Aggregation Constants Research Articles

Aggregation and Capacity Limiting Effects in Anthraquinone-Based Flow Battery Negolytes

Anthraquinone-based molecules are promising electroactive materials for use in aqueous organic flow batteries. At high concentrations in aqueous solutions, the well-known negolyte molecule anthraquinone disulfonic acid (AQDS) molecule has been observed to aggregate under near-neutral and acidic conditions. Aggregation has been hypothesized to be directly linked to observed concurrent capacity reduction. In this study, we investigated three different water-soluble anthraquinones in electrolytes of varying compositions and pH to gain further insight into the possible causes of capacity loss. We used low-field benchtop 1H-NMR and diffusion NMR measurements directly in non-deuterated aqueous flow-battery electrolytes to investigate molecular aggregation. Single-cell testing was performed under exhaustive electrolysis conditions to determine the number of electrons exchanged per molecule. We observed a decrease in the number of electrons exchanged per molecule in the presence of carbonate ions due to CO2 adduct formation. The aggregation constants were determined from both concentration-dependent chemical shifts and self-diffusion coefficients. We show that aggregation of the oxidized form of all three molecules studied here occurs under near-neutral and alkaline conditions and does not affect the number of accessible e−.

Properties and structural features of native and modified proteins of concentrates from white and brown rice

The physicochemical properties and structural features of proteins of concentrates (PC) from white and brown rice are described. Differences in a degree of proteolysis and the relationship between the functional and technological properties of proteins and their structure were established. Proteins from white rice had lower molecular weights (MW) than proteins from brown rice: 1.3–199.5 kDa versus 1.5–251.1 kDa. Most proteins were grouped in three low-molecular-weight fractions (МW 11.7–27.5 kDa) in PC from white rice and in one fraction with the high molecular weight (182–251.1 kDa) in PC from brown rice. In the process of hydrolysis, the MW of proteins (high-molecular-weight) of PC from white rice decreased from 199.5 to 120.2 kDa with generation of peptides with a molecular weight of <1.3 kDa, while the MW remained unchanged upon hydrolysis of proteins of PC from brown rice. In the composition of PC from white rice, flavonoids interacted with protein fractions with molecular weights of 131, 10, and 4.0 kDa; while in PC from brown rice, they interacted with only one fraction with a molecular weight of 216 kDa. The quantity of flavonoids in PC from white rice was 2.3 times less than that in PC from brown rice. Elements of protein secondary structure were established for PC: α–helix, 310 — helix, β-structure, β-bends, and irregular shape. Proteolysis of proteins was accompanied by a decrease in the number of α-helices, increase in the proportion of β-structures and irregular regions, weakening of the hydrophobic properties of proteins and an increase in the number of S–S bonds; PC from brown rice was characterized by a higher content of –СН2 groups from the flavonoid family, as well as by a higher degree of unsaturation of groups of benzene nuclei, ketone and ester groups. A negative correlation was found between the foaming ability of PC and the upper boundaries of molecular weights (r = — 0.95), quantity of high-molecular weight proteins (r = — 0.80) and aggregation constants, and a positive correlation (r = + 0.8) with the number of S–S bonds. The high foaming ability is interrelated with proteins with a molecular weight of no more than 120 kDa. The results are intended to regulate the functional properties of protein products based on the characteristics of the physicochemical properties of native and modified proteins.

