Reflectin needs its intensity amplifier: Realizing the potential of tunable structural biophotonics
Underwater camouflage and communication in a family of recently evolved squids are controlled, in part, by the reflectin proteins, which regulate dynamic changes in the color and intensity of light reflected from intracellular Bragg reflectors and Mie scatterers. The reflectins act as a signal transducer, precisely tuning the dimensions and refractive index of the membrane-bounded structural reflectors containing them to an extent exactly proportional to neuronal signaling. This calibration between the activating signal and the output color and intensity of reflectance is governed by the physics of the liquid state of reflectin assemblies, which osmotically fine-tune the dimensions and refractive index of the membrane-bounded structural reflectors containing them. While thin-film or genetically engineered assemblies of reflectin deliver changes in the colors of reflection approximating those of the biological system (as do other proteins and polymers), none has delivered the signal-activated increase in the intensity of reflectance characteristic of the native system. The membrane envelopes (Bragg reflector lamellae and Mie-scattering vesicles) containing the reflectins apparently serve as amplifiers, functionally “impedance matched” to the range of tunable sizes of the reflectin assemblies; reflectin films or assemblies lacking such an amplifier fail to deliver signal-activated increases in reflectance intensity.