Adaptable Porous Materials

Porous materials find widespread application in storage, separation and catalytic technologies. The sorption of a guest molecule by a rigid porous material such as a zeolite or active carbon is controlled by the fixed size and shape of the pores.

Nature catalyses chemical processes and manipulates molecules using proteins. Proteins are characterised by an adaptable response to their environment, produced by conformational selection of an appropriate functional structure (e.g. for enzyme catalysis, or pore opening of the mechanosensitive channel of small conductance in an ion channel) from a large ensemble of energetically low-lying and kinetically accessible states. This is enabled by the manifold torsions available to polypeptide chains, which allow folding into the required structures.

The project team have recently (Rabone et al Science 2010) used a simple dipeptide linker to assemble a crystalline porous framework through metal-binding. The resulting material combines pre-formed pores with the degrees of freedom from a peptide linker required for conformational selection. This peptide-based open framework displays adaptable porosity that evolves continuously from an open to a partially disordered closed structure in response to the guest content. The functional porous behavior is unconventional, displaying cooperative feedback characteristic of cooperative interactions as the pore topography changes in response to the number of guests occupying the pore volume of the host. The peptide-based material displays adaptable porosity. It undergoes dynamical structural changes on guest loading because there are many accessible sorption states with low energy barriers between them. This energy landscape arises because of the low energies required for torsional changes to the structure of the peptide linker.

This opens up the possibility of designed adaptable porous materials which respond to guests in a manner analogous to that of a biomolecule undergoing conformational selection, produced by the modular assembly of multiple amino acid residues around several metal centres to access large and functionally diverse unit cells. This vision cannot be presently realized due to the large numbers of potential chemical constituents of such materials and the absence of computational tools to understand and predict how they would respond to guests. Such materials would though be unprecedented and offer new and potentially useful sorption and catalysis functionality.

The proposed research aims to develop the tools to allow the isolation of such materials by focussing on adaptable porous materials derived from chemically simple di- and tripeptide linkers containing two and three amino acid residues respectively. We will identify the characteristics of both individual peptide linkers and metal-based units which give adaptable porous behaviour. This will allow the development of second-generation systems in which multiple peptide and metal units are used, making the key advance of demonstrating modular amino acid residue assembly in a functional porous solid. Rigid linkers will be introduced together with the peptides to produce structures where rigid sub-units are repositioned by the flexible peptide-based units, in a manner analogous to the repositioning of rigid helix and sheet units in protein folding.

Given the diversity of possible peptide components, descriptor-based computational methods including machine-learning will be developed as a complementary approach to the selection of synthetic targets in the second- and third-generation families.

The response of adaptable porous materials to guests does not follow classical models, and will be evaluated from experimental sorption, dynamics and structural data coupled with computational models appropriate to the dynamical restructuring of the adaptable porous host around a guest, to move beyond the current static view of host-guest interactions in synthetic porous materials.

Who will benefit?

Adaptable porous materials have the potential to expand the applications of porous solids. This programme has long-term benefits in sorption (analytical and preparative selective sorption; clean-up of gas streams; energy-intensive separations) and catalysis for processes of relevance to the chemical and pharmaceutical industries. The industrial beneficiaries will be catalyst and sorbent manufacturers (in this project we engage initially with Johnson Matthey (JM)) and users (Defence Science and Technology Laboratory (Dstl)), and pharmaceutical and speciality chemicals companies. JM will benefit from the impact of chiral naturally-derived adaptable porous materials on catalysis and separation technologies - sorption-based separations reduce energy requirements, and metal-organic frameworks are now under pilot plant evaluation for several key separations. The public sector research organization Dstl will benefit through the development of new sorption mechanisms as their personnel protection technologies largely depend on sorption removal and catalytic destruction of hazards.
Society will benefit from the trained personnel emerging from the programme equipped to contribute to UK industry in a high-tech sector. Longer term benefits will arise from the scientific advances enabling reduced energy use for processes such as separations, the development of new sensors (either through applications of the materials themselves or the understanding of adaptable host response to guests) with associated health and environmental benefits, and through the generic impact of enhanced control of the assembly of highly chemically-functionalised molecules in effect materials.

What will be done to ensure they have the opportunity to benefit?

Day 1 partners JM and Dstl will receive regular six-monthly project updates for discussion with the project team, leading to personnel exchange and materials evaluation where appropriate. Beyond this day 1 group, we will work with the Knowledge Centre for Materials Chemistry (KCMC) to ensure the widest possible dissemination of relevant developments to UK chemicals-using and broader industry sectors. MJR is the Liverpool lead and founder member of KCMC, which is an applied materials chemistry collaboration between the Universities of Bolton, Liverpool (UoL), Manchester and STFC Daresbury Laboratories - 50% of the KCMC partners are involved in the present proposal. KCMC has supported 71 companies in over 100 projects, generating over £6M of industrial funding since March 2009. KCMC thus has strong collaborative relationships with many UK-based chemical companies, providing a mechanism for advances in science emerging from the project to be evaluated and where appropriate taken forward for exploitation through engagement of the KCMC Knowledge Transfer (KT) team via individual discussion with companies and themed industry days, using case-study type summaries of both materials and methodologies emerging from the programme prepared by the KT team to maximize impact on potential users. IP will be protected by Business Gateway at UoL. The team currently has 8 patents filed via this mechanism from EPSRC-funded research in the past 3 years.
Project advances of societal interest will be disseminated via the UoL press office, working with EPSRC as appropriate. The new modelling methodology will be showcased on the UoL chemtube3d.com website (>160,000 visitors from >180 countries spending >2.5 mins per visit), and together with experimental advances, incorporated in new experiments for the UoL Schools Lab. Policymaker engagement is key to societal benefit and KCMC will enable this through events such as the 2 year Anniversary event at the Houses of Parliament in May 2011.
The skills and contact network of the two project RAs will be strongly enhanced by close experiment/theory co-working in an emerging science area, and engagement with JM, Dstl and the industry network of KCMC.