The relative stability of components in a complex dynamic equilibrium mixture can be determined by quantifying a single leftover reference compound
Systems chemistry aims to understand how chemicals behave and interact in complex mixtures rather than in isolation. The determination of a large number of equilibrium constants in such systems relies on the quantification of a large number of products. This is problematic because the analytical protocol (for example, a UV absorbance profile) must be optimized for each product — requiring access to each component in pure form — and because characteristic signals for a particular product in a complex mixture of similar products may be difficult to identify.
Now, Leonard Prins and co-workers from the University of Padova in Italy have developed1 an indirect optical method for performing such an analysis. They illustrate the principle of their method using a series of acyl hydrazones — formed from mixtures of hydrazides and an aldehyde. Rather than attempting to quantify the amounts of various products, they measure the amount of a specific unreacted hydrazide by scavenging it from the mixture with another aldehyde — chosen so that the product of this reaction has a characteristic UV absorbance.
Prins and co-workers used the method to determine the relative thermodynamic stabilities of a mixture of six hydrazones — a system that involves 15 separate equilibrium reactions in concert. The technique may be useful for assessing how such combinatorial libraries adapt to external stimuli.