A Mini-Review on Reflectins, from Biochemical Properties to Bio-Inspired Applications

Abstract

Some cephalopods (squids, octopuses, and cuttlefishes) produce dynamic structural colors, for camouflage or communication. The key to this remarkable capability is one group of specialized cells called iridocytes, which contain aligned membrane-enclosed platelets of high-reflective reflectins and work as intracellular Bragg reflectors. These reflectins have unusual amino acid compositions and sequential properties, which endows them with functional characteristics: an extremely high reflective index among natural proteins and the ability to answer various environmental stimuli. Based on their unique material composition and responsive self-organization properties, the material community has developed an impressive array of reflectin- or iridocyte-inspired optical systems with distinct tunable reflectance according to a series of internal and external factors. More recently, scientists have made creative attempts to engineer mammalian cells to explore the function potentials of reflectin proteins as well as their working mechanism in the cellular environment. Progress in wide scientific areas (biophysics, genomics, gene editing, etc.) brings in new opportunities to better understand reflectins and new approaches to fully utilize them. The work introduced the composition features, biochemical properties, the latest developments, future considerations of reflectins, and their inspiration applications to give newcomers a comprehensive understanding and mutually exchanged knowledge from different communities (e.g., biology and material).

Keywords: bio-inspired applications; biochemical properties; origin; reflectin; self-organization.

Figure 1

Timeline and roadmap for reflectin-associated studies, including their initial component identification, fundamental studies about their working mechanism, and novel inspiring bio-inspired applications [5,13,18,19,25,26,32,33,34,35,36,38,39,40,41]. Copyright 2007, Nature Publishing Group. Copyright 2009, Elsevier Ltd. Copyright 2004, Science Publishing Group. Copyright 2009, The Royal Society. Copyright 2018, Science Publishing Group. Copyright 2018, IOP Publishing Ltd. Copyright 2013, National Academy of Sciences of the United States of America. Copyright 2013, John Wiley & Sons, Inc. Copyright 2014, Nature Publishing Group. Copyright 2021, the American Chemical Society. Copyright 2022, Frontiers Publishing Group. Copyright 2016, the American Society for Biochemistry and Molecular Biology. Copyright 2017, AIP Publishing LLC. Copyright 2021, Nature Publishing Group. Copyright 2016, John Wiley & Sons, Inc.

Figure 2

(A) Net charge neutralized progressive and reversible hierarchical assembly driven by pH-titration (in vitro) or phosphorylation (in vivo) [14]. Copyright 2017, AIP Publishing LLC. (B–E) TEMs of reflectin A1 assemblies at pH6.5 and pH7.5 at low and high magnifications [38]. Copyright 2015, American Society for Biochemistry and Molecular Biology.

Figure 3

Proposed ACh-activated reflectin platelet condensation and iridescence tuning [32]. Copyright 2013, National Academy of Sciences of the United States of America.

Figure 4

(A) The response changes of the reflection spectrum of the RA1 film layer before and after the acetic acid treatment; (B) Changes in the electrostatic state of RA1 nanoparticles before and after the acetic acid treatment and the expansion of the film layer [33]. Copyright 2013, WILEY-VCH Verlag.

Figure 5

(A) RefCBA films of different layers (as red numbers in the figure) prepared by the spin-coating method; (B) RefCBA films of different thicknesses; (C) Dynamic color response of RefCBA films of different thicknesses under different relative humidity [23]. Copyright 2013, Wiley Periodicals, Inc.

Figure 6

(A) Structure of the reflectin/BSA composite sheet layer inspired by cephalopods; (B) Light response of the phytochrome/reflectin composite sheet layer [40]. Copyright 2021, Nature Publishing Group.

Figure 7

(A) Surface reflection before and after the mechanical stretching of the TiO₂/SiO₂ composite sheet layer [25]; Copyright 2018, Science Publishing Group. (B) Biological semiconductor based on the film layer of reflectins [34]. Copyright 2014, Nature Publishing Group.

Figure 8

(A) Electron-dense structure of RA1 particles based on the cross-sectional view of the cells expressing RA1 observed by TEM; (B) Optical characteristics of the cells after expressing RA1 proteins (compared with the Petri dish substrate, the contrast increases); (C) changes in the transparency of cells expressing RA1 proteins before and after the treatment with sodium chloride solutions with the scale bar = 25 µm [37]. Copyright 2020, Nature Publishing Group.

Figure 9

(A) Co-positioning of reflectins and the cytoskeleton in the cell center, in interphase cells, with the scale bars = 10 μm. (B) Positioning of reflectins in the spindle center at the late stage of cell division, with the scale bars = 5 μm. Nuclei and microtubules are stained with DAPI (blue) and Tubulin-Tracker Red (red); RA1 is indicated by tandem EGFP (green); Yellow comes from the merging of green and red [36]. Copyright 2022, Frontiers Publishing Group.