Voltage-calibrated, finely tunable protein assembly.

Neuronally triggered phosphorylation drives the calibrated and cyclable assembly of the reflectin signal transducing proteins, resulting in their fine tuning of colours reflected from specialized skin cells in squid for camouflage and communication. In close parallel to this physiological behaviour, we demonstrate for the first time that electrochemical reduction of reflectin A1, used as a surrogate for charge neutralization by phosphorylation, triggers voltage-calibrated, proportional and cyclable control of the size of the protein's assembly. Electrochemically triggered condensation, folding and assembly were simultaneously analysed using in situ dynamic light scattering, circular dichroism and UV absorbance spectroscopies. The correlation of assembly size with applied potential is probably linked to reflectin's mechanism of dynamic arrest, which is controlled by the extent of neuronally triggered charge neutralization and the corresponding fine tuning of colour in the biological system. This work opens a new perspective on electrically controlling and simultaneously observing reflectin assembly and, more broadly, provides access to manipulate, observe and electrokinetically control the formation of intermediates and conformational dynamics of macromolecular systems.

A new electrochemical method that mimics phosphorylation of the core tau peptide K18 enables kinetic and structural analysis of intermediates and assembly.

Tau protein's reversible assembly and binding of microtubules in brain neurons are regulated by charge-neutralizing phosphorylation, while its hyperphosphorylation drives the irreversible formation of cytotoxic filaments associated with neurodegenerative diseases. However, the structural changes that facilitate these diverse functions are unclear. Here, we analyzed K18, a core peptide of tau, using newly developed spectroelectrochemical instrumentation that enables electroreduction as a surrogate for charge neutralization by phosphorylation, with simultaneous, real-time quantitative analyses of the resulting conformational transitions and assembly. We observed a tipping point between behaviors that paralleled the transition between tau's physiologically required, reversible folding and assembly and the irreversibility of assemblies. The resulting rapidly electroassembled structures represent the first fibrillar tangles of K18 that have been formed in vitro at room temperature without using heparin or other charge-complementary anionic partners. These methods make it possible to (i) trigger and analyze in real time the early stages of conformational transitions and assembly without the need for preformed seeds, heterogenous coacervation, or crowding; (ii) kinetically resolve and potentially isolate never-before-seen early intermediates in these processes; and (iii) develop assays for additional factors and mechanisms that can direct the trajectory of assembly from physiologically benign and reversible to potentially pathological and irreversible structures. We anticipate wide applicability of these methods to other amyloidogenic systems and beyond.

Low Voltage Voltammetry Probes Proton Dissociation Equilibria of Amino Acids and Peptides.

Platinum-catalyzed electrochemical reduction of dissociable protons at low potentials was used to investigate proton dissociation equilibria of freely diffusing and peptide-incorporated charged amino acids. We first demonstrate with five charged essential amino acids and their analogs that the electrochemically induced deprotonation of each amino acid occurs at distinct formal reduction potential. Moreover, the observed direct reduction for all the charged species, excluding arginine, occurs at low potentials suitable for investigation under aqueous conditions (-0.4 to -0.9 V vs Ag/AgCl). The direct proton reduction was resolved via deconvolution of the observed differential pulse voltammogram (DPV) from background hydronium reduction and water electrolysis. A linear correlation was found between the formal reduction potentials and the pKa values of the dissociable protons hosted by various molecular moieties in the amino acids and their analogs and further verified with tripeptides. DPV of poly(l-lysine) decamer (Lys10) distinctively resolved the pKa values of the amino groups in the side chains and N-terminus, at a resolution not possible by conventional acid-base titration. This work demonstrates selective electrochemical titration of dissociable protons in charged amino acids in the free state and as residues in biomolecules, as well as the utility of DPV to indirectly interrogate local electrostatic environments that are essential to the stability and function of biomolecules.

Reversible electrochemical triggering and optical interrogation of polylysine α-helix formation.

