Light-induced Aggregation Research Articles

Unprecedented light induced aggregation of cationic 1,4,5,8-naphthalenediimide amphiphiles

Naphthalenediimide amphiphiles (NDI-as) with quaternary ammonium groups (DC4, DaP, and DaO) display unprecedented UV light-induced aggregation in solutions of water, acetonitrile and THF.

Aggregation of alpha-synuclein disrupts mitochondrial metabolism and induce mitophagy via cardiolipin externalization

Accumulation of α-synuclein aggregates in the substantia nigra pars compacta is central in the pathophysiology of Parkinson’s disease, leading to the degeneration of dopaminergic neurons and the manifestation of motor symptoms. Although several PD models mimic the pathological accumulation of α-synuclein after overexpression, they do not allow for controlling and monitoring its aggregation. We recently generated a new optogenetic tool by which we can spatiotemporally control the aggregation of α-synuclein using a light-induced protein aggregation system. Using this innovative tool, we aimed to characterize the impact of α-synuclein clustering on mitochondria, whose activity is crucial to maintain neuronal survival. We observed that aggregates of α-synuclein transiently and dynamically interact with mitochondria, leading to mitochondrial depolarization, lower ATP production, mitochondrial fragmentation and degradation via cardiolipin externalization-dependent mitophagy. Aggregation of α-synuclein also leads to lower mitochondrial content in human dopaminergic neurons and in mouse midbrain. Interestingly, overexpression of α-synuclein alone did not induce mitochondrial degradation. This work is among the first to clearly discriminate between the impact of α-synuclein overexpression and aggregation on mitochondria. This study thus represents a new framework to characterize the role of mitochondria in PD.

Photoredox Processes in the Aggregation and Gelation of Electron-Responsive Supramolecular Polymers Based on Viologen

Viologen-based ditopic bis-pyridinyl-triazole bidentate ligands self-assemble in the presence of palladium ions into supramolecular polymers whose structure is imposed by the directed formation of coordination bonds. Light-irradiation of these electron-responsive supramolecular materials triggers a photo-induced electron transfer yielding isolated π-radicals and dimers of radicals. The photoreduction events and the associated dimerization steps trigger a large-scale reorganization occurring within the supramolecular network yielding aggregates or gels depending on the irradiation conditions (power, duration). Detailed electrochemical, spectro-electrochemical and photochemical analyses were conducted to understand the mechanisms at stakes in these light-induced aggregation and gelation.

Visualizing light-induced dynamic structural transformations of Au clusters-based photocatalyst via in situ TEM

Ultrasmall gold (Au) clusters have been regarded as one of the prototypes materials for solar energy conversion due to their unique strong molecular-like light absorption properties. However, the light-induced aggregation of Au clusters into nanoparticles is one of the most important factors that restricts its application in photocatalysis. Although Au clusters aggregation has been widely demonstrated, the underlying mechanism for cluster fusion is still unclear due to the lack of experimental evidence. Herein, we report the direct observation of Au clusters on TiO2 nanosheets aggregating when used as visible light photocatalysts for the reduction of nitroaromatics. Through in situ high-resolution transmission electron microscopy (TEM), the coexistence of two fusion mechanisms of Au clusters on TiO2 under ultraviolet-visible (UV-Vis) light irradiation in air is identified, i.e., the migration and coalescence (MC) and Ostwald ripening (OR). Additionally, the correlation between the photostability of Au clusters and reaction atmospheres has been investigated, among which Au clusters have higher stability in an inert N2 atmosphere or vacuum than the oxidizing atmospheres (i.e., air and O2). These results indicate the inherent stability of Au cluster during photocatalysis, and instability comes from the consuming of ligand layer. This work not only discloses the underlying mechanism of Au cluster sintering but also provides guidelines for enhancing metal clusters-based photocatalysts stability.

Efficient Molecular Aggregation of Rhodamine 6G and Pseudoisocyanine by Light-Induced Force

A highly efficient light-induced aggregation of porphyrin molecules in solution was recently reported for 4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid (TPPS). Here, we demonstrate that rhodamine 6G (R6G) and pseudoisocyanine (PIC) also show efficient light-induced aggregation in unsaturated aqueous solution, being detected with a multichannel lock-in amplifier as the absorbance decrease/increase in the monomers/aggregates, induced by a laser at 633 nm, which is non-resonant off their main monomer absorption wavelengths. The light-induced aggregation states are H-aggregates that are hardly formed in the thermal equilibrium. The similar absorbance changes are absent in the monomer molecules fixed in polyvinyl alcohol (PVA) films. The aggregation efficiency defined as ( Δ A / A ) / ( U t r a p / k B T ) , where A is the absorbance of the monomers, Δ A is the absorbance increase in the aggregates, U t r a p is the optical gradient force potential, and k B T is the thermal energy at room temperature, is approximately 100 for R6G and 500 for PIC, which are much smaller than that of TPPS.

