Fibre formations of peptides cause diseases. Famous examples are Prion disease and Alzheimer's disease. The origin of such diseases have been investigated intensively so far and concluded that it is related to fibres of β-sheet forming peptide. There are two kinds of peptide which forms β-sheet and α-helix, however, there is little research for the latter.
In this study, we synthesized five kinds of amino acids (Leu, Glu, Thr, Ala and Lys), and designed α3 peptide: (Leu-Glu-Thr-Leu-Ala-Lys-Ala)3 to form an amphiphilic α-helical structure. Structures of peptides were observed by means of atomic force microscopy (AFM) in air at room temperature. All AFM measurements were performed on cleaved mica(001) surface covered by a 20µl of peptide solutions.
Experimentally obtained AFM images showed that α3 peptide forms fibre structures at pH=3-6. Typical dimensions of a single fibre are 1000x40x2.5 nm3. Roughly, a single fibre includes more than 400000 peptides. The height of the fibre is independent of pH, while the width of the fibre gradually changes from 50 nm (pH=3) to 30 nm (pH=6). However, such fibre structure was destroyed when pH changed. At pH=2, α3 peptide forms dots with a size of about 50 nm. Also, at pH=7 and 8, no fibre was observed.
Since we found fibre formation of the α-helix forming peptide, we focused to study a mechanism of the α3 fibre formation. In a helix of α3 peptide there are two Ala residues in addition to Leu residues in the hydrophobic surface. Ala at the fifth or the seventh position in the repeating sequence in the α3 peptide was changed to hydrophilic amino acid (Gln), and we synthesized two new hydrophiphilic peptides at pH=5.5: 5Q (Leu-Glu-Thr-Leu-Gln-Lys-Ala)3 and 7Q (Leu-Glu-Thr-Leu-Ala-Lys-Glu)3.
AFM images showed that 5Q peptide forms fibres. Its single fibre has a dimension of 450x60x3.9 nm3 including about 40000 peptides. However, 7Q peptide did not form fibres. These indicate that the hydrophobic Ala residue at the seventh position in the unit sequence in the α3 is a key residue to form a fibre structure.
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