Reflectin proteins are widely distributed in reflective structures in cephalopods. However, only in loliginid squids are they and the subwavelength photonic structures they control dynamically tunable, driving changes in skin color for camouflage and communication. The reflectins are block copolymers with repeated canonical domains interspersed with cationic linkers. Neurotransmitter-activated signal transduction culminates in catalytic phosphorylation of the tunable reflectins' cationic linkers; the resulting charge neutralization overcomes coulombic repulsion to progressively allow condensation, folding, and assembly into multimeric spheres of tunable well-defined size and low polydispersity. Here, we used dynamic light scattering, transmission EM, CD, atomic force microscopy, and fluorimetry to analyze the structural transitions of reflectins A1 and A2. We also analyzed the assembly behavior of phosphomimetic, deletion, and other mutants in conjunction with pH titration as an in vitro surrogate of phosphorylation. Our experiments uncovered a previously unsuspected, precisely predictive relationship between the extent of neutralization of a reflectin's net charge density and the size of resulting multimeric protein assemblies of narrow polydispersity. Comparisons of mutants revealed that this sensitivity to neutralization resides in the linkers and is spatially distributed along the protein. Imaging of large particles and analysis of sequence composition suggested that assembly may proceed through a dynamically arrested liquid-liquid phase-separated intermediate. Intriguingly, it is this dynamic arrest that enables the observed fine-tuning by charge and the resulting calibration between neuronal trigger and color in the squid. These results offer insights into the basis of reflectin-based biophotonics, opening paths for the design of new materials with tunable properties.

Magnetic barium phosphate nanoflakes with embedded iron oxide nanoparticles, Fe3O4@Ba3(PO4)2 (denoted FBP), were prepared through a facile and inexpensive two-step process. FBP was used to purify water heavily contaminated with E. coli (initial concentration of 5 × 108 CFU mL-1). FBP exhibited high removal efficiency (97%) within 30 min at 25 °C and pH 6. We investigated the effects of factors such as pH, ionic strength, co-existing anions, temperature, contact time, material dosage, and initial concentration of bacterial suspension, and developed optimized treatment conditions. The negligible effect of solution ionic strength on bacterial removal efficiency of FBP indicates its potential for microbial control even in high salinity water. Importantly, FBP can maintain a high bacterial removal efficiency of 87% after being reused for five cycles. FBP's magnetic properties allow an easy recovery from water. Several types of forces and mechanisms are thought to be involved in the bacterial removal process by FBP: electrostatic interactions, adhesion to FBP's planar surface, flocculation by polyvalent cations on FBP's surface, oxidation sterilization from Fe3O4 in FBP, irreversible cell structural damage by FBP's edges and corners, and magnetic aggregation under a magnetic field. Thus, FBP is a promising material for effectively treating water with high microbial contamination.