Aggregation and Capacity Limiting Effects in Anthraquinone-Based Flow Battery Negolytes

Anthraquinone-based molecules constitute promising electroactive materials for use in aqueous organic flow batteries. It has been observed that in high concentration in aqueous solutions these molecules tend to aggregate. It has been speculated that the aggregation causes a decrease in capacity as the number of electrons partaking in the electrochemical processes gets smaller. Three water-soluble anthraquinones were studied in different electrolytes in order to have a more meaningful insight into the possible causes, aggregation phenomena and electrochemical performance. 1H-NMR and PGSTE measurements were performed in non-deuterated solvents. Single cell testing was performed to assess the number of electrons. The aggregation constants have been determined using data from both 1H-NMR and PGSTE at different concentrations. Those data were fit into the Isodesmic model wherein the diffusion coefficient of the monomers and aggregation constants were extracted and compared. In all cases NMR signals showed aggregation taking place but it did not translate to reduced number of electrons. Capacity retention was very profound in the case of disodium anthraquinone-2,7-disuflonate (AQDS) in carbonate solution, wherein a reduced amount of electrons from 2 to 1 was observed. 2,6 dihydroxyanthraquinone (DHAQ), to a less extent, also showcased a reduced amount of electrons in carbonate solution.

Bowl-Shaped Symmetric and Non-symmetric Bis-functionalized Indacenopicenes.

Regioselective arrangement of two groups of orthogonal reactivity in bowl-shaped as-indaceno[3,2,1,8,7,6-pqrstuv]picene (Idpc) was key for the synthesis of hetero-bis-functionalized Idpc derivatives. Halogen and methyl groups were positioned at specific positions in the rim area of Idpc at an early stage during the synthesis by functionalization of suitable precursors. Regioselective functionalization of the bowl's rim was then finally achieved either via consecutive Cu(I)-catalyzed azide-alkyne coupling (CuAAC) and C-C cross-coupling reactions or by C-C cross-coupling alone, giving access to either symmetric or non-symmetric bis-functionalized Idpc derivatives. The self-aggregation behavior of 9c in solution was investigated by recording a series of concentration-dependent NMR spectra. The aggregation constant of 9c was determined by a nonlinear least-squares treatment of the 1H NMR shift data to be 2.9 ± 0.2 M-1, and the formation of dimers was found to be the prevailing process.

Further investigation of the irreversible floc breakup in flocculation kinetics modelling

Abstract Recent research has shown that the equilibrium between aggregation and breakup in flocculation kinetics modelling may not hold due to irreversible floc breakup. This work investigated the influence of the coagulant/particle ratio and average velocity gradient () in the occurrence of this phenomenon with a different water matrix than previous research. Jar test assays were conducted with low (≈15 NTU), medium (≈50 NTU) and high (≈100 NTU) turbidity synthetic raw water, using alum as coagulant and kaolin as primary particles. Results were analyzed using Argaman and Kaufman's model and values for KA (aggregation constant) and KB (breakup coefficient) were determined with the evolutionary convergence method. Model fitting was satisfactory when the Al/particle ratio varied between 10 and 27 mg Al3+/g. Outside this range, residual turbidity increased after equilibrium, attributed to the irreversible floc breakup process and not adequately described by the model. A two-factor analysis of variance (ANOVA) was conducted, and the results indicated a plausible correlation between pKA (–log KA) and Al/particle ratio. As for pKB (–log KB), the results indicated a possible correlation with both Al/particle ratio and . The results suggest that the irreversible floc breakup may occur regardless of the water matrix composition.

Self-aggregation of stilbazolium ion pairs in liquid chloroform. A molecular dynamics study

Molecular aggregation plays an important role on optical phenomena, and optimizing the latter requires understanding the aggregation behavior as a function of the molecular structures. The formation of molecular aggregates composed of stilbazolium chromophores and small to medium-size anions is studied by carrying out classical molecular dynamics (MD) simulations, shedding light on the dynamics of spontaneous self-aggregation of anion-cation (AC) pairs and estimating their equilibrium aggregation constants. First, analyses based on atom-atom distances and on contact mapping (Voronoi tessellation) highlight that the interactions occur at the level of the methylpyridinium groups of the cations, which enabled monitoring the clustering dynamics. Then, simulations show that clusters containing up to 5 AC pairs dominate the populations. Among the AC pairs, two of them (where the stilbazolium bears a strong donor substituent and when the anion is SCN- or pTS-) aggregate into small clusters even at high concentration, corroborating related experimental results [ChemPhysChem. 11 (2010) 495–507]. Still, these MD simulations do not reveal strict relationships between the aggregate size and the nature of the anion. Nevertheless, AC pairs bearing iodide anions require longer MD simulation times to reach an asymptotic equilibrium tendency of the average number of clusters. Finally, the aggregates are shown to adopt a preferential Λ-shape with contributions from head-to-head and stacked forms. Complementarily, the analyses of the isoperimetric quotient reinforce the AC pairs are not forming spherical aggregates even for clusters including more than 10 AC pairs. So, when the stilbazolium bears a strong donor substituent and when the anion is pTS-, stacked aggregates are formed, at both low and high concentrations, resulting in excellent agreement with the measured aggregation equilibrium constant.

