Do molecules behave like people in a crowd?

Researchers from the ICIQ have studied the binding behaviour of molecules that are immobilised at a surface versus those free in bulk solution. Anthea Blackburn explains further…

If you have ever been stuck in a crowd, you may have noticed that your range of motion and the speed at which you can move is highly dependent not only on whether you are leaving a sports game or a pop concert, but also on where you are positioned in the mass of people. The same is true of a solution of molecules – the molecules that are located in the bulk of the solution would be expected to have different properties from those that are surface-immobilised. This is especially true if we consider the supramolecular association of a guest within a host, where thermodynamics and kinetics play an important role in whether a complex will form or not.

Pablo Ballester and his team from the Institute of Chemical Research of Catalonia (ICIQ) set out to study this phenomenon and prove whether or not there was a difference in the binding of a guest, pyridine N-oxide derivatives, with a host molecule, α,α,α,α-calix[4]pyrrole, in bulk solution or tethered to a gold surface. To achieve this goal, the team employed a surface plasmon resonance (SPR) technique, a technique that is sensitive to the accumulation or release of mass, and has been used previously to study large biomolecular systems in real time.

The binding of calix[4]pyrrole to guests immobolised on a surface is, kinetically, slower than in bulk solution

It was found that thermodynamically, binding events between two molecules are similar in bulk solution and at an interface. This is perhaps not surprising, as the changes in enthalpy and entropy to a calix[4]pyrrole in bulk solution or tethered to a surface will be similar; therefore so too will be the binding event. Differences were observed, however, when considering the kinetic aspect of binding, such that binding was much slower when the molecule was on a surface than when it was in bulk solution. This was attributed to the presence of a matrix hindering the motion of the surface-bound calix[4]pyrrole, thereby providing a barrier to complexation.

This work presents an interesting method of studying the binding events that occur in the different regions of a solution. It also shows that the events that occur on the macroscale, such as in a crowd of people, can, in some cases, be analogous to those that occur on the molecular level.

Read this HOT ChemSci article in full!

Binding of calix[4]pyrroles to pyridine N-oxides probed with surface plasmon resonance
Louis Adriaenssens, Josep Lluís Acero Sánchez, Xavier Barril, Ciara K. O’Sullivan and Pablo Ballester
Chem. Sci., 2014, Edge Article
DOI: 10.1039/C4SC01745E


Anthea Blackburn is a guest web writer for Chemical Science. Anthea is a graduate student hailing from New Zealand, studying at Northwestern University in the US under the tutelage of Prof. Fraser Stoddart (a Scot), where she is exploiting supramolecular chemistry to develop multidimensional systems and study the emergent properties that arise in these superstructures. When time and money allow, she is ambitiously attempting to visit all 50 US states before graduation.

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