Ionic Gating of Liquid-Crystal-Like Pteridine Assemblies Enables Tunable Light Scattering.

The bottom-up assembly of small organic molecules into dynamic optical materials remains a major challenge in materials chemistry. Biological systems, however, demonstrate that hierarchical organization of such molecules can generate diverse optical functionalities. Here we show that zebrafish pigment cells undergo a developmentally regulated phase transition in which pteridine-rich organelles, known as pterinosomes, transform from disordered ultraviolet (UV)-absorbing compartments into highly reflective, liquid-crystal-like scattering structures. Cryogenic STEM diffraction mapping and electron microscopy reveal that this transition corresponds to the reorganization of a disordered fibrous matrix into a hierarchical, concentrically ordered mesophase. We show that pterinosomes are built from a composite pteridine material whose relative composition is developmentally tuned. We further identify ionic gating as a key regulator of this process: potassium (K+) depletion enables π-π stacking-driven ordering, thereby decoupling metabolite accumulation from crystallization. Optical modeling indicates that the resulting mesoscale organization generates strong dielectric anisotropy and refractive-index contrast, enabling efficient broadband scattering through collective effects in organelle arrays. Our findings establish ionic gating and hierarchical assembly as strategies for controlling the phase behavior of confined small-molecule systems and provide design principles for tunable bio-inspired photonic materials.