Study of aggregation of O^N^N^O Pt(II) complexes in solution

The newly-synthesized bidentate and tridentate Pt(II)-complexes containing the ONNO core and differing by the length of aliphatic side chain, were studied in the context of their aggregation ability in chlorophorm using 1H, 195Pt and diffusion-ordered NMR spectroscopy. It was found that the elongation of the aliphatic chain causes increase in the value of aggregation constant, whereas the addition of the third benzene ring to the structure of bidentate Pt(II) complexes results in lowering of K. These observations were explained in terms of the interplay of various physico-chemical factors influencing the stability of the tridentate dimers in solution, which is important for further understanding of the aggregation behaviour of the Pt-ONNO complexes in liquid crystalline phase.

Study Interionic Attraction of anionic dyes by Electrolytic Conductivity Measurement

Measurements of conductivity in aqueous solution are recommended for studying the behavior of some dyes. The conductivity of two azo dyes C.I. Acid Red 151 (AR 151) and C.I. Mordant green 17 (MG17) have been studied. The ionization degree, α and Van,t Hoff’s factor ,i, were calculated; their values decrease with increasing dye concentration and increase with increasing temperature. The mobility, µ, the activation energy, E, were also calculated. The findings indicate that interionic attractions and connections are present. Dyes ions tend to self-associate. Since these textile dyes were applied from aqueous systems, it is important to understand the association of dyes. The molecular aggregation of two sulphonated dyes studied by measuring the electrolytic conductivity in aqueous solutions as a function of dye concentration and temperature. Aggregation number, aggregation constant and shape of aggregate were calculated. As expected the aggregation number decreases with increasing temperature, and that tends to J-aggregation which enhance the devices performance such as xerography, sensors in presence of different additives.

Nanomaterials fusing with the skin: Alpha-tocopherol phosphate delivery into the viable epidermis to protect against ultraviolet radiation damage

Vitamin E (alpha tocopherol, α-T) is an important skin antioxidant, but its penetration into the viable epidermis, where it acts, is very limited. This study investigated if phosphorylating α-tocopherol (α-TP) to form a provitamin, improved its interactions with skin, its passage into the tissue, and thus its ability to protect the skin from ultraviolet radiation (UVR) damage. At pH 7.4, when the α-TPO4−1 microspecies predominated in solution, dynamic light scattering measurements showed that α-TP formed nanoaggregates with a median hydrodynamic diameter of 9 nm (Critical aggregation constant, CAC, – 4.2 mM). At 9.0 when the α-TPO4−2 microspecies predominated there was no aggregation. The passage of α-TP nanoaggregates through regenerated cellulose membranes was significantly slower than the α-TP monomers (at pH 9) suggesting that aggregation slowed diffusion. However, a lotion formulation containing the nanoaggregates delivered more α-TP into the skin compared to the formulation containing the monomers. In addition, the nanosized α-TP aggregates delivered 8-fold more active into the stratum corneum (SC) (252.2 μg/cm2 vs 29.5 μg/cm2) and 4 fold more active into the epidermis (85.1 μg/cm2 vs 19 μg/cm2, respectively, p < 0.05) compared to α-T. Langmuir subphase injection studies at pH 7.4 (surface pressure 10 mN m−1) showed that the α-TP nanoaggregates more readily fused with the SC compared to the monomers and the membrane compression studies demonstrated that α-TP fluidised the SC lipids. Together the fusion with the SC and its fluidisation were proposed as the causes of the better α-TP penetration into the skin, which enhanced potential of α-TP to protect from UVR-induced skin damage compared to α-T.

