Peptide Self-assembly: Controlling Conformation and Mechanical Properties

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Abstract

In recent years a great deal of research has focussed on understanding and exploiting self assembling peptides as they form fibrillar hydrogels for use in a variety of different applications, such as tissue engineering and drug delivery. A particular class of such peptide systems are ionic-complementary peptides, composed of alternating hydrophobic and hydrophilic amino acids. Their simple structure is generally seen to assemble into β sheet rich fibrils, and easy modification of the primary structure is possible to allow the inclusion of recognition motifs tailored for a specific use. This can be done simply via physical mixing. To maximise the potential of such systems it is important to understand the interactions that govern the self-assembly behaviour. Here a variety of different peptides have been studied to elucidate control of peptide conformation and fibril morphology. The ability to easily tune the mechanical strength of the hydrogel has been explored by mixing peptide systems.The peptide FEFEFKFK (FEKII) was seen to assemble into β sheet rich fibrils of ~3 nm in diameter. Control of pH and hence the charge state of the E and K side chains altered sample properties. Gelation at pH 2.8 occurred at a concentration between 20 30 mg ml 1. At pH 4, 5 and 10 where the peptide has a lower net charge gelation was lowered to ~10 mg ml 1. Mechanical properties varied with G′ values of 20-1200 Pa as pH was altered. Stronger gels were formed with lower net peptide charge. Hierarchical fibre assembly was observed for positively charged peptides, with fibres forming from lateral association of fibrils. Negatively charged peptides at pH 10 showed no such hierarchical assembly, and lower fibril persistence length. This was related to the change in charge along the fibril structure. At pH 7, where the peptide has no net charge, precipitation occurred. This showed a net charge was required on the peptide to disperse fibrils and prevent aggregation. The work showed the importance of ionic-interactions in determining both network morphology and bulk properties, and also elucidated control of such behaviour.AEAEAKAK (AEKII) was shown to assemble into α helix fibres. Alanine (A) is less hydrophobic than F, and is a known helix former. The role of F and A in assembly was assessed by the design of peptides FEAEFKAK (FAIEKII) and FEFEAKAK (FAIIEKII). Mixing A with F disrupted the peptides’ ability to form a β sheet network by lowering the driving force for assembly given by the F residues. Trace amounts of β sheet were observed at low concentration, but at a critical concentration β sheet content increased and gelation occurred. This was found to be pH dependent. FAIEKII formed β-sheet fibrils at a lower concentration than FAIIEKII. While FAIEKII was able to assemble into different fibril structures, FAIIEKII showed no specific aggregation. This not only highlighted the importance of Hydrophobicity as a key driving force to assembly but also how the grouping of these amino acids in the primary sequence can determine the overall assembly characteristics of the peptide. The peptides FEFEFKFKGGFEFEFKFK (FEKII18-1) and FEFEFKFKGGFKFKFEFE (FEKII18-2) were designed to co-assemble with FEKII. Individually both peptides were seen to assemble into β sheet fibrils. FEKII18-1 formed fibrils of 2.3 3.1 nm in size, a result of folding along the chain caused by intra molecular attractive ionic interactions. FEKII18-2 formed larger fibrils of 4.4 5.2 nm from a straightened peptide chain given by the change in charge distribution. When co assembled with FEKII mechanical properties were enhanced, with G′ increasing from 40 Pa at 20 mg ml 1 to 2400 Pa, depending on the concentration of FEKII18-1/FEKII18-2 added to the system. This was a result of these peptides providing fibril connections acting as cross links.This work has detailed control over the assembly process via peptide conformation and fibril interactions and the effect this has on overall macroscopic sample properties. This is vital in determining the viability of such systems in various biomedical applications.

Keyword(s)

Conformation; Hydrogel; Mechanical Properties; Peptide; Self-assembly

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