On Crystal versus Fiber
Formation in Dipeptide Hydrogelator
Systems
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Abstract
Naphthalene dipeptides have been shown to be useful low-molecular-weight
gelators. Here we have used a library to explore the relationship
between the dipeptide sequence and the hydrogelation efficiency. A
number of the naphthalene dipeptides are crystallizable from water,
enabling us to investigate the comparison between the gel/fiber phase
and the crystal phase. We succeeded in crystallizing one example directly
from the gel phase. Using X-ray crystallography, molecular modeling,
and X-ray fiber diffraction, we show that the molecular packing of
this crystal structure differs from the structure of the gel/fiber
phase. Although the crystal structures may provide important insights
into stabilizing interactions, our analysis indicates a rearrangement
of structural packing within the fibers. These observations are consistent
with the fibrillar interactions and interatomic separations promoting
1D assembly whereas in the crystals the peptides are aligned along
multiple axes, allowing 3D growth. This observation has an impact
on the use of crystal structures to determine supramolecular synthons
for gelators