Effect of aggregation on the simple ion transfer across oil|water interfaces

The theory of cyclic voltammetry of simple ion transfer across a liquid|liquid (L|L) interface including the effect of aggregation is developed. We focus on two different situations: ion transfer followed by an aggregation process in the organic phase, and a disaggregation process in the aqueous phase followed by ion transfer. We use parameters typical of the areas of study of molecular aggregation and polymer formation, namely average aggregate size and polydispersity index. These parameters allow us to analyse, in a simple and rigorous way, the interconversion between monomer and aggregate species, both at the aqueous or organic side of the L|L interface and in the interfacial region during the potential scan. Our results show that aggregation occurring in the organic phase facilitates ion transfer across the L|L interface. In contrast, when the aggregates are present in the aqueous phase, they must be disaggregated so that the monomers can be transferred, hence the transfer energy increases. Both processes significantly alter the shape of voltammograms. The peak current and mid-peak potential are analysed as a function of initial concentrations, aggregation constants and maximum number of monomers forming each aggregate. These results are compared with approximate analytical equations, which can be used to guide further experiments and extract information from experimental responses.

The interplay of enthalpic/entropic factors in nanoparticles' aggregation in solution: The case of fullerene C60

In the present work we accomplished analysis of interplay of enthalpic (van der Waals and electrostatic) and entropic (solvophobic) factors into the aggregation process of fullerene C60 in solution. We showed that in order to maintain the fullerene clusters stable their internal structure must be irregular, viz. dense packing of fullerene molecules inside (the nucleus) and loose packing on the outer shell. The stabilization of the nucleus is governed by van der Waals factor with negligible contribution from electrostatics, whereas the stability of the outer shell is mainly determined by electrostatics. We demonstrated, that the outer shell of clusters is made of the fullerene mantle with water molecules incorporated, required for reduction the electrostatic repulsion of surface charges. Such irregularity enabled to explain the origin of well-known intrinsic fractality of large fullerene clusters, the existence of barrier of solvent polarity required to maintain the clusters stable, and predicted the attenuation of the aggregation constant for large clusters. And, finally, we showed that fullerene binding to clusters is accompanied by small magnitude of van der Waals and electrostatic interactions, thus explaining the purely entropic character of fullerene aggregation with nearly zero experimental enthalpy change.

Fractal C60 fullerene aggregation: Equilibrium thermodynamics approach

Using dynamic light scattering (DLS) data we showed that the main physico-chemical parameters characterizing C60 fullerene aggregation in solution, viz. equilibrium aggregation constant and fractal number, may be obtained from equilibrium thermodynamics approach. We implemented the standard isodesmic model of aggregation for approximation of experimental particle size distribution and obtained the magnitude of fractal number, df, previously known from other physico-chemical methods. We propose that the suggested DLS-based approach may be applied as a method of analysis of fractal aggregation of other types of nanoparticles, such as silver, gold, or nanodiamonds, alternative to standard spectroscopic (SAXS/SANS/SLS) methods.

