β Sheets Not Required: Combined Experimental and Computational Studies of Self-Assembly and Gelation of the Ester-Containing Analogue of an Fmoc-Dipeptide Hydrogelator

In
our work toward developing ester-containing self-assembling
peptides as soft biomaterials, we have found that a fluorenylmethoxycarbonyl
(Fmoc)-conjugated alanine-lactic acid (Ala-Lac) sequence self-assembles
into nanostructures that gel in water. This process occurs despite
Fmoc-Ala-Lac’s inability to interact with other Fmoc-Ala-Lac
molecules via β-sheet-like amide–amide hydrogen bonding,
a condition previously thought to be crucial to the self-assembly
of Fmoc-conjugated peptides. Experimental comparisons of Fmoc-Ala-Lac
to its self-assembling peptide sequence analogue Fmoc-Ala-Ala using
a variety of microscopic, spectroscopic, and bulk characterization
techniques demonstrate distinct features of the two systems and show
that while angstrom-scale self-assembled structures are similar, their
nanometer-scale size and morphological properties diverge and give
rise to different bulk mechanical properties. Molecular dynamics simulations
were performed to gain more insight into the differences between the
two systems. An analysis of the hydrogen-bonding and solvent-surface
interface properties of the simulated fibrils revealed that Fmoc-Ala-Lac
fibrils are stronger and less hydrophilic than Fmoc-Ala-Ala fibrils.
We propose that this difference in fibril amphiphilicity gives rise
to differences in the higher-order assembly of fibrils into nanostructures
seen in TEM. Importantly, we confirm experimentally that β-sheet-type
hydrogen bonding is not crucial to the self-assembly of short, conjugated
peptides, and we demonstrate computationally that the amide bond in
such systems may act mainly to mediate the solvation of the self-assembled
single fibrils and therefore regulate a more extensive higher-order
aggregation of fibrils. This work provides a basic understanding for
future research in designing highly degradable self-assembling materials
with peptide-like bioactivity for biomedical applications.