Beyond Fmoc: a review of aromatic peptide
capping groups
Adam D. Martin *a and Pall Thordarson *b
Self-assembling short peptides have attracted widespread interest due to their tuneable, biocompatible
nature and have potential applications in energy materials, tissue engineering, sensing and drug delivery.
The hierarchical self-assembly of these peptides is highly dependent on the selection of not only amino
acid sequence, but also the capping group which is often employed at the N-terminus of the peptide to
drive self-assembly. Although the Fmoc (9H-fluorenylmethyloxycarbonyl) group is commonly used due
to its utility in solid phase peptide synthesis, many other aromatic capping groups have been reported
which yield functional, responsive materials. This review explores recent developments in the utilisation
of functional, aromatic capping groups beyond the Fmoc group for the creation of redox-responsive,
fluorescent and drug delivering hydrogel scaffolds.
Self-assembly is a phenomenon which is ubiquitous in nature,
from the complementarity of DNA base pairs,1 to the transport
of ions across membranes,2,3 the folding of proteins into
functional tertiary structures,4,5 self-assembly underlies many
of themost fundamental biological processes. The self-assembly of
proteins is of particular interest, as the structure of the protein is
intricately linked with its function. Two well-known examples of
this are observed in Alzheimer’s disease, where minor modifications
to the amyloid and tau proteins result in the self-assembly
and the apparent pathological accumulation of these proteins,
resulting in a gain of toxic function.6,7
Originally identified as an aggregation-prone region of the
amyloid protein, the diphenylalanine sequence has been used
in a number of applications, from semi-conductors to nanophotonics,
to optics, with excellent reviews available on these
topics.8–10 Perhaps one of the most popular applications of the
diphenylalanine motif has been its incorporation into short
hydrogel-forming peptides. These hydrogels can be engineered
to mimic the physical and mechanical properties of the extracellular
matrix and have been extensively reviewed.11–15
Often, these diphenylalanine containing, self-assembling
peptide hydrogels are ‘‘capped’’ at their N-terminus with an
aromatic group. The choice of this capping group plays a key
role in the subsequent self-assembly of the peptide. It is known
that the diphenylalanine sequence alone (i.e., NH2-Phe-Phe-OH)
will not form hydrogels, instead they tend to form crystalline
nanotubes which have excelled in a number of applications.16–18
The introduction of the fluorenylmethyloxycarbonyl (Fmoc) group
to the N-terminus of the diphenylalanine sequence (Fmoc-FF),
as first reported by Gazit,19 resulted in the formation of a selfsupporting
hydrogel, formed through the dilution of Fmoc-FF
dissolved in hexafluoroisopropanol with water. Since this initial
study, extensive research effort has been applied to elucidating
different ways to initiate gelation,20,21 minimizing the variability
in resultant hydrogel networks (which are gelation-method
dependent),22 and developing applications for these nanostructured
scaffolds in tissue engineering, electronics and drug
One reason for the popularity of Fmoc-FF in various applications
is its ease of synthesis. Fmoc-FF can be synthesised either
using solution or solid phase peptide synthesis methods, owing
to the common use of the Fmoc group as an amine protecting
group in solid phase peptide synthesis (SPPS) and is also
commercially available through Bachem. However, this also
means that the Fmoc group is susceptible to cleavage at pH
values above 10, which can be problematic, as Fmoc-containing
peptide gelators are often dissolved in basic aqueous solutions
prior to initialising gelation. Upon cleavage of the Fmoc-group
from a peptide chain, a highly reactive dibenzofulvalene is
formed. While the toxicity of dibenzofulvalene coming from
Fmoc-based peptides has not been determined directly, our
studies have indicated that Fmoc-FF degradation products show
some cytotoxicity.28