Glycine-X-Glycine peptide gels: self-assembly mechanism and rheological properties

Lavenia Josiane Thursch

doi:10.17918/00000524

Low Molecular Weight Gels (LMWG) are a versatile class of materials used in a wide-range of applications. Within these materials, short peptide gels have gained momentum these recent years for bioapplications such as drug-delivery and tissue engineering due to their biocompatibility. The aggregation of peptide sequences has been the focus of many studies in the framework of amyloid assemblies, a structure involved in many human diseases. The propensity for peptide aggregation has been argued to be mainly due to the presence of aromatic peptide residues, even in the case of tripeptides. However, our team has found that glycine-alanine-glycine (GAG), an aliphatic tripeptide, could self-assemble at the concentration of 200 mM in a 55 mol% ethanol / 45 mol % water solution. This gel presents a set of interesting characteristics: - the formation of large fibrils with a micron-size diameter and a millimeter length, as opposed to most peptide gels which form nanofibrils, - the properties of the gel network with a storage modulus in the 100 kPa range, - unlike many peptide assemblies, its structure is distinct from [beta]-sheets, - the formation of two chiral phases: phase I at temperatures < 16°C and phase II at room temperature. We used rheology, microscopy, spectroscopy, and X-ray scattering techniques to investigate the gel self-assembly from molecular to macroscopic level. First, we investigated further the formations conditions of GAG gels as a function of peptide concentration and ethanol fraction. Our findings underline the prominent role of solubility in the self-assembly mechanism. The increase of peptide concentration or ethanol fraction decreases the solubility and induces self-assembly. At high concentrations, the network assembles faster (jamming) which decreases the network's homogeneity and the mechanical properties. This mechanism can be used to control the gel strength, formation kinetics, and melting temperature. Then we then focused on the thermal behavior of the gel. We found that the gel was thermally irreversible with no reformation after long times at elevated temperature (50°C) with the formation of stable amorphous oligomers. This suggests the fibrillar phase is thermodynamically unfavored. The residence time at 50°C was shown to control the gelation kinetics and gel properties, including thermal stability. This study confirmed the role of solubility in the self-assembly process. We investigated the crystalline arrangements in GAG and found them to be different in pure peptide and gel phase with no impact of the chiral phases on the crystalline structure of the fibrils. Our team also found that Glycine-histidine-glycine (GHG) self-assembled in its deprotonated state, forming large fibrils with a high storage modulus, similar to GAG. The assembly was triggered by pH-switching and without the addition of ethanol. Similar to GAG, we investigated the formation of GHG fibrils under various conditions. Our results clearly show that at a given concentration, the parameters that determines the gelation kinetics and gel properties seems depend on the concentration of available deprotonated peptide, which is a function of pH. This also denotes the tunability of the GHG gel networks. Finally, we screened additional GxG molecules for self-assembly in water and ethanol/water mixtures. We found that Glycine-phenylalanine-glycine (GFG), glycine-tyrosine-glycine (GYG),glycine-tryptophan-glycine (GWG) and glycine-aspartic acid-glycine (GDG) can form hydrogels via pH-switching, while GFG and GDG aggregate in ethanol/water solutions. We focused primarily on GFG, GWG and GYG hydrogels. The first two were underlined by a fibrillar network similar to GAG and GHG and high storage modulus that exceeds 1 MPa in the case of GFG. GYG had an interesting microstructure with spherical aggregates. We were able to obtain crystalline structures from XRD data for most of the pure peptides and peptide gels. GHG forms a monoclinic crystalline system while the other GxG are orthorhombic. All gels studied here can be reformed upon shear which makes them potential candidates for injectable bio-systems. This class of GxG peptides offer many advantages to traditional hydrogel systems, such as polymers and longer peptide sequences, as they are potentially more biocompatible, easy to synthesize, and allow for a wide range of tunable gel properties. Furthermore, this work defines new self-assemble structures that are not intuitive from previous literature findings, e.g. the formation of self-assembled fibrils from aliphatic peptide sequences.

Glycine-X-Glycine peptide gels: self-assembly mechanism and rheological properties

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Glycine-X-Glycine peptide gels: self-assembly mechanism and rheological properties

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