All-atom MD simulations of heterochiral peptides' self-assembly revealed a general rigidification of the system after heating that persisted upon subsequent cooling to rt, with peptides sampling a significantly smaller range of conformations (see ESI Section S7). Initially (Fig. ), the supramolecular assemblies at rt gave rise to a 3D network containing water channels along three directions and compatible with the observed fibrillar hydrogel. During heating, solubility decreased, the hydrophobic peptides aggregated, and 5-to-9 water molecules per peptide molecule were released from solvation shells, leading to an increase in entropy and the formation of aggregates that are fully separated from each other along one direction (Fig. ). These aggregates could serve as seeds for the onset of plates, that is structures likely corresponding to a thermodynamic sink and thus leading to an irreversible transition. Re-assembly was driven by the entropic gain of the system due to the release of water molecules during the heating phase. Due to the temperature dependence of this term (TΔS) in the overall free energy balance of the process, this gain increases with increasing temperatures, leading the system to a deeper free energy minimum and rendering the process virtually irreversible. This picture is corroborated by several analyses including the calculation of the Solvent Accessible Surface Area (SASA), which decreases in the morphological transition, and the number of water molecules set free in the bulk phase, correlated with that reduction. The reduction in SASA (Table ), leading also to a reduction in the number of water-peptide H-bonds is, however, partly compensated by the increase in the number of such bonds involving only the peptides (Table ). Moreover, the overall number of bridging waters did not change before and after the heating/cooling steps. This is likely because virtually all the possible H-bonds of the termini and backbone were saturated in the MD simulation before the heating of the system, a picture consistent with the very minor changes in the nature of the surface exposed to the solvent (Figure ). Interestingly, the β-structure content was reduced, but no dramatic change to a different conformation was seen, as confirmed by MD simulations, FT-IR, circular dichroism (CD), and Thioflavin T fluorescence analyses (Fig. and). An increase of the temperature up to 363 K leads the peptide to sample different conformations, some corresponding to L-α-helices. However, this difference almost vanishes when simulating self-assembly of hundreds of tetrapeptides in explicit solvent (Fig. ), thus supporting the role of water thermodynamics in the process and the high stability of β-structures. Layered βstructures have been predicted to be the most stable secondary conformation for amyloid superstructures, due to the preference of backbones to engage in extended H-bonding to form sheets. Figure . Representative conformations extracted from MD simulations of heterochiral tetrapeptide assemblies at rt before (a) and after heating to 363 K and cooling to 298 K . Surfaces are colored by atom type (grey, white, blue, and red for C, H, N, and O respectively). In conclusion, we describe the first uncapped D,L-tetrapeptide self-assembly into a hydrogel that, upon heating, undergoes an irreversible morphological transition from fibrils to plates, in marked contrast with all the short peptide assemblies described thus far. This investigation reveals that the peptide undergoes only minor conformational changes, while water plays a key role in the transition, whereby water-peptide interactions are replaced with peptide-peptide interactions, and the entropic gain of the water molecules that are set free locates the system in an energetic sink. These results shed new light on the modulation of peptide assemblies and their morphology.