Crystallization of L‐Cysteine in Heavy Water Induces Intrinsic Fluorescence

Developing noninvasive techniques that can probe how solvents modulate the nucleation pathways of bioorganic molecules in solution remains an active and open area of research. Herein, we investigate the crystallization of the amino acid L-Cysteine and show that both the structure of the crystal and its intrinsic fluorescence can be drastically altered by the solvent. Crystals formed in heavy water exhibit markedly different intermolecular packing as well as strikingly different monomer conformations compared to those in light water. Remarkably, these differences in the supramolecular packing result in significantly elevated intrinsic fluorescence in the crystal that is formed in heavy water. Using a combination of experimental techniques and advanced electronic structure approaches, we elucidate the molecular interactions within the crystals that govern both the electronic origins and the intensity of their emission. These findings demonstrate how tuning the solvent by changing its isotope leads to the emergence of design principles for new intrinsic fluorophores that could serve as novel sensing probes for biomedical applications.