An introduction to color-changing systems from the cephalopod protein reflectin

Abstract

Cephalopods possess unrivaled camouflage and signaling abilities that are enabled by their sophisticated skin, wherein multiple layers contain chromatophore pigment cells (as part of larger chromatophore organs) and different types of reflective cells called iridocytes and leucophores. The optical functionality of these cells (and thus cephalopod skin) critically relies upon subcellular structures partially composed of unusual structural proteins known as reflectins. Herein, we highlight studies that have investigated reflectins as materials within the context of color-changing coatings. We in turn discuss these proteins' multi-faceted properties, associated challenges, and future potential. Through our presentation of selected case studies, we hope to stimulate additional dialogue and spur further research on photonic technologies based on and inspired by reflectins.

Cephalopod-inspired optical engineering of human cells

“…octopuses, squids, and cuttlefish) demonstrating remarkable camouflage capabilities and even performing literal vanishing acts [12][13][14][15][16][17] . Indeed, these animals can dynamically alter how their skin transmits, absorbs, and reflects light through the functionality of unique natural optical components, which include pigmented organs called chromatophores, typically narrowband-reflecting cells called iridophores, and broadband-reflecting cells called leucophores [12][13][14][15][16][17] . As one specific example, the female Doryteuthis opalescens squid can avoid unwanted aggression by switching a stripe on its mantle from nearly transparent (i.e.…”

“…Many of the internalized photonic architectures that enable the optical functionalities of cephalopod skin cells (including leucophores) are composed of proteins known as reflectins 13,21,22 . With a few exceptions, reflectins' amino acid sequences consist of variable linker regions that are separated by conserved motifs with the highly general form (M/F-D-X 5 )(M-D-X 5 ) n (M-D-X 3/4 ) 13,21,22 . These sequences are unusual because they have a low percentage of common aliphatic amino acids, e.g.…”

“…alanine, leucine, isoleucine, and a high percentage of aromatic amino acids, e.g. tyrosine and tryptophan, while also being enriched in arginine, asparagine, and methionine 13,21,22 . This peculiar composition is thought to be directly responsible both for reflectins' remarkably diverse self-assembly properties 13,22 and for their unusually high refractive indices 23,24 .…”

“…tyrosine and tryptophan, while also being enriched in arginine, asparagine, and methionine 13,21,22 . This peculiar composition is thought to be directly responsible both for reflectins' remarkably diverse self-assembly properties 13,22 and for their unusually high refractive indices 23,24 . In particular, reflectins not only form the spheroidal leucosomes found in leucophores 19,20 and the membrane-enclosed platelets found in iridophores [25][26][27] in vivo, but they also readily assemble into nanoparticles 24,[28][29][30] , microfibers 24 , hexagonal plates 31 , and thin films 13,24,[32][33][34] in vitro.…”

Structure, self-assembly, and properties of a truncated reflectin variant

“…Altogether, our findings address multiple challenges associated with the development of reflectins as materials, furnish molecular-level insight into the mechanistic underpinnings of cephalopod skin cells' color-changing functionalities, and may inform new research directions across biochemistry, cellular biology, bioengineering, and optics. reflectin | self-assembly | proteins | biomaterials | optical properties M aterials from naturally occurring and recombinant proteins are frequently employed for the study of fundamental biological processes and leveraged for applications in fields as diverse as electronics, optics, bioengineering, medicine, and fashion (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). Such broad utility is enabled by the numerous advantageous characteristics of protein-based materials, which include sequence modularity, controllable self-assembly, stimuli-responsiveness, straightforward processability, inherent biological compatibility, and customizable functionality (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13).…”