Reversible electrochemical triggering of the random coil to α-helix conformational transition of polylysine (Lys10, Lys20, Lys50) was accomplished at a Pt electrode at potentials < |1| V vs. Ag/AgCl. Direct electroreduction of the N-terminus vs ε-amino groups in Lys sidechains, as well as hydronium reduction and electrolysis, could be easily distinguished and deconvolved using differential pulse voltammetry. Electrochemistry was coupled with in situ UV absorbance and circular dichroism spectroscopies to dynamically follow the evolution of α-helix formation at different potentials. Isotope experiments in H2O vs. D2O unequivocally confirm that direct electroreduction of ε-NH3+/ND3+ groups in Lys sidechains, rather than electrochemically generated pH gradient-induced deprotonation, leads to subsequent α-helix formation. The site-selective electrochemistry and optical methodologies presented herein can be generalized and extended to interrogate other protonation-sensitive biomolecular systems, and potentially provide access to early intermediates and control over the dynamic structural evolution of peptides and proteins.

Electrochemistry as a surrogate for protein phosphorylation: voltage-controlled assembly of reflectin A1.

Phosphorylation is among the most widely distributed mechanisms regulating the tunable structure and function of proteins in response to neuronal, hormonal and environmental signals. We demonstrate here that the low-voltage electrochemical reduction of histidine residues in reflectin A1, a protein that mediates the neuronal fine-tuning of colour reflected from skin cells for camouflage and communication in squids, acts as an in vitro surrogate for phosphorylation in vivo, driving the assembly previously shown to regulate its function. Using micro-drop voltammetry and a newly designed electrochemical cell integrated with an instrument measuring dynamic light scattering, we demonstrate selective reduction of the imidazolium side chains of histidine in monomers, oligopeptides and this complex protein in solution. The formal reduction potential of imidazolium proves readily distinguishable from those of hydronium and primary amines, allowing unequivocal confirmation of the direct and energetically selective deprotonation of histidine in the protein. The resulting 'electro-assembly' provides a new approach to probe, understand, and control the mechanisms that dynamically tune protein structure and function in normal physiology and disease. With its abilities to serve as a surrogate for phosphorylation and other mechanisms of charge neutralization, and to potentially isolate early intermediates in protein assembly, this method may be useful for analysing never-before-seen early intermediates in the phosphorylation-driven assembly of other proteins in normal physiology and disease.

Reflectin Proteins Bind and Reorganize Synthetic Phospholipid Vesicles.

The reflectin proteins have been extensively studied for their role in reflectance in cephalopods. In the recently evolved Loliginid squids, these proteins and the structural color they regulate are dynamically tunable, enhancing their effectiveness for camouflage and communication. In these species, the reflectins are found in highest concentrations within the structurally tunable, membrane enclosed, periodically stacked lamellae of subcellular Bragg reflectors and in the intracellular vesicles of specialized skin cells known as iridocytes and leuocophores, respectively. To better understand the interactions between the reflectins and the membrane structures that encompass them, we analyzed the interactions of two purified reflectins with synthetic phospholipid membrane vesicles similar in composition to cellular membranes, using confocal fluorescence microscopy and dynamic light scattering. The purified recombinant reflectins were found to drive multivalent vesicle agglomeration in a ratio-dependent and saturable manner. Extensive proteolytic digestion terminated with PMSF of the reflectin A1-vesicle complexes triggered energetic membrane rearrangement, resulting in vesicle fusion, fission, and tubulation. This behavior contrasted markedly with that of vesicles complexed with reflectin C, from which PMSF-terminated proteolysis only released the original size vesicles. Clues to the basis for this difference, residing in significant differences between the structures of the two reflectins, led to the suggestion that specific reflectin-membrane interactions may play a role in the ontogenetic formation, long-term maintenance, and/or dynamic behavior of their biophotonically active host membrane nanostructures. Similar energetic remodeling has been associated with osmotic stress in other membrane systems, suggesting a path to reconstitution of the biophotonic system in vitro.

Calibration between trigger and color: Neutralization of a genetically encoded coulombic switch and dynamic arrest precisely tune reflectin assembly.

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.

Sparkling Reflective Stacks of Purine Crystals in the Nudibranch Flabellina iodinea.

