What a difference a state makes: NMR curiosity

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  • Published: Mar 15, 2017
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: What a difference a state makes: NMR curiosity

State to state

Structurally different G-quartet-based assemblies are formed in chloroform depending on the nature of the cation, anion and the salt concentration, as characterized by circular dichroism and time course diffusion-ordered NMR spectroscopy data. (Credit: Wiley/Chem Eur J/Brown et al)

Nuclear magnetic resonance (NMR) spectroscopy, notably fast magic-angle spinning experiments at the UK 850 MHz Solid-State NMR Facility, has revealed how molecular self-assembly processes can take different forms of a supramolecular structure from the solution state to the solid state and back again, a phenomenon identified for the first time by researchers at the University of Warwick, UK.

Physicist Steven Brown and his colleagues in the Department of Chemistry, together with collaborators in Italy and the USA, have observed supramolecular structures containing a guanosine derivative as changing when passing from the solid state into the solution state and back again. This phenomenon, they suggest, defies chemical precedent and may require a rethink on many well-known systems that science thought it understood. After all, self-assembled structures are commonly driven by the formation of specific intermolecular hydrogen bonds in solution and chemists and physicists alike had assumed that these structures would persist in the transition to the solid state, this seems not always to be the case at least for one example and may well prove problematic in the simpler understanding of such systems in others.

Concentrate, now

Writing in Chemistry - A European Journal, the team explains that: "Structurally different G-quartet-based assemblies are formed in chloroform depending on the nature of the cation, anion and the salt concentration, as characterized by circular dichroism and time course diffusion-ordered NMR spectroscopy data." Chemical intuition would normally suggest that this quartet should remain in harmony in the solid state too. Surprisingly, the jump to the solid state from solution leads the supramolecular assembly to form quartet and ribbon structures. The researchers identified the distinct supramolecular states from the obvious differences between the two NMR spectra, as one might expect. Additional evidence highlighted the differences too.

"Access to state-of-the-art NMR infrastructure has enabled us to see with chemical precision how the guanosine-based molecules self-assemble, thus revealing the surprising phenomenon of a change in self-assembly upon going changing from solution to solid and from solid to solution," Brown explains.

Contrariwise

The researchers point out that even with the addition of potassium ions to the system that would otherwise stabilize G-quartets in chloroform, their presence was insufficient to maintain these assemblies uniquely in the solid state. The NMR spectra showed peaks characteristic of distinct N−H⋅⋅⋅N and N−H⋅⋅⋅O intermolecular hydrogen bonding interactions that lead to quartet and ribbon-like self-assembly. The team adds that dissolving the systems in solution again leads to quartet and ribbon interconversion. "A systematic study for complexation with different cations (potassium and strontium) and anions (picrate, ethanoate and iodide) emphasizes that the existence of a stable solution or solid-state structure may not reflect the stability of the same supramolecular entity in another phase," the team writes. The work showcases how a combination of solid state and solution NMR spectroscopic techniques can distinguish between different supramolecular structures in the solid, solution, and gel states. It also beggars the question, what have earlier studies said about supramolecular structures, particularly biological ones that have been investigated in the solid state, that might prove contrary in the solution or gel phases.

Related Links

Chem Eur J 2017, online: "Co-existence of Distinct Supramolecular Assemblies in Solution and in the Solid State"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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