Azobenzene-Based Ionic Liquid Switches Phase Separation of Poly(N-isopropylacrylamide) Aqueous Solutions as a Molecular Trigger, Leading to UV Shutdown of Ionic Transport.

This paper presents a light-switchable phase separation of poly(N-isopropylacrylamide) (PNIPAm) in aqueous solutions using an azobenzene-based ionic liquid as a molecular trigger. A different UV response was obtained with use of an azobenzene molecular trigger. A UV-induced aggregation of PNIPAm was observed in the current system, which is opposite to that reported for azobenzene-functionalized PNIPAm systems. According to in situ FT-IR, cis-isomers destabilized the hydrogen bonding between water and PNIPAm, contributing to a light-induced aggregation of PNIPAm. This polarity effect was quite similar to salting effects of hofmeister anions on clouding points. Furthermore, a UV shutdown of ion transport of the azobenzene-based ionic liquid was demonstrated with a significant reduction factor of 0.03 at ambient temperature, which can be potentially applied in biotechnology, optoelectronics, and ionic circuits.

Time-resolved emission microscopy of light-induced aggregation of luminescent polymers

Photon pressure has been used to induce the aggregation from solution of a series of photoluminescent conjugated polyelectrolytes containing tetraphenylethene units. These polymers show steady-state and time-resolved emission properties that are dependent on the local chromophore environment that can be influenced by the degree of intra- and inter-molecular interactions, which enables the photoaggregation process to be monitored by time-resolved fluorescence imaging techniques. Structural differences in the polymer lead to variations in the photo-induced aggregation behaviour.

Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation

It has been reported that when irradiated with laser light non-resonant with the main absorption peaks, porphyrin molecules (4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid, TPPS) in an aqueous solution become 10,000 to 100,000 times more efficient in light-induced molecular aggregation than expected from the ratio of gradient force potential to the thermal energy of molecules at room temperature. To determine the mechanism of this phenomenon, experiments on the light-induced aggregation of TPPS in alcohol solutions (methanol, ethanol, and butanol) were performed. In these alcohol solutions, the absorbance change was orders of magnitude smaller than in the aqueous solution. Furthermore, it was found that the absorbance change in the aqueous solution tended to be saturated with the increase of the irradiation intensity, but in the ethanol solution, the absorbance change increased linearly. These results can be qualitatively explained by the model in which intermolecular light-induced interactions between molecules within a close distance among randomly distributed molecules in the laser irradiation volume are highly relevant to the signal intensity. However, conventional dipole–dipole interactions, such as the Keesom interaction, are not quantitatively consistent with the results.

Sunlight induced aggregation of dissolved organic matter: Role of proteins in linking organic carbon and nitrogen cycling in seawater

Organic matter export from the euphotic zone is a key component of oceanic carbon (C) and nitrogen (N) cycles. Although interactions between these two cycles are important, studies on geochemical processes to directly connect them are limited. Here we show that sunlight can induce chemical aggregation of dissolved organic matter (DOM) into high N containing photo-aggregates. The size of microgels in natural coastal seawaters increased by18~25% compared to corresponding dark controls. Within a relatively short time (1 h), the C and N sequestered into the photo-aggregates accounted for 10% and 13% of the bulk particulate C and N, respectively. The N/C ratio of the photo-aggregates was two times higher after sunlight irradiation. Furthermore, we show that the aggregation process was dependent on reactive oxygen species (ROS). To accommodate for the different organic material in the marine environment, we monitored the particle size in various extracellular polymeric substances (EPS) and model biopolymers using flow cytometry, dynamic laser scattering, and scanning electron microscopy. We found that proteins play important roles in light-induced aggregation, which is in contrast to previous views that sunlight can break down DOM and interrupt aggregation. The photo-flocculation process involving organic N provides new insights into DOM assembly, bioavailability, and sedimentation, and thus potentially link the C and N cycles.

Light-Induced Aggregation and Disaggregation of Stimuli-Responsive Latex Particles Depending on Spiropyran Concentration: Kinetics of Photochromism and Investigation of Reversible Photopatterning.

