Porous materials have wide ranging applications in areas such as hydrogen storage, catalysis and drug delivery. In rigid porous materials such as zeolites, the ability of the material to adsorb depends on the fixed size and shape of the pores. Until the mid 1990s most synthetic porous materials were either zeolites and their analogs or activated carbons. The discovery of metal-organic frameworks (MOFs) opened the possibility of more flexible frameworks, where the geometry of the pores can change in response to guest species. Scientists from the University of Liverpool have been using Diamond as part of an extensive project to describe an adaptable peptide-based porous material that will help us understand biological processes such as ion channel behaviour and protein folding. This research has been published in the journal Science.
The ability of MOFs to adapt depends on the metal coordination geometry and the degrees of freedom of the linkers. Typically the metal coordination geometry is fixed, connected by rigid linkers that restrict adaptability to rotations around a single axis and displacement of the linkers. In contrast, proteins are characterised by their ability to adapt to their environment, enabled by polypeptide chains which allow them to fold into the required structures.
The Liverpool group have been studying a MOF where zinc cations are connected by simple dipeptide linkers that allow the porosity of the MOF to change smoothly from zero with the loading of guest molecules. They combined experimental data from high resolution powder X-ray diffraction from Beamline I11 at Diamond’s I11 beamline and solid state NMR with molecular dynamics simulations. They also used Beamline I19 to develop the chemistry to access peptide porous frameworks. They found that when no guest molecules are present the pores block, gradually opening when triggered by small guest molecules with polar bonds. This adaptability is the direct result of the flexible dipeptide linkers. The torsional degrees of freedom they possess allows the framework to be maintained while the space available to guest molecules is changed. This is in contrast to MOFs with rigid links, which have a binary open/close response.
“This research has shown that it is possible to synthesise a porous solid with adaptable porosity, where the peptide linker plays a pivotal role due to its torsional degrees of freedom. The resulting proposed mechanism for guest response could be seen as analogous to conformational selection in proteins. Further investigation of peptide-based crystalline frameworks will show how far it is possible to take this analogy.”
Prof Matt Rosseinsky, University of Liverpool
“An Adaptable Peptide-Based Porous Material”, J. Rabone, Y.-F. Yue, S. Y. Chong, K. C. Stylianou, J. Bacsa, D. Bradshaw, G. R. Darling, N. G. Berry, Y. Z. Khimyak, A. Y. Ganin, P. Wiper, J. B. Claridge, M. J. Rosseinsky, Science, 329 (5995), pp. 1053 – 1057
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