Thermodynamics of the aggregation of imidazolium ionic liquids with sodium alginate or hydroxamic alginate in aqueous solution

Ionic liquids (ILs) were examined as new components in polymer/IL mixtures for their potential applications as advanced materials. This work reports the thermodynamics of the aggregation of imidazolium ionic liquids, CnMIMBr (n = 10, 12, 14, and 16), and sodium alginate (ALG) or hydroxamic alginate (AHX) in aqueous solutions. The goal of the study was to evaluate the effect of the concentration and alkyl chain length of the ILs and the concentrations and charge density of the polysaccharides on the formation of polyelectrolyte complexes. The aggregation constant (Kagg) was determined from electrical conductivity measurements, and we found that the polymer–IL interactions increase with increasing alkyl chain length of the ILs. Furthermore, an increase in surface tension close to the critical aggregation concentration (C2) indicates that the adsorbed layer thickness at liquid–air interface is much thicker for the AHX/CnMIMBr systems than for the ALG/CnMIMBr systems (n = 12, 14, and 16). Phase separation was observed for ILs with high hydrophobicity (n = 12,14, and 16) at zeta potentials close to zero. However, in the presence of C10MIMBr, only a slightly turbidity was observed, and the dispersion became transparent with further IL addition. The aggregation of ILs on alginate and its derivative is favorable for longer IL alkyl chains, meaning that the hydrophobic interactions act cooperatively with the electrostatic interactions in the formation of the complexes. The values of the standard Gibbs energy of aggregation (ΔGagg°) indicate that the interactions between C12MIMBr with ALG are favorable compared to those with AHX because the polymer charge density is higher. On the other hand, C14MIMBr and C16MIMBr prefer to aggregate with the more hydrophobic polymer (AHX). These outcomes highlight the role of ILs in the design of new polymer formulations.

Coupled Cooperative Supramolecular Polymerization: A New Model Applied to the Competing Aggregation Pathways of an Amphiphilic aza-BODIPY Dye into Spherical and Rod-Like Aggregates.

Based on our studies on biphasic self-assembly behavior of an amphiphilic BF2 -azadipyrromethene (aza-BODIPY) dye 1, a new analytical model to quantitatively describe the thermodynamic properties of the aggregation involving two competing supramolecular polymerization processes is proposed. In this model, the formation of the metastable as well as the thermodynamically stable aggregates was considered to follow a nucleated polymerization mechanism. The numerical calculation based on the new model gives insight into the formation of different species in such complicate aggregate systems. Moreover, the aggregation of the biphasic self-assembly processes for dye 1 was investigated by concentration-dependent UV/Vis spectroscopy. The experimental data were analyzed by using the new model to evaluate the thermodynamic parameters including aggregation constants, the size of nuclei, and the cooperativity the two types of aggregates.

Watching proteins' weight

Biophysics Careful measurements of light scattering can provide information on individual macromolecules and complexes. Young et al. used a light-scattering approach for accurate mass determination of proteins as small as 20 kDa (see the Perspective by Lee and Klenerman). Movies of protein complex association and dissociation were analyzed to extract biophysical parameters from single molecules and assemblies without labeling. Using this approach, the authors determined in vitro kinetics of fibril and aggregate growth and association constants for a complex protein-glycoprotein assembly. Science , this issue p. [423][1]; see also p. [378][2] [1]: /lookup/doi/10.1126/science.aar5839 [2]: /lookup/doi/10.1126/science.aat5851

Evaluation of Necrosis Avidity and Potential for Rapid Imaging of Necrotic Myocardium of Radioiodinated Hypocrellins.

Rapid noninvasive delineation of necrotic myocardium in ischemic regions is very critical for risk stratification and clinical decision-making but still challenging. This study aimed to evaluate the necrosis avidity of radioiodinated hypocrellins and its potential for rapidly imaging necrotic myocardium. The aggregation constants of four natural hypocrellins were analyzed by UV/vis spectroscopy. Then, they were radiolabeled with iodine-131 by iodogen oxidation method. Necrosis avidity of iodine-131-labeled hypocrellins was evaluated in rat models with reperfused liver infarction and muscular necrosis by gamma counting, autoradiography, and histopathology. Their pharmacokinetic properties were examined in normal rats. The potential of iodine-131-labeled hypomycin A ([131I]HD) for early imaging of necrotic myocardium was explored in rat models with reