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Peptide co-assembly is attractive for creating biomaterials with new forms and functions. Emergence of these properties depends on the peptide content of the final assembled structure, which is difficult to predict in multicomponent systems. Here using experiments and simulations we show that charge governs content by affecting propensity for self- and co-association in binary CATCH(+/−) peptide systems. Equimolar mixtures of CATCH(2+/2−), CATCH(4+/4−), and CATCH(6+/6−) formed two-component β-sheets. Solid-state NMR suggested the cationic peptide predominated in the final assemblies. The cationic-to-anionic peptide ratio decreased with increasing charge. CATCH(2+) formed β-sheets when alone, whereas the other peptides remained unassembled. Fibrillization rate increased with peptide charge. The zwitterionic CATCH parent peptide, “Q11”, assembled slowly and only at decreased simulation temperature. These results demonstrate that increasing charge draws complementary peptides together faster, favoring co-assembly, while like-charged molecules repel. We foresee these insights enabling development of co-assembled peptide biomaterials with defined content and predictable properties.

 

Prion diseases are infectious neurodegenerative diseases that are capable of cross‐species transmission, thus arousing public health concerns. Seed‐templating propagation of prion protein is believed to underlie prion cross‐species transmission pathology. Understanding the molecular fundamentals of prion propagation is key to unravelling the pathology of prion diseases. In this study, we use coarse‐grained molecular dynamics to investigate the seeding and cross‐seeding aggregation of three prion protein fragments PrP(120–144) originating from human (Hu), bank vole (BV), and Syrian hamster (SHa). We find that the seed accelerates the aggregation of the monomer peptides by eliminating the lag phase. The monomer aggregation kinetics are mainly determined by the structure of the seed. The stronger the hydrophobic residues on the seed associate with each other, the higher the probability that the seed recruits monomer peptides to its surface/interface. For cross‐seeding aggregation, we show that Hu has a strong tendency to adopt the conformation of the BV seed and vice versa; the Hu and BV monomers have a weak tendency to adopt the conformation of the SHa seed. These two findings are consistent with Apostolet al.'s experimental findings on PrP(138–143) and partially consistent with Joneset al.'s finding on PrP(23–144). We also identify several conformational mismatches when SHa cross‐seeds BV and Hu peptides, indicating the existence of a cross‐seeding barrier between SHa and the other two sequences. This study sheds light on the molecular mechanism of seed‐templating aggregation of prion protein fragments underlying the sequence‐dependent transmission barrier in prion diseases.

 

Co-assembling peptides can be crafted into supramolecular biomaterials for use in biotechnological applications, such as cell culture scaffolds, drug delivery, biosensors, and tissue engineering. Peptide co-assembly refers to the spontaneous organization of two different peptides into a supramolecular architecture. Here we use molecular dynamics simulations to quantify the effect of anionic amino acid type on co-assembly dynamics and nanofiber structure in binary CATCH(+/-) peptide systems. CATCH peptide sequences follow a general pattern: CQCFCFCFCQC, where all C’s are either a positively charged or a negatively charged amino acid. Specifically, we investigate the effect of substituting aspartic acid residues for the glutamic acid residues in the established CATCH(6E-) molecule, while keeping CATCH(6K+) unchanged. Our results show that structures consisting of CATCH(6K+) and CATCH(6D-) form flatter β-sheets, have stronger interactions between charged residues on opposing β-sheet faces, and have slower co-assembly kinetics than structures consisting of CATCH(6K+) and CATCH(6E-). Knowledge of the effect of sidechain type on assembly dynamics and fibrillar structure can help guide the development of advanced biomaterials and grant insight into sequence-to-structure relationships.

 

Injectable hydrogels are attractive for therapeutic delivery because they can be locally administered through minimally-invasive routes. Charge-complementary peptide nanofibers provide hydrogels that are suitable for encapsulation of biotherapeutics, such as cells and proteins, because they assemble under physiological temperature, pH, and ionic strength. However, relationships between the sequences of charge-complementary peptides and the physical properties of the hydrogels that they form are not well understood. Here we show that hydrogel viscoelasticity, pore size, and pore structure depend on the pairing of charge-complementary “CATCH(+/−)” peptides. Oscillatory rheology demonstrated that co-assemblies of CATCH(4+/4−), CATCH(4+/6−), CATCH(6+/4−), and CATCH(6+/6−) formed viscoelastic gels that can recover after high-shear and high-strain disruption, although the extent of recovery depends on the peptide pairing. Cryogenic scanning electron microscopy demonstrated that hydrogel pore size and pore wall also depend on peptide pairing, and that these properties change to different extents after injection. In contrast, no obvious correlation was observed between nanofiber charge state, measured with ζ-potential, and hydrogel physical properties. CATCH(4+/6−) hydrogels injected into the subcutaneous space elicited weak, transient inflammation whereas CATCH(6+/4−) hydrogels induced stronger inflammation. No antibodies were raised against the CATCH(4+) or CATCH(6−) peptides following multiple challenges in vehicle or when co-administered with an adjuvant. These results demonstrate that CATCH(+/−) peptides form biocompatible injectable hydrogels with viscoelastic properties that can be tuned by varying peptide sequence, establishing their potential as carriers for localized delivery of therapeutic cargoes. 

Water + elastin-like polypeptides (ELPs) exhibit a transition temperature below which the chains transform from collapsed to expanded states, reminiscent of the cold denaturation of proteins. This conformational change coincides with liquid–liquid phase separation. A statistical-thermodynamics theory is used to model the fluid-phase behavior of ELPs in aqueous solution and to extrapolate the behavior at ambient conditions over a range of pressures. At low pressures, closed-loop liquid–liquid equilibrium phase behavior is found, which is consistent with that of other hydrogen-bonding solvent + polymer mixtures. At pressures evocative of deep-sea conditions, liquid–liquid immiscibility bounded by two lower critical solution temperatures (LCSTs) is predicted. As pressure is increased further, the system exhibits two separate regions of closed-loop of liquid–liquid equilibrium (LLE). The observation of bimodal LCSTs and two re-entrant LLE regions herald a new type of binary global phase diagram: Type XII. At high-ELP concentrations the predicted phase diagram resembles a protein pressure denaturation diagram; possible “molten-globule”-like states are observed at low concentration.