Fibrils Emerging from Droplets: Molecular Guiding Principles behind Phase Transitions of a Short Peptide‐Based Condensate Studied by Solid‐State NMR**

Abstract

AbstractBiochemical reactions occurring in highly crowded cellular environments require different means of control to ensure productivity and specificity. Compartmentalization of reagents by liquid‐liquid phase separation is one of these means. However, extremely high local protein concentrations of up to 400 mg/ml can result in pathological aggregation into fibrillar amyloid structures, a phenomenon that has been linked to various neurodegenerative diseases. Despite its relevance, the process of liquid‐to‐solid transition inside condensates is still not well understood at the molecular level. We thus herein use small peptide derivatives that can undergo both liquid‐liquid and subsequent liquid‐to‐solid phase transition as model systems to study both processes. Using solid‐state nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), we compare the structure of condensed states of leucine, tryptophan and phenylalanine containing derivatives, distinguishing between liquid‐like condensates, amorphous aggregates and fibrils, respectively. A structural model for the fibrils formed by the phenylalanine derivative was obtained by an NMR‐based structure calculation. The fibrils are stabilised by hydrogen bonds and side‐chain π‐π interactions, which are likely much less pronounced or absent in the liquid and amorphous state. Such noncovalent interactions are equally important for the liquid‐to‐solid transition of proteins, particularly those related to neurodegenerative diseases.