properties. This allows for the quantification of the initial binding interaction by indirectly measuring the outcome through the secondary reaction, enhancing the ability to analyze and interpret the binding kinetics and affinities accurately.

The other options, while they touch on important aspects of using secondary reactions, do not directly address the primary concern of lacking spectroscopic properties in the substrate-product pairs, thus making them less relevant in this context. For instance, utilizing non-specifically bound substrates may not pertain to the specificity and sensitivity of the measured interactions, and speeding up the primary reaction or reducing costs is generally not the main motivation for implementing secondary reactions in binding assays.