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A 20-residue peptide E5 containing five glutamates, an
analog of the fusion peptide of influenza virus hemagglutinin
(HA) exhibiting fusion activity at acidic pH lower than
6.0–6.5 was studied by circular dichroism (CD), Fourier
transform infrared, and 1H-NMR spectroscopy
in water, water/trifluoroethanol (TFE) mixtures, dodecylphosphocholine
(DPC) micelles, and phospholipid vesicles. E5 became structurally
ordered at pH ≤6 and the helical content in the peptide
increased in the row: water < water/TFE < DPC ∼
phospholipid vesicle while the amount of β-structure
was approximately reverse. 1H-NMR data and line-broadening
effect of 5-, 16-doxylstearates on proton resonances of
DPC bound peptide showed E5 forms amphiphilic α-helix
in residues 2–18, which is flexible in 11–18
part. The analysis of the proton chemical shifts of DPC
bound and CD intensity at 220 nm of phospholipid bound
E5 showed that the pH dependence of helical content is
characterized by the same pKa ≈5.6.
Only Glu11 and Glu15 in DPC bound peptide showed such elevated
pKas, presumably due to transient hydrogen
bond(s) Glu11 (Glu15) δCOO−(H+)
… HN Glu15 that dispose(s) the side chain of Glu11
(Glu15) residue(s) close to the micelle/water interface.
These glutamates are present in the HA-fusion peptide and
the experimental half-maximal pH of fusion for HA and E5
peptides is ∼5.6. Therefore, a specific anchorage of
these peptides onto membrane necessary for fusion is likely
driven by the protonation of the carboxylate group of Glu11
(Glu15) residue(s) participating in transient hydrogen
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