Light-controlling the physical and chemical properties of smart polymers by using photochromic compounds has been an interesting research subject. Incorporation of spiropyran (SP) on the surface of particles can induce photoswitchable aggregation/disaggregation to stimuli-responsive colloids. Herein, we developed a novel class of stimuli-responsive latex particles bearing SP with different contents (0, 0.5, 1, 3, and 5 wt %) by semicontinuous emulsifier-free emulsion copolymerization, which is able to change the particle size by light-induced aggregation/disaggregation in response to ultraviolet (UV) irradiation and visible light. The scanning electron microscopy images revealed the spherical morphology of the latex particles, with the size in the range of 400-900 nm. Light-induced aggregation and disaggregation of stimuli-responsive latex particles were investigated by dynamic light scattering and also confirmed by variation of transmittance during UV illumination time using ultraviolet-visible spectroscopy. The range of the light-induced shift in the particle size is about 200-600 nm (depending on the concentration of SP), where the reduction of transmittance upon UV irradiation (and conversely upon visible light) confirms the ability of latex particles for displaying reversible photoswitchable aggregation/disaggregation and also light-controlling the particle size. The kinetics of SP to merocyanine (MC) and MC to SP isomerizations were experimentally investigated and fitted by exponential equations. The photochromic latexes displayed remarkable photoswitchability and photofatigue resistant properties under alternating UV and visible light irradiation cycles. Additionally, these stimuli-responsive latexes displayed potential applications such as anticounterfeiting inks in erasable and rewritable writings on cellulosic papers for increasing safety in security documents.

Synthesis and light-induced aggregation of benzoate-stabilized silver nanoparticles

We report the influence of LED light irradiation on the colloids of silver nanoparticles synthesized in the presence of sodium benzoate. Scanning electron and atomic force microscopy measurements showed progressive aggregation and agglomeration of the particles during the irradiation with the light which energy is close to particle plasmon resonance. We consider local heating phenomenon caused due to the plasmon excitation as the main reason for the aggregation of the particles. A specific role of the benzoate and irradiation wavelength in these processes are discussed. This plasmon-induced aggregation opens new opportunities for the creation of metallic nanoparticle clusters.

Visible light-induced insulin aggregation on surfaces via photoexcitation of bound thioflavin T

Insulin is known to form amyloid aggregates when agitated in a hydrophobic container. Amyloid aggregation is routinely measured by the fluorescence of the conformational dye thioflavin T, which, when incorporated into amyloid fibers, fluoresces at 480 nm. The kinetics of amyloid aggregation in general is characterized by an initial lag-phase, during which aggregative nuclei form on the hydrophobic surface. These nuclei then lead to the formation of fibrils presenting a rapid growth during the elongation phase. Here we describe a novel mechanism of insulin amyloid aggregation which is surprisingly devoid of a lag-time for nucleation. The excitation of thioflavin T by visible light at 440 nm induces the aggregation of thioflavin T-positive insulin fibrils on hydrophobic surfaces in the presence of strong agitation and at physiological pH. This process is material surface-induced and depends on the fact that surface-adsorbed insulin can bind thioflavin T. Light-induced insulin aggregation kinetics is thioflavin T-mediated and is based on an energy transfer from visible light to the protein via thioflavin T. It relies on a constant supply of thioflavin T and insulin from the solution to the aggregate. The growth rate increases with the irradiance and with the concentration of thioflavin T. The supply of insulin seems to be the limiting factor of aggregate growth. This light-induced aggregation process allows the formation of local surface-bound aggregation patterns.

Biophysics of Collective Phototaxis of Euglena gracilis

High-density suspensions of actively swimming cells in fluids often self-organize into complex dynamic patterns due to the interplay between microscopic force generation and macroscopic hydrodynamics. Though various biophysical models have been developed to describe the dynamics of these systems, experimental verification has been difficult to obtain. This is due to the complex interaction rules which govern the swimmer behavior, and the lack of tools for delivering controlled perturbations to a defined subset of cells inside such a suspension. Here, we consider concentrated suspensions of the unicellular, photoresponsive algae, Euglena gracilis. We propose an integrated experimental and theoretical platform for predicting the collective behavior of Euglena under light stimulation. By varying the spatiotemporal light patterns, we are able to finely adjust cell densities and achieve arbitrary non-homogeneous distributions, including compression into high-density aggregates of varying geometries. Our models were able to account for the light-induced aggregation that result from a complex interplay between negative phototaxis, hydrodynamic interactions, and shading effects between the cells. This work will deepen our general understanding of how relatively simple mechanical feedback interactions can generate multi-cell behaviors in microswimmer suspensions, and how to bridge such understanding over different length and time scales. Furthermore, it may also aid the engineering of microfluidic devices and advance concepts in active matter physics.

Light-Induced Aggregation of Gold Nanoparticles and Photoswitching of Silicon Surface Potential

Hybrid nanomaterials having tunable properties that can be reversibly conducted by external stimuli, in particular light, are of great importance since they enable synergetic behavior between their components and enable the design of stimuli responsive smart materials and surfaces. Here we describe the formation of organic-inorganic hybrid nanoparticles that photochemically aggregate and their effect on the electronic properties of a semiconducting surface, as a function of external irradiation. The inorganic component consists of 3 nm gold nanoparticles while the organic component is a covalently attached, photochromic spiropyran derivative. Aggregation/deaggregation patterns in solution were obtained and analyzed by UV-Vis spectroscopy and transmission electron microscopy upon photoswitching. The assembly of spiropyran-modified gold nanoparticles on Si/SiO2 surface proved useful in photo-tuning the electronic properties of semiconductors measured by contact potential difference.

Particle aggregation behavior during photocatalytic ethanol reforming reaction and its correlation with the activity of H2 production

When dispersed in water, nanoscale photocatalysts tend to aggregate due to their high surface energy. Complex interaction of the incident light with the aggregated particles is expected due to their comparable length scales. In this study, the aggregation behavior of TiO2 nanoparticles during photocatalytic ethanol reforming was investigated by the dynamic light scattering (DLS). DLS results demonstrated that UV light can significantly inhibit the aggregation of TiO2 compared to that under visible light or in the dark. Considering that the ethanol photo-reforming over TiO2 can only occur under UV light, we correlated the significant light-induced particle aggregation with the photocatalytic reactions. Three different aggregation degrees for TiO2 were employed to study the effect of aggregation on its activity for photocatalytic H2 production. The H2 production is found to decrease with the increase of aggregates in the suspension. Our results should be of great value for the understanding of the dynamics in photocatalyst particle suspension during the photocatalytic reaction. More importantly, it is supposed that one can improve the activity of the photocatalytic reaction by controlling the particle aggregation through carefully adjusting the external physical fields such as the intensity of the incident light at certain wavelength interval.

Structure-Based Correlation of Light-Induced Histidine Reactivity in A Model Protein.

Light is known to induce covalently linked aggregates in proteins. These aggregates can be immunogenic and are of concern for drug product development in the biotechnology industry. Histidine (His) is proposed to be a key residue in cross-link generation ( Pattison , D. I. Photochem. Photobiol. Sci. 2012 , 11 , 38 - 53 ). However, the factors that influence the reactivity of His in proteins, especially the intrinsic factors are little known. Here, we used rhDNase, which only forms His-His covalent dimers after light treatment to determine the factors that influence the light-induced reactivity of His. This system allowed us to fully characterize the light-induced covalent dimer and rank the reactivities of the His residues in this protein. The reactivities of these His residues were correlated with solvent accessibility-related parameters both by crystal structure-based calculations of solvent-accessible surface area and by hydrogen-deuterium exchange (HDX) experiments. Through this correlation, we demonstrate that the photoreactivity of His is determined by both solvent accessibility and structural flexibility. This new insight can explain the highly complex chemistry of light-induced aggregation and help predict the aggregation propensity of protein under light treatment.

Light-induced aggregation of microbial exopolymeric substances

Sunlight can inhibit or disrupt the aggregation process of marine colloids via cleavage of high molecular weight compounds into smaller, less stable fragments. In contrast, some biomolecules, such as proteins excreted from bacteria can form aggregates via cross-linking due to photo-oxidation. To examine whether light-induced aggregation can occur in the marine environment, we conducted irradiation experiments on a well-characterized protein-containing exopolymeric substance (EPS) from the marine bacterium Sagitulla stellata. Our results show that after 1 h sunlight irradiation, the turbidity level of soluble EPS was 60% higher than in the dark control. Flow cytometry also confirmed that more particles of larger sized were formed by sunlight. In addition, we determined a higher mass of aggregates collected on filter in the irradiated samples. This suggests light can induce aggregation of this bacterial EPS. Reactive oxygen species hydroxyl radical and peroxide played critical roles in the photo-oxidation process, and salts assisted the aggregation process. The observation that Sagitulla stellata EPS with relatively high protein content promoted aggregation, was in contrast to the case where no significant differences were found in the aggregation of a non-protein containing phytoplankton EPS between the dark and light conditions. This, together with the evidence that protein-to-carbohydrate ratio of aggregates formed under light condition is significantly higher than that formed under dark condition suggest that proteins are likely the important component for aggregate formation. Light-induced aggregation provides new insights into polymer assembly, marine snow formation, and the fate/transport of organic carbon and nitrogen in the ocean.

