Skip to content

The Concept of Chemical Reversibility and Implications for Commercial Aptamer Diagnostics

    This blog starts with a portrait of Claude Louis Berthollet (1748 – 1872), who was the first to demonstrate and elaborate on the concept of chemical reversibility. As an aside, it should be noted that he accompanied Napoleon to Egypt and, along with Joseph Fourier, investigated means of converting the sodium carbonate found there into gunpowder. The concept of chemical reversibility is simple enough. The relation between the on rate and the off rate of reactions determines the direction that a chemical reaction proceeds in. 

    The implications, however, are profound. When two molecules bind to each other this is a reversible reaction. Especially as is the case with aptamer and antibody interaction, the binding event does not involve the formation of a covalent bond. When we describe the affinity of these binding reactions we use the terms on rate (k on) and off rate (k off). Note that these terms are defined as concentration per unit time (M-1 sec-1). This means that individual molecules are forming complexes and disassociating from such complexes on a microsecond scale. Binding affinities measured in terms of k on and k off represent the proportion of the molecules that are in either state at a given time. Non-covalent binding, even strong non-covalent binding, is not ever a fixed event. It is a measurement of a dynamic state. 

    I said above that the implications are profound. Why does it matter that we are actually measuring a dynamic state? It matters in terms of the design of commercial products for several reasons.

    1.  Every time that there is a wash in a diagnostic test, there will necessarily be a loss of bound complex. Dynamic states are governed by the mathematical rules of equilibrium, and these rules must be obeyed. If all the free, unbound target is removed from a system, then a portion of the bound target must become unbound in order to satisfy the equilibrium.  
    2. The volume of solution that the bound complex is present in determines the proportion that is bound versus the proportion that is unbound. The lower the volume, the smaller the off-state room and the higher the proportion bound. 
    3. The farther a system is from equilibrium the more noise there will be from measurement to measurement. 

    At NeoVentures we love mathematical models of systems. When we interact with our clients on projects we apply our understanding of the binding coefficients of aptamers to the development of models of diagnostic systems. This then enables us to understand how to achieve the performance that we need. 

    The discovery of the rules of chemistry by Lavoisier, Bertholett, Priestly and many others was an incredible feat of human intellect. It just makes sense to stand on the shoulders of these giants as we create the next generation of diagnostic products.