Although pigments contribute to much of the brilliant purple and orange coloration of the aeolid nudibranch Flabellina iodinea, the optical appearance of the animal was found to be augmented by dynamically sparkling, brightly reflective material in cells located throughout its epidermis. Electron microscopy revealed that specialized cells most abundant near the epithelial basal lamina contain numerous multilayer stacks of crystals, each within a fragile membrane capsule. High-resolution light microscopy of tissue sections showed that these crystalline stacks intermittently reflect light, with a temporally dynamic, sparkling appearance, suggesting that they are free to move-a phenomenon also observed in the live, intact whole animal and in the purified crystal stacks as well. Thin-layer chromatography and ultraviolet spectrometry show that the crystals isolated from all epithelial tissues are identical in composition, with guanine being the major component and its derivative, hypoxanthine, a minor component, regardless of the tissue's pigmentary color. Electron diffraction of the crystals purified separately from the orange and purple tissues exhibits nearly identical lattice parameters that closely match those measured for guanine crystals, which are widely distributed in other biophotonic systems ranging from marine invertebrates to terrestrial vertebrates. Heterogeneity of the thickness and spacing of the crystals within their stacks accounts for their broadband silvery reflectance. The optical appearance of the epidermis of this nudibranch thus results from the interaction of incident light with mobile stacks of purine crystals, augmenting the effects of its pigmentary colors.

Silicatein: A Unique Silica-Synthesizing Catalytic Triad Hydrolase From Marine Sponge Skeletons and Its Multiple Applications.

Silicatein, a silica-synthesizing, catalytic triad hydrolase, was discovered in the silica spicules comprising the skeletons of certain marine sponges. Sequence similarity is closest to that of the mammalian cathepsin L, a catalytic triad hydrolase and protease. Genetic substitutions of residues in the catalytic triad, the predictive activities of polymeric and small-molecule analogs of the enzyme, and the wide range of structures accepted as substrates all support a reaction mechanism closely analogous to that established for the classical catalytic triad hydrolases. In this mechanism, hydrogen bonding of residues in the catalytic site is required to enhance nucleophilic attack and consequent hydrolysis of silicon alkoxide (and a wide range of other precursors), enabling subsequent polycondensation. Experimental and computational analyses revealed a novel pathway of self-assembly, in which the silicatein subunits first form a fractally patterned intermediate before entropic rearrangement to the hexagonally close-packed, macroscopic filament that serves both as the catalyst of silica synthesis in the sponge, and as a template guiding the deposition and emergent structure of the macroscopic silica filaments that form the sponge skeleton. Silicatein also proves capable of catalyzing the synthesis of organic silicones, metal oxides, metal phosphates, polylactides, and polymeric materials composed of organic metal compounds from their corresponding precursors, suggesting an evolutionary relaxation of structural substrate specificity that may have been necessary to accommodate the organic adducts of silicic acid suggested to comprise the natural precursor of the biogenic silica. Methods for purification, characterization, assay, and multiple uses of the enzyme are described.

Moth eye-inspired anti-reflective surfaces for improved IR optical systems & visible LEDs fabricated with colloidal lithography and etching.

Near- and sub-wavelength photonic structures are used by numerous organisms (e.g. insects, cephalopods, fish, birds) to create vivid and often dynamically-tunable colors, as well as create, manipulate, or capture light for vision, communication, crypsis, photosynthesis, and defense. This review introduces the physics of moth eye (ME)-like, biomimetic nanostructures and discusses their application to reduce optical losses and improve efficiency of various optoelectronic devices, including photodetectors, photovoltaics, imagers, and light emitting diodes. Light-matter interactions at structured and heterogeneous surfaces over different length scales are discussed, as are the various methods used to create ME-inspired surfaces. Special interest is placed on a simple, scalable, and tunable method, namely colloidal lithography with plasma dry etching, to fabricate ME-inspired nanostructures in a vast suite of materials. Anti-reflective surfaces and coatings for IR devices and enhancing light extraction from visible light emitting diodes are highlighted.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring.

