VARPA: In Silico Additive Screening for Protein-Based Lighting Devices

Protein optoelectronics is an emerging field facing implementation and stabilization challenges of proteins in harsh non-natural environments, such as dry polymers, inorganic materials, etc., operating at high temperatures/irradiations. In this context, additives promoting structural and functional protein stabilization are paramount to realize highly performing devices. On one hand, trial-error experimental assays based on previous knowledge of classical additives in aqueous solutions are effort/time-consuming, while their translation to water-less matrices is uncertain. On the other hand, computational simulations (molecular dynamics, electronic structure methods, etc.) are limited by the system size and time. Herein, ligand-binding affinity and atomic perturbations to create a day-fast computational method combining Vina And Rosetta for Protein Additives (VARPA) to simulate the stabilization effect of sugars for the archetypal enhanced green fluorescent protein embedded in a standard dry polymer color-converting filter for bio-hybrid light-emitting diodes is merged. The VARPA's sugar additive prediction trend for protein stabilization is nicely validated by thermal and photophysical studies as well as lighting device analysis. The device stability followed the predicted enhanced stability trend, reaching a 40-fold improvement compared to reference devices. Overall, VARPA can be adapted to a myriad of additives and proteins, driving first-step experimental efforts toward highly performing protein devices.

Supercharged Fluorescent Protein-Apoferritin Cocrystals for Lighting Applications

The application of fluorescent proteins (FPs) in optoelectronics is hindered by the need for effective protocols to stabilize them under device preparation and operational conditions. Factors such as high temperatures, irradiation, and organic solvent exposure contribute to the denaturation of FPs, resulting in a low device performance. Herein, we focus on addressing the photoinduced heat generation associated with FP motion and rapid heat transfer. This leads to device temperatures of approximately 65 °C, causing FP-denaturation and a subsequent loss of device functionality. We present a FP stabilization strategy involving the integration of electrostatically self-assembled FP-apoferritin cocrystals within a silicone-based color down-converting filter. Three key achievements characterize this approach: (i) an engineering strategy to design positively supercharged FPs (+22) without compromising photoluminescence and thermal stability compared to their native form, (ii) a carefully developed crystallization protocol resulting in highly emissive cocrystals that retain the essential photoluminescence features of the FPs, and (iii) a strong reduction of the device's working temperature to 40 °C, leading to a 40-fold increase in Bio-HLEDs stability compared to reference devices.

Core-Shell Structured Fluorescent Protein Nanoparticles: New Paradigm Toward Zero-Thermal-Quenching in High-Power Biohybrid Light-Emitting Diodes

Stable and efficient high-power biohybrid light-emitting diodes (Bio-HLEDs) using fluorescent proteins (FPs) in photon downconverting filters have not been achieved yet, reaching best efficiencies of 130 lm W^-1 stable for >5 h. This is related to the rise of the device temperature (70-80 °C) caused by FP-motion and quick heat-transmission in water-based filters, they lead to a strong thermal emission quenching followed by the quick chromophore deactivation via photoinduced H-transfer. To tackle both issues at once, this work shows an elegant concept of a new FP-based nanoparticle, in which the FP core is shielded by a SiO₂ -shell (FP@SiO₂ ) with no loss of the photoluminescence figures-of-merit over years in foreign environments: dry powder at 25 °C (ambient) or constant 50 °C, as well as suspensions in organic solvents. This enables the preparation of water-free photon downconverting coatings with FP@SiO₂ , realizing on-chip high-power Bio-HLEDs with 100 lm W^-1 stable for >120 h. Both thermal emission quenching and H-transfer deactivation are suppressed, since the device temperature holds <40 °C and remote high-power Bio-HLEDs exhibit final stabilities of 130 days compared to reference devices with water-based FP@SiO₂ (83 days) and FP-polymer coatings (>100 h). Hence, FP@SiO₂ is a new paradigm toward water-free zero-thermal-quenching biophosphors for first-class high-power Bio-HLEDs.

Croconaine-based nanoparticles enable efficient optoacoustic imaging of murine brain tumors

Contrast enhancement in optoacoustic (photoacoustic) imaging can be achieved with agents that exhibit high absorption cross-sections, high photostability, low quantum yield, low toxicity, and preferential bio-distribution and clearance profiles. Based on advantageous photophysical properties of croconaine dyes, we explored croconaine-based nanoparticles (CR780RGD-NPs) as highly efficient contrast agents for targeted optoacoustic imaging of challenging preclinical tumor targets. Initial characterization of the CR780 dye was followed by modifications using polyethylene glycol and the cancer-targeting c(RGDyC) peptide, resulting in self-assembled ultrasmall particles with long circulation time and active tumor targeting. Preferential bio-distribution was demonstrated in orthotopic mouse brain tumor models by multispectral optoacoustic tomography (MSOT) imaging and histological analysis. Our findings showcase particle accumulation in brain tumors with sustainable strong optoacoustic signals and minimal toxic side effects. This work points to CR780RGD-NPs as a promising optoacoustic contrast agent for potential use in the diagnosis and image-guided resection of brain tumors.

