Supramolecular Devices and Materials

Abstract

This thesis describes work towards the development of a range of supramolecular devices and materials based on cyclodextrin host-guest interactions, and/or short peptide structures.

The first such device targeted was a photoswitchable molecular lariat composed of a [c2] (cyclic, two-component) asymmetric daisy chain. Upon combining the two hetero monomer units it was found that a [c2]-dimer was not formed. The next phase of research involved the design of a second type of supramolecular device, also based on cyclodextrin host-guest inclusion complexes, this time serving to produce a change in peptide secondary structure. A range of devices were synthesised giving several modifications; increasing the stability of hexavaline beta sheets, reducing the PPII (polyproline type II) helicity in pentaalanine and introducing beta sheet character to pentaalanine. Furthermore, an azobenzene moiety was introduced as a photo-switchable cyclodextrin guest, enabling PPII helicity to be switched between states of increased and decreased structure via photoirraditation.

This principle was then developed to give a device which not only produced a change in peptide structure but also brought about a change in function, in this case metal-binding ability. Upon cyclodextrin host-guest [c2]-dimer formation, beta sheet structure was induced in a short peptide strand which facilitated the arrangement of histidine residues in order to bind to a metal ion in a pseudo-chelating arrangement. The unmodified peptide, with no cyclodextrin or guest attached did not bind to zinc ions, whereas the cyclodextrin dimer was found to bind to Zn2+ with a Ka of 2,223 M-1. The system was then investigated in the solid-phase and a crystal was grown wherein [c1]-complexes were bound to Zn2+ in tetrameric assemblies. In addition, the same crystal structure was observed in the absence of Zn2+ showing that the system pre-assembles to form a crystal lattice with vacant metal binding sites.

A number of smaller studies were also performed to explore the possibility of the application of peptide chemistry techniques and principles to supramolecular concepts. The solid-phase synthesis technique used to make peptide sequences was investigated as a method for the preparation of rotaxanes. Despite several attempts no interlocked structures were synthesised, however a method was established for the mobilisation of a cyclodextrin onto a stationary phase. -Sheet peptide-peptide interactions, in conjunction with cyclodextrin host-guest interactions were then explored as a method for the generation of self-assembling polymers. A range of polymeric structures were considered, however within all the systems examined it was found that none formed.

Finally, it was observed that a peptide amphiphile (PA), synthesized during the preparation of a metal-binding cyclodextrin dimer, formed a gel in acetonitrile/water mixtures, DMF and acetone at just 0.06 wt.%. The compound did not form a gel in water, despite its structural similarity to previously reported PAs, but did obey the structure-property relationships determined for these hydrogels. This suggests that despite the difference in solvents, assembly of these organo and hydrogels is the same and indicates a new strategy for the modification of PAs in order to gel target solvents.