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Self assembly of a model amphiphilic phenylalanine peptide/polyethylene glycol block copolymer in aqueous solution

DOI: 10.1016/j.bpc.2009.01.008 DOI Help

Authors: Valeria Castelletto (University of Reading) , Ian Hamley (University of Reading, Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Biophysical Chemistry , VOL 141 , PAGES 169-174

State: Published (Approved)
Published: May 2009

Abstract: There has been great interest recently in peptide amphiphiles and block copolymers containing biomimetic peptide sequences due to applications in bionanotechnology. We investigate the self-assembly of the peptide-PEG amphiphile FFFF-PEG5000 containing the hydrophobic sequence of four phenylalanine residues conjugated to PEG of molar mass 5000. This serves as a simple model peptide amphiphile. At very low concentration, association of hydrophobic aromatic phenylalanine residues occurs, as revealed by circular dichroism and UV/vis fluorescence experiments. A critical aggregation concentration associated with the formation of hydrophobic domains is determined through pyrene fluorescence assays. At higher concentration, defined ?-sheets develop as revealed by FTIR spectroscopy and X-ray diffraction. Transmission electron microscopy reveals self-assembled straight fibril structures. These are much shorter than those observed for amyloid peptides, the finite length may be set by the end cap energy due to the hydrophobicity of phenylalanine. The combination of these techniques points to different aggregation processes depending on concentration. Hydrophobic association into irregular aggregates occurs at low concentration, well-developed ?-sheets only developing at higher concentration. Drying of FFFF-PEG5000 solutions leads to crystallization of PEG, as confirmed by polarized optical microscopy (POM), FTIR and X-ray diffraction (XRD). PEG crystallization does not disrupt local ?-sheet structure (as indicated by FTIR and XRD). However on longer lengthscales the ?-sheet fibrillar structure is perturbed because spherulites from PEG crystallization are observed by POM.

Journal Keywords: Peptide Copolymers ; Self-Assembly ; Beta Sheets ; Fibrils

Subject Areas: Biology and Bio-materials

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