H E A L T H I N N O V A T I O N , O V E R C O M I N G D I S E A S E S A N D P A N D E M I C S

S C I E N T I F I C H I G H L I G H T S

4 0 H I G H L I G H T S 2 0 2 3 I

X-rays and neutrons unravel the unusual stability of peptide assemblies

X-ray and neutron scattering experiments show that self-assembled nanostructures based on peptides can be engineered with superior stability with respect to ordinary synthetic molecules such as surfactants. These can be utilised as robust assembled structures for drug-delivery systems, or as antibiotics against resistant bacteria, with enhanced stability towards enzymatic degradation.

Molecular self-assembly is a fascinating process that leads to the formation of well-defined nanostructures, from spherical or cylindrical micelles in surfactant or polymeric systems, to precisely folded protein complexes and lipid membranes in biological systems. This assembly is driven by, in particular, hydrophobic interactions, where the molecules shield their water-insoluble residues, resulting in spontaneous ordering into nanostructures. For regular amphiphilic systems, small structures have a larger critical micelle concentration that are prone to disintegrate upon dilution or changes in temperature. This renders them inappropriate as drug carriers in pharmaceutical applications or as biomaterials, for example. Enhanced stability can be obtained by block copolymer micelles. However, polymeric structures are dynamically slow, leading to long equilibration times and pathway-dependent (non-ergodic) structures.

Fig. 25: a) Schematic drawing of the peptide. b) Computer simulation results. c) SAXS scattering curves showing the effect of PEGylation, temperature and pH respectively.

In the realm of proteins and peptides, the physical stability, such as the tendency to disassemble or dissolve, is less coupled to the size of the molecules. This is because the assembly process is controlled by multiple molecular interactions. The primary amino acid sequences and motifs lead to a wider range of interactions and orchestrate the formation of intricate and tuneable nanostructures. Although molecular self-assembly has been studied for decades, there is still a lack of a predictive capability for their assembled structure. As an example, even simple dipeptides (i.e., peptides consisting of only two amino acids), may assemble into a variety of nanostructures that cannot easily be foreseen.

This work investigates a family of simple peptides based on lysine (K), glutamine(Q) and leucine(L); Kx(QL)yKz multidomain motif [1]. Small-angle X-ray scattering (SAXS) at beamline BM29 and small-angle neutron scattering (SANS) was used to assess both the nanostructure and exchange dynamics of assemblies [2]. The experiments show that K3W(QL)6K2 peptides self-assemble into well- defined elongated sandwich nanostructures. This is driven by hydrophobic leucine residues pointing towards the core while the structure is reinforced through hydrogen bonds between the glutamine units (Figure 25a). This leads to twisted elongated nanosheet structures, which could be confirmed in detail using computer simulations (Figure 25b). Interestingly, the nanosheets are not susceptible to conjugation of polymers ( PEGylation ), variations in temperature and pH (Figure 25c). This was confirmed by detailed modelling of the peptide sheets using core-shell sheet models with Gaussian chains attached.