Structure-Function Relationships of Cephalopod Proteins Called Reflectins

Cephalopods are known for their remarkable ability to camouflage by rapidly changing the color and reflectance of their skin, a property that is enabled by the presence of optically active subcellular structures composed of unique structural proteins called reflectins. Besides playing a critical role in regulating the optical behavior of these organisms, reflectins have also emerged as potential candidates for applications in biophotonics and bioelectronics. However, despite their interesting optical (e.g., high refractive index) and electrical (e.g., excellent proton conductivity) properties, the development of reflectin-based materials has been impeded because of a lack of complete understanding of the relationships between their structures and properties. Moreover, an incomplete understanding of such relationships also limits our grasp over the exact mechanisms by which reflectins enable cephalopods’ optical functionalities. Herein, results from the investigation of the structure-function relationships of reflectin proteins are presented with the aims of (a) obtaining insights into the role of such relationships in cephalopods’ structural coloration and in the design of reflectin-based functional materials, (b) obtaining molecular-level insights into the structure of reflectin proteins, (c) developing new strategies to precisely control the assembly and optical and/or electrical properties of these proteins, and (d) validating new approaches for the fabrication of reflectin-based electrical devices. First, we have elucidated the molecular structure of a prototypical truncated reflectin variant that was designed to recapitulate the key characteristics of the parent full-length reflectin. Second, we have demonstrated a straightforward mechanical agitation-based methodology for controlling the reflectin variant’s secondary structure and assembly and established a direct correlation between the protein’s structural characteristics and intrinsic optical properties. Subsequently, we have explored the self-assembly and the in vitro optical properties (i.e., refractive indices) of a full-length reflectin isoform that is found in all three types of optically active cephalopod skin cells. Next, we have investigated the concentration-dependent assembly and the aggregated-state structural characteristics of the prototypical truncated reflectin variant followed by the fabrication and electrical interrogation of films from the same protein. Finally, we have validated inkjet printing as an excellent processing strategy for the fabrication of reflectin-based proton-conducting devices. Taken together, findings from this work afford exciting insights into the structure-function relationships of reflectins, reveal previously unknown critical considerations for the processing of this class of proteins, address multiple challenges associated with the development of reflectins as materials, furnish new perspectives on the mechanistic underpinnings of the optical functionality of cephalopod skin cells, and hold the potential to inform new strategies for the development of cephalopod-inspired biophotonic and bioelectronic platforms from the reflectin family of proteins