Journal of the American Chemical Society
Article
carbodiimides,8−10 or biomolecules like adenosine triphos-
phate (ATP).11,12 In alternative designs, these cycles harvest
their energy by oxidizing reducing agents13−15 or vice versa.
These cycles have been coupled to a rich diversity of
assemblies like micelles,13,16,17 vesicles,18 droplets,9,19 col-
loids,8,14 nano- or microparticle clusters,20,21 and fibers.7,8,11
Due to their dynamic nature, a variety of unique behaviors is
observed such as the emergence of oscillations, the control
over growth rates by reaction speeds, and simultaneous growth
and collapse.
assembly and disassembly of peptides into fibers. Reciprocally,
the assembly regulates the activation and deactivation of the
peptides. With a library of peptides, we found that the catalytic
activity depends on the degree of the internal structure of the
assembly, i.e., the greater the degree of organization, the faster
both reactions proceed. Consequently, the assembly can
accelerate its building block activation five-fold and its
deactivation two-fold. The underlying mechanism relies on
pKa shifts induced by the self-assembly. Finally, we
demonstrate that these findings are generalizable for a range
of chemically fueled peptide precursors.
However, systems in which the chemical reaction cycle and
the emerging assembly are reciprocally coupled remain rare. In
one strategy, the activated building blocks phase separate. Due
to the physical separation from the solvent, the deactivation
reaction via solvolysis is severely inhibited.8−10,22 In another
strategy, the opposite behavior occurs, i.e., the assembly
solubilizes poorly soluble precursor molecules by micelle or
vesicle formation.13,16,17 Consequently, the activation reaction
is drastically accelerated. Both strategies of feedback of the
assembly on their reaction cycle rely on phase separation,
which up- or- downconcentrate reactants in the reaction
cycle.8−10,13,16,22 However, such mechanisms differ vastly from
the feedback mechanism observed, for example, in micro-
tubules, in which the assembly catalyzes the deactivation
reaction. While catalytic activity by molecular assemblies has
been explored extensively,23−28 examples of molecular
assemblies accelerating the kinetics of their building block
activation or deactivation via catalysis are underexplored. In
one recent example, Das and coworkers made use of histidine-
based peptides to accelerate building block deactivation via
catalysis in a chemically fueled assembly.29 In another recent
example, Otto and co-workers show that assembled lysine-
based self-replicators can catalyze reactions that generate
molecules that promote formation of their building blocks,
thereby accelerating building block activation.15
RESULTS AND DISCUSSION
■
Self-Assembly Accelerates Activation and Deactiva-
tion. The chemical reaction cycle we used in this work is based
on formation of a transient anhydride at the expense of a
carbodiimide (fuel, Scheme 2).8 In the activation reaction, a
dicarboxylate-based precursor is condensed into its corre-
sponding cyclic anhydride by irreversibly reacting with EDC
([1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide], fuel).
As the reaction cycle takes place in water, the anhydride
product is exposed to water and spontaneously hydrolyzes,
which constitutes the deactivation reaction. Thus, a dicarbox-
ylate precursor is temporarily converted into a transient
anhydride at the expense of a molecule of carbodiimide.
Depending on the design of the precursor, conversion of the
anionic dicarboxylate into a noncharged product can induce
self-assembly. Alternately, the deactivation reaction reinstates
the negative charges on the precursor, and the increased
electrostatic repulsions can induce disassembly.
We first used the precursor Fmoc-AVD and compared it to
Ac-FAVD (in which Fmoc means fluorenylmethoxycarbonyl,
Ac means acetyl, F stands for phenylalanine, A stands for
alanine, V stands for valine, and D stands for aspartic acid, see
Figure 1A and 1B). The C-terminal aspartic acids of both of
these derivatives can react with carbodiimide-based condensing
agents like EDC to form their corresponding anhydrides. The
Fmoc-AVD was used because it is known to self-assemble into
hydrogel-forming fibers upon activation with EDC.8 We
designed the Ac-FAVD derivative, which lacks the hydro-
phobic and aromatic Fmoc group, such that it was not able to
self-assemble in the precursor or product state. Thus, Ac-
FAVD serves as a control to test the effect of the peptide self-
assembly on the kinetics of the reaction cycle. We prepared
both precursors on a 0.1 g scale using solid-phase peptide
synthesis and preparative HPLC (high-performance liquid
chromatography). The peptide purity was assessed by NMR,
soluble at 10 mM in 200 mM MES buffer at pH 6.0.
We found molecular assemblies regulated through chemi-
cally fueled reaction cycles that simultaneously catalyze their
building block activation and their building block deactivation
(Scheme 2). Thus, the chemical reaction cycle regulates
Scheme 2. Chemically Fueled Self-Assembly of Peptide
a
Fibers
We added 100 mM EDC to 10 mM Ac-FAVD and found no
evidence of self-assembly by the naked eye, confocal
mM Fmoc-AVD yielded a self-standing hydrogel (Figure 1C)
that collapsed with time. Confocal microscopy showed that a
dense network of bundled fibers formed upon EDC addition
(Figure 1 D). On the basis of the results above, we conclude
that Fmoc-AVD self assembles into fibers upon addition of
EDC. In contrast, due to the lack of the large Fmoc group, Ac-
FAVD does not assemble.
a
In this work, we find that activation and deactivation regulate self-
Next, we compared the kinetics of the reaction cycle of the
nonassembling Ac-FAVD and the fiber-forming Fmoc-AVD.
assembly and are both accelerated by the microenvironment of the
assembly.
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX