Atomic force microscopy investigations of peptide self-assembly
The ability of short peptide fragments to self-assemble in isolation as amyloid and amyloid-like structures has prompted their use as model systems for the study of amyloid formation and recently also for their utilisation as novel nanofibrillar material. The atomic force microscope (AFM) is used here to investigate the self-assembly of two peptide systems and the development of strategies to directly manipulate and control the structures they form. The studies presented in Chapter 2 address the self-assembly of a peptide fragment of the human amylin polypeptide; amylin (20-29). In the opening study we use ex situ AFM imaging to characterise the early stages of amylin (20-29) fibril formation. High-resolution images reveal that following an initial lag phase, fibrils displaying a globular appearance are formed, which over time are replaced by flat ribbon-like fibrils with no periodicity displaying a range of polymorphic structures and assemblies. Following on from these findings, we investigate the influence of solution conditions on amylin (20-29) fibril formation utilising in situ AFM imaging. Altering the pH and electrolyte composition affords a range of morphologies including, truncated and long branched or unbranched flexible fibrils and globular aggregates. Following on from this characterisation chapter, in Chapter 3 a strategy to assemble specifically functionalised fibrillar material from chemically modified amylin (20-29) peptides was investigated. Azide and alkyne moieties were successfully coupled to the amylin (20-29) peptides. Ex situ AFM imaging and Congo red binding confirmed that the additional steric bulk had no detrimental effects on the fibril forming capacity of the peptides. Finally, in Chapter 4 the focus turns to the self-assembly of a dipeptide of phenylalanine which corresponds to the core recognition motif of the beta-amyloid polypeptide. Here, the AFM is used to study the physical properties of the well-ordered, discrete, hollow nanotubes which are formed. Their chemical stability in organic solvents and considerable thermal stability under both dry and wet heating conditions is revealed. Finally, the use of strong magnetic fields to directly control and orientate the diphenylalanine nanotubes was examined by AFM. The results presented throughout this thesis demonstrate the versatility of self-assembling peptides for the generation of fibrillar nanostructures that can be directly modified and controlled to generate novel architectures and functionalised well ordered nanomaterials.