The Effects of Light-Accelerated Degradation on the Aggregation of Marketed Therapeutic Monoclonal Antibodies Evaluated by Size-Exclusion Chromatography With Diode Array Detection

Research into the effects that exposure to light can have on therapeutic proteins is essential for ensuring the quality and safety of the medicines in which they are used. It is important to understand the effects of light on aggregation to help avoid undesirable colloidal instabilities, both in the original medicines and in the formats in which they are finally administered. In this study, 5 marketed therapeutic mAbs, namely bevacizumab, cetuximab, infliximab, rituximab, and trastuzumab, were investigated for this purpose. The medicines and 2 diluted preparations in 0.9 NaCl (2 mg/mL and 5 mg/mL)—commonly used in clinical practice—were subjected to controlled light-accelerated degradation. The formation of aggregates was monitored by size-exclusion chromatography. The results indicated that light induced protein aggregation. This process of protein damage was influenced above all by mAb concentration, although the particular characteristics of each mAb were also important. Photodegradation also produced the fragmentation of the mAbs. The damage caused to the mAbs as a result of light-induced aggregation and/or fragmentation was demonstrated both in the medicines and in the diluted preparation forms. These findings should be carefully considered when handling the medicines for administration and when recommending beyond-use dates in normal hospital conditions.

Melanopsin photoreception in the eye regulates light-induced skin colour changes through the production of α-MSH in the pituitary gland.

How skin colour adjusts to circadian light/dark cycles is poorly understood. Melanopsin (Opn4) is expressed in melanophores, where in vitro studies suggest it regulates skin pigmentation through a 'primary colour response' in which light photosensitivity is translated directly into pigment movement. However, the entrainment of the circadian rhythm is regulated by a population of melanopsin-expressing retinal ganglion cells (mRGCs) in the eye. Therefore, in vivo, melanopsin may trigger a 'secondary colour response' initiated in the eye and controlled by the neuro-endocrine system. We analysed the expression of opn4m and opn4x and melanin aggregation induced by light (background adaptation) in Xenopus laevis embryos. While opn4m and opn4x are expressed at early developmental times, light-induced pigment aggregation requires the eye to become functional. Pharmacological inhibition of melanopsin suggests a model whereby mRGC activation lightens skin pigmentation via a secondary response involving negative regulation of alpha-melanocyte-stimulating hormone (α-MSH) secretion by the pituitary.

Cyanobacteriochrome SesA is a diguanylate cyclase that induces cell aggregation in Thermosynechococcus.

Cyanobacteria have unique photoreceptors, cyanobacteriochromes, that show diverse spectral properties to sense near-UV/visible lights. Certain cyanobacteriochromes have been shown to regulate cellular phototaxis or chromatic acclimation of photosynthetic pigments. Some cyanobacteriochromes have output domains involved in bacterial signaling using a second messenger cyclic dimeric GMP (c-di-GMP), but its role in cyanobacteria remains elusive. Here, we characterize the recombinant Tlr0924 from a thermophilic cyanobacterium Thermosynechococcus elongatus, which was expressed in a cyanobacterial system. The protein reversibly photoconverts between blue- and green-absorbing forms, which is consistent with the protein prepared from Escherichia coli, and has diguanylate cyclase activity, which is enhanced 38-fold by blue light compared with green light. Therefore, Tlr0924 is a blue light-activated diguanylate cyclase. The protein's relatively low affinity (10.5 mM) for Mg(2+), which is essential for diguanylate cyclase activity, suggests that Mg(2+) might also regulate c-di-GMP signaling. Finally, we show that blue light irradiation under low temperature is responsible for Thermosynechococcus vulcanus cell aggregation, which is abolished when tlr0924 is disrupted, suggesting that Tlr0924 mediates blue light-induced cell aggregation by producing c-di-GMP. Given our results, we propose the name "sesA (sessility-A)" for tlr0924. This is the first report for cyanobacteriochrome-dependent regulation of a sessile/planktonic lifestyle in cyanobacteria via c-di-GMP.