Challenging a Preconception: Optoacoustic Spectrum Differs from the Optical Absorption Spectrum of Proteins and Dyes for Molecular Imaging

Optoacoustic (photoacoustic) imaging has seen marked advances in detection and data analysis, but there is less progress in understanding the photophysics of common optoacoustic contrast agents. This gap blocks the development of novel agents and the accurate analysis and interpretation of multispectral optoacoustic images. To close it, we developed a multimodal laser spectrometer (MLS) to enable the simultaneous measurement of optoacoustic, absorbance, and fluorescence spectra. Herein, we employ MLS to analyze contrast agents (methylene blue, rhodamine 800, Alexa Fluor 750, IRDye 800CW, and indocyanine green) and proteins (sfGFP, mCherry, mKate, HcRed, iRFP720, and smURFP). We found that the optical absorption spectrum does not correlate with the optoacoustic spectrum for the majority of the analytes. We determined that for dyes, the transition underlying an aggregation state has more optoacoustic signal generation efficiency than the monomer transition. For proteins we found a favored optoacoustic relaxation that stems from the neutral or zwitterionic chromophores and unreported photoswitching behavior of tdTomato and HcRed. We then crystalized HcRed in its photoswitch optoacoustic state, confirming structurally the change in isomerization with respect to HcReds' fluorescence state. Finally, on the example of the widely used label tdTomato and the dye indocyanine green, we show the importance of correct photophysical (e.g., spectral and kinetic) information as a prerequisite for spectral-unmixing for in vivo imaging.

Challenging a Preconception: Optoacoustic Spectrum Differs from the Optical Absorption Spectrum of Proteins and Dyes for Molecular Imaging

Optoacoustic (photoacoustic) imaging has seen marked advances in detection and data analysis, but there is less progress in understanding the photophysics of common optoacoustic contrast agents. This gap blocks the development of novel agents and the accurate analysis and interpretation of multispectral optoacoustic images. To close it, we developed a multimodal laser spectrometer (MLS) to enable the simultaneous measurement of optoacoustic, absorbance, and fluorescence spectra. Herein, we employ MLS to analyze contrast agents (methylene blue, rhodamine 800, Alexa Fluor 750, IRDye 800CW, and indocyanine green) and proteins (sfGFP, mCherry, mKate, HcRed, iRFP720, and smURFP). We found that the optical absorption spectrum does not correlate with the optoacoustic spectrum for the majority of the analytes. We determined that for dyes, the transition underlying an aggregation state has more optoacoustic signal generation efficiency than the monomer transition. For proteins we found a favored optoacoustic relaxation that stems from the neutral or zwitterionic chromophores and unreported photoswitching behavior of tdTomato and HcRed. We then crystalized HcRed in its photoswitch optoacoustic state, confirming structurally the change in isomerization with respect to HcReds' fluorescence state. Finally, on the example of the widely used label tdTomato and the dye indocyanine green, we show the importance of correct photophysical (e.g., spectral and kinetic) information as a prerequisite for spectral-unmixing for in vivo imaging.

Structure-Based Mutagenesis of Phycobiliprotein smURFP for Optoacoustic Imaging

Photo- or optoacoustics (OA) imaging is increasingly being used as a non-invasive imaging method that can simultaneously reveal structure and function in deep tissue. However, the most frequent transgenic OA labels are current fluorescent proteins that are not optimized for OA imaging. Thus, they lack OA signal strength, and their absorption maxima are positioned at short wavelengths, thus giving small penetration depths and strong background signals. Here, we apply insights from our recent determination of the structure of the fluorescent phycobiliprotein smURFP to mutate a range of residues to promote the nonradiative decay pathway that generates the OA signal. We identified hydrophobic and aromatic substitutions within the chromophore-binding pocket that substantially increase the intensity of the OA signal and red-shift the absorption. Our results demonstrate the feasibility of structure-based mutagenesis to repurpose fluorescent probes for OA imaging, and they may provide structure-function insights for de novo engineering of transgenic OA probes.

Light-Responsive Size of Self-Assembled Spiropyran-Lysozyme Nanoparticles with Enzymatic Function

Novel light-responsive nanoassemblies with switchable size and enzymatic activity are built from a protein and a water-soluble spiropyran. Assemblies are created by electrostatic self-assembly in aqueous solution such that the photochromic property of the spiropyran enables light responsiveness. Upon visible light exposure, the aggregate size increases from 200 to 400 nm. The enzyme retains its activity upon aggregation into the assembly, while it decreases through visible light irradiation. Fundamentally, we show how the two different spiropyran isomers, the open-ring merocyanine form and the closed-ring spiropyran form, bind differently to the protein, which triggers the assembly size and use of thermodynamic data to understand the binding process and the size response. Thus, as a proof of concept, a self-assembly driven light-tunable enzyme activity in conjunction with a triggerable assembly size is demonstrated for a model system. The concept bears future potential for various possible biological applications ranging from genetic control over vaccine applications to the detection of certain proteins.

New insights into the nature of the Cibacron brilliant red 3B-A – Chitosan interaction

Cibacron brilliant red 3B-A (CBR) has been introduced to determine chitosan (CS) concentrations in solution, and several studies applied it to measure chitosan content in pharmaceutical formulations. So far, studies have relied on the absorbance band shift to 570 nm to determine the extent of the CBR – CS interaction. In this study, we show that CBR forms micro- to nanometer sized aggregates with CS, depending on their charge ratio and that other photophysical changes in CBR are induced by this interaction. We found that, besides the bathochromic band shift, aggregation induces emission at 600 nm and emission quenching at 360 nm. We compared changes CS induced in absorbance and fluorescence emission of CBR with the CS monomer glucosamine and poly(allylamine) hydrochloride, which both contain amino groups, and found that similar but less intense photophysical changes also occur. Furthermore, CS-induced circular dichroism in CBR suggests a twisted, chiral structure of these aggregates that should match with the previously published in silico simulations of the structure of CS in solution. The low linear charge density of CS and its chiral conformation are considered responsible for the enhanced photophysical response of CBR interacting with the polycation.