posted on 2020-03-16, 14:33authored byJuliane N.B.D. Pelin, Charlotte J. C. Edwards-Gayle, Valeria Castelletto, Andrea M. Aguilar, Wendel A. Alves, Jani Seitsonen, Janne Ruokolainen, Ian W. Hamley
The
self-assembly of the amphiphilic lipopeptide PAEPKI-C16 (P = proline, A = alanine, E = glutamic acid, K = lysine, I = isoleucine,
and C16 = hexadecyl) was investigated using a combination
of microscopy, spectroscopy, and scattering methods and compared to
that of C16-IKPEAP with the same (reversed) peptide sequence
and the alkyl chain positioned at the N-terminus and lacking a free
N-terminal proline residue. The catalytic activity of these peptides
was then compared using a model aldol reaction system. For PAEPKI-C16, the cryo-TEM images showed the formation of micrometer-length
fibers, which by small-angle X-ray scattering (SAXS) were found to
have radii of 2.5–2.6 nm. Spectroscopic analysis shows that
these fibers are built from β-sheets. This behavior is in complete
contrast to that of C16-IKPEAP, which forms spherical micelles
with peptides in a disordered conformation [Hutchinson J. Phys. Chem. B 2019, 123, 613]. In PAEPKI-C16, spontaneous
alignment of fibers was observed upon increasing pH, which was accompanied
by observed birefringence and anisotropy of SAXS patterns. This shows
the ability to form a nematic phase, and unprecedented nematic hydrogel
formation was also observed for these lipopeptides at sufficiently
high concentrations. SAXS shows retention of an ultrafine (1.7 nm
core radius) fibrillar network within the hydrogel. PAEPKI-C16 with free N-terminal proline shows enhanced anti:syn diastereoselectivity and better conversion compared to C16-IKPEAP. The cytotoxicity of PAEPKI-C16 was also lower
than that of C16-IKPEAP for both fibroblast and cancer
cell lines. These results highlight the sensitivity of lipopeptide
properties to the presence of a free proline residue. The spontaneous
nematic phase formation by PAEPKI-C16 points to the high
anisotropy of its ultrafine fibrillar structure, and the formation
of such a phase at low concentrations in aqueous solution may be valuable
for future applications.