We explored whether we could tune the degree of kinetically trapping of precursors in fibers by mixing Fmoc-AVD with Fmoc-GGD. We assume that both peptides coassemble and form weaker hydrogen bonds resulting in a milder shift in the pKa of the carboxylates, thereby increasing the amount of negatively charged precursor molecules per fiber. Cryogenic transmission electron microscopy (cryo-TEM) showed that Fmoc-GGD assembled into vesicles in response to fuel (Fig. ), while a mixture of 25 mol% of Fmoc-AVD and 75 mol% Fmoc-GGD resulted in twisted tapes instead of vesicles; a good indication that the two coassemble. Moreover, the tapes of the mixture were more twisted compared to the tapes formed by 100 mol% Fmoc-AVD, which we explain by the decrease in the ability to form flat β-sheets. Indeed, a ThT-assay showed that the maximum emission intensity observed after the addition of EDC declined with the increasing ratio of Fmoc-GGD to Fmoc-AVD (Fig. ; Fig. ). This observation was further corroborated by the center of the amide-I band shifting from 1632 cm -1 to 1647 cm -1 upon gradually increasing the ratio of Fmoc-GGD to Fmoc-AVD (Fig. , Fig. ). More excitingly, the decrease in βsheet's rigidity enabled us to tune the pKa of the precursor between 5.8 and pKa 4.4 (Fig. , Fig. ). Thus, tuning the peptide content allowed us to change the microenvironment of the precursor and, with that, its degree of deprotonation. 1 H-NMR and fluorescence spectroscopy confirmed a decrease of kinetically trapped precursor from 48% to 18% for pure Fmoc-AVD to 50 mol% Fmoc-AVD with Fmoc-GGD content (Fig. , Fig. , Fig. ). From the combined experiments, we can create a simplified energy landscape of the transitions occurring in our chemically fueled system (Scheme 2). The peptide in this landscape can be in four states: it can be activated or not, and it can be in solution or the microenvironment of the fiber. Consequently, four reversible transitions are possible, i.e., the non-assembled precursor can be activated, the activated building block can assemble, the assembled activated product can deactivate, and the deactivated precursor can disassemble. The thermodynamically most favored state is the non-activated, non-assembled peptide. Upon activation, the product assembles into a fiber. Deactivation leads to the deactivated peptide in a fiber. The disassembly of the precursor is endowed with a high energy barrier (Eb) which leads to the precursor being kinetically trapped within the fiber. Due to the kinetic trapping, the deactivated peptide is more likely to become re-activated than disassemble. Thus, the peptide spends most of its time in the peptide microenvironment, and this is also where most fuel is consumed. Through changes in molecular design and pH, the Eb can be decreased. Scheme 2. Schematic and simplified representation of the free energy landscape of chemically fueled assembly.