Improved methodologies are provided to synthesize (1R,2S)-2-aminocyclobutane-1-carboxylic acid derivatives and their incorporation into beta-peptides of 2-8 residues bearing different N-protecting groups. The conformational analysis of these oligomers has been carried out by using experimental techniques along with theoretical calculations. This study shows that these oligomers adopt preferentially a strand-type conformation in solution induced by the formation of intra-residue six-membered hydrogen-bonded rings, affording cis-fused [4.2.0]octane structural units that confer high rigidity on these beta-peptides. Moreover, all of them are prone to self-assemble producing nano-sized fibres, as evidenced by TEM, AFM and SPFM, and, in some instances, they also form gels. These techniques and molecular modelling allowed us to suggest an aggregation model for the assembly structures in which a parallel molecular-arrangement is preferred and the conformation is similar to that observed in solution. According to this model, both hydrogen-bonding and hydrophobic interactions would account for formation of the assemblies.

Three new bis(cyclobutane) beta-dipeptides have been synthesized from appropriate derivatives of cis- and trans-2-aminocyclobutane-1-carboxylic acid, respectively. The predominance of eight-membered hydrogen-bonded rings has been manifested for (trans,trans)- and (trans,cis)-beta-dipeptides while the formation of six-membered rings is preferred for the (cis,trans)- beta-dipeptide similarly to the previously described (cis,cis)-diastereomer.

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.

A homochiral synthetic dipeptide incorporating two cyclobutyl rings has been used as an assembling unit for the pi-electron-rich tetrathiafulvalene (TTF) moiety. The molecule was prepared and characterised to show all the features of the two components, whereby chirality and pi-function are incorporated in the same species. Supramolecular fibres are formed by the compound, as proven by atomic force microscopy (AFM) and transmission electron microscopy. The dimensions of the nanostructures suggest that the molecules pack into dimeric tapes with the peptide head groups at the centre. Current-sensing AFM shows that once doped, films of the material are capable of conducting electricity.

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