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl2/N2-based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

Wavelength-specific forward scattering of light by Bragg-reflective iridocytes in giant clams.

A surprising recent discovery revealed that the brightly reflective cells ('iridocytes') in the epithelia of giant clams actually send the majority of incident photons 'forward' into the tissue. While the intracellular Bragg reflectors in these cells are responsible for their colourful back reflection, Mie scattering produces the forward scattering, thus illuminating a dense population of endosymbiotic, photosynthetic microalgae. We now present a detailed micro-spectrophotometric characterization of the Bragg stacks in the iridocytes in live tissue to obtain the refractive index of the high-index layers (1.39 to 1.58, average 1.44 ± 0.04), the thicknesses of the high- and low-index layers (50-150 nm), and the numbers of pairs of layers (2-11) that participate in the observed spectral reflection. Based on these measurements, we performed electromagnetic simulations to better understand the optical behaviour of the iridocytes. The results open a deeper understanding of the optical behaviour of these cells, with the counterintuitive discovery that specific combinations of iridocyte diameter and Bragg-lamellar spacing can produce back reflection of the same colour that is also scattered forward, in preference to other wavelengths that are scattered at higher angles. We find for all values of size and wavelength investigated that more than 90% of the incident energy is carried by the photons that are scattered in the forward direction; while this forward scattering from each iridocyte shows very narrow angular dispersion (ca ±6°), the multiplicative scattering from a layer of ca 20 iridocytes broadens this dispersion to a cone of approximately ±90°. This understanding of the complex biophotonic dynamics enhances our comprehension of the physiologically, ecologically and evolutionarily significant light environment inside the giant clam, which is diffuse and nearly white at small tissue depths and downwelling, relatively monochromatic, and can be the same colour as the back-reflected light at greater depths in the tissue. Originally thought to be unique, cells of similar structure and photonic activity are now recognized in other species, where they serve other functions. The behaviour of the iridocytes opens possible new considerations for conservation and management of the valuable giant clam resource and new avenues for biologically inspired photonic applications.

Cyclable Condensation and Hierarchical Assembly of Metastable Reflectin Proteins, the Drivers of Tunable Biophotonics.

Reversible changes in the phosphorylation of reflectin proteins have been shown to drive the tunability of color and brightness of light reflected from specialized cells in the skin of squids and related cephalopods. We show here, using dynamic light scattering, electron microscopy, and fluorescence analyses, that reversible titration of the excess positive charges of the reflectins, comparable with that produced by phosphorylation, is sufficient to drive the reversible condensation and hierarchical assembly of these proteins. The results suggest a two-stage process in which charge neutralization first triggers condensation, resulting in the emergence of previously cryptic structures that subsequently mediate reversible, hierarchical assembly. The extent to which cyclability is seen in the in vitro formation and disassembly of complexes estimated to contain several thousand reflectin molecules suggests that intrinsic sequence- and structure-determined specificity governs the reversible condensation and assembly of the reflectins and that these processes are therefore sufficient to produce the reversible changes in refractive index, thickness, and spacing of the reflectin-containing subcellular Bragg lamellae to change the brightness and color of reflected light. This molecular mechanism points to the metastability of reflectins as the centrally important design principle governing biophotonic tunability in this system.

Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells.

The reversible assembly of reflectin proteins drives dynamic iridescence in cephalopods. Squid dynamically tune the intensity and colors of iridescence generated by constructive interference from intracellular Bragg reflectors in specialized skin cells called iridocytes. Analysis of the tissue specificity of reflectin subtypes reveals that tunability is correlated with the presence of one specific reflectin sequence. Differential phosphorylation and dephosphorylation of the reflectins in response to activation by acetylcholine, as well as differences in their tissue-specific and subcellular spatial distributions, further support the suggestion of different roles for the different reflectin subtypes.

Photosymbiotic giant clams are transformers of solar flux.

'Giant' tridacnid clams have evolved a three-dimensional, spatially efficient, photodamage-preventing system for photosymbiosis. We discovered that the mantle tissue of giant clams, which harbours symbiotic nutrition-providing microalgae, contains a layer of iridescent cells called iridocytes that serve to distribute photosynthetically productive wavelengths by lateral and forward-scattering of light into the tissue while back-reflecting non-productive wavelengths with a Bragg mirror. The wavelength- and angle-dependent scattering from the iridocytes is geometrically coupled to the vertically pillared microalgae, resulting in an even re-distribution of the incoming light along the sides of the pillars, thus enabling photosynthesis deep in the tissue. There is a physical analogy between the evolved function of the clam system and an electric transformer, which changes energy flux per area in a system while conserving total energy. At incident light levels found on shallow coral reefs, this arrangement may allow algae within the clam system to both efficiently use all incident solar energy and avoid the photodamage and efficiency losses due to non-photochemical quenching that occur in the reef-building coral photosymbiosis. Both intra-tissue radiometry and multiscale optical modelling support our interpretation of the system's photophysics. This highly evolved 'three-dimensional' biophotonic system suggests a strategy for more efficient, damage-resistant photovoltaic materials and more spatially efficient solar production of algal biofuels, foods and chemicals.

Experimental determination of refractive index of condensed reflectin in squid iridocytes.

Loliginid squid dynamically tune the structural iridescence of cells in their skin for active camouflage and communication. Bragg reflectors in these cells consist of membrane-bound lamellae periodically alternating with low refractive index extracellular spaces; neuronal signalling induces condensation of the reflectin proteins that fill the lamellae, consequently triggering the expulsion of water. This causes an increase in refractive index within the lamellae, activating reflectance, with the change in lamellar thickness and spacing progressively shifting the wavelength of reflected light. We used micro-spectrophotometry to measure the functionally relevant refractive index of the high-index lamellae of the Bragg reflectors containing the condensed reflectins in chemically fixed dermal iridocytes of the squid, Doryteuthis opalescens. Our high-magnification imaging spectrometer allowed us to obtain normalized spectra of optically distinct sections of the individual, subcellular, multi-layer Bragg stacks. Replacement of the extracellular fluid with liquids of increasing refractive index allowed us to measure the reflectivity of the Bragg stacks as it decreased progressively to 0 when the refractive index of the extracellular medium exactly matched that of the reflectin-filled lamellae, thus allowing us to directly measure the refractive index of the reflectin-filled lamellae as ncondensed lamellae ≈ 1.44. The measured value of the physiologically relevant ncondensed lamellae from these bright iridocytes falls within the range of values that we recently determined by an independent optical method and is significantly lower than values previously reported for dehydrated and air-dried reflectin films. We propose that this directly measured value for the refractive index of the squid's Bragg lamellae containing the condensed reflectins is most appropriate for calculations of reflectivity in similar reflectin-based high-index layers in other molluscs.

Mesostructure from hydration gradients in demosponge biosilica.

Organisms of the phylum Porifera, that is, sponges, utilize enzymatic hydrolysis to concatenate bioavailable inorganic silicon to produce lightweight, strong, and often flexible skeletal elements called spicules. In their optical transparency, these remarkable biomaterials resemble fused silica, despite having been formed under ambient marine biological conditions. Although previous studies have elucidated the chemical mechanisms of spicule formation and revealed the extensive hydration of these glasses, their precise composition and local and medium-range structures had not been determined. We have employed a combination of compositional analysis, (1) H and (29) Si solid-state nuclear magnetic resonance spectroscopy, and synchrotron X-ray total scattering to characterize spicule-derived silica produced by the demosponge Tethya aurantia. These studies indicate that the materials are highly hydrated, but in an inhomogeneous manner. The spicule-derived silica is, on average, perfectly dense for the given extent of hydration and regions of fully condensed and unstrained SiO networks persist throughout each monolithic spicule. To accommodate chemical strain and defects, the extensive hydration is concentrated in distinct regions that give rise to mesostructural features. The chemistry responsible for producing spicule silica resembles hydrolytic sol-gel processing, which offers exceptional control over the precise local atomic arrangement of materials. However, the specific processing involved in forming the sponge spicule silica further results in regions of fully condensed silica coexisting with regions of incomplete condensation. This mesostructure suggests a mechanism for atomistic defect tolerance and strain relief that may account for the unusual mechanical properties of the biogenic spicules.

Efficient selection of biomineralizing DNA aptamers using deep sequencing and population clustering.

DNA-based information systems drive the combinatorial optimization processes of natural evolution, including the evolution of biominerals. Advances in high-throughput DNA sequencing expand the power of DNA as a potential information platform for combinatorial engineering, but many applications remain to be developed due in part to the challenge of handling large amounts of sequence data. Here we employ high-throughput sequencing and a recently developed clustering method (AutoSOME) to identify single-stranded DNA sequence families that bind specifically to ZnO semiconductor mineral surfaces. These sequences were enriched from a diverse DNA library after a single round of screening, whereas previous screening approaches typically require 5-15 rounds of enrichment for effective sequence identification. The consensus sequence of the largest cluster was poly d(T)30. This consensus sequence exhibited clear aptamer behavior and was shown to promote the synthesis of crystalline ZnO from aqueous solution at near-neutral pH. This activity is significant, as the crystalline form of this wide-bandgap semiconductor is not typically amenable to solution synthesis in this pH range. High-resolution TEM revealed that this DNA synthesis route yields ZnO nanoparticles with an amorphous-crystalline core-shell structure, suggesting that the mechanism of mineralization involves nanoscale coacervation around the DNA template. We thus demonstrate that our new method, termed Single round Enrichment of Ligands by deep Sequencing (SEL-Seq), can facilitate biomimetic synthesis of technological nanomaterials by accelerating combinatorial selection of biomolecular-mineral interactions. Moreover, by enabling direct characterization of sequence family demographics, we anticipate that SEL-Seq will enhance aptamer discovery in applications employing additional rounds of screening.

Importance of diffuse scattering phenomena in moth-eye arrays for broadband infrared applications.

Moth-eye (ME) arrays with varying aspect ratios and profile heights were fabricated in Si using a general colloidal lithography and reactive ion etching technique. Antireflective (AR) properties of the arrays were rigorously assessed from the near to far infrared (λ=2-50 µm) using transmission and reflection measurements via dispersive and Fourier transform infrared spectroscopy and modeled using an effective medium approximation (EMA). Infrared transmission of low aspect ratio structures (~2) matched the EMA model, indicating that the most important factor for AR at higher wavelengths is structure height. High aspect ratio structures (>6) were highly transmissive (>90% of theoretical maximum) over a large bandwidth in the mid-infrared (20-50 µm). Specular reflectance, total transmission, and diffuse reflectance (DR) measurements indicate that ME structures do not reach the theoretical maximum at near-infrared wavelengths due to DR and forward scattering phenomena. Ultimately, correlating optical performance with feature geometry (pitch, profile, height, etc.) over multiple length scales allows intelligent design of ME structures for broadband applications.

Dynamic biophotonics: female squid exhibit sexually dimorphic tunable leucophores and iridocytes.

Loliginid squid use tunable multilayer reflectors to modulate the optical properties of their skin for camouflage and communication. Contained inside specialized cells called iridocytes, these photonic structures have been a model for investigations into bio-inspired adaptive optics. Here, we describe two distinct sexually dimorphic tunable biophotonic features in the commercially important species Doryteuthis opalescens: bright stripes of rainbow iridescence on the mantle just beneath each fin attachment and a bright white stripe centered on the dorsal surface of the mantle between the fins. Both of these cellular features are unique to the female; positioned in the same location as the conspicuously bright white testis in the male, they are completely switchable, transitioning between transparency and high reflectivity. The sexual dimorphism, location and tunability of these features suggest that they may function in mating or reproduction. These features provide advantageous new models for investigation of adaptive biophotonics. The intensely reflective cells of the iridescent stripes provide a greater signal-to-noise ratio than the adaptive iridocytes studied thus far, while the cells constituting the white stripe are adaptive leucophores--unique biological tunable broadband scatterers containing Mie-scattering organelles activated by acetylcholine, and a unique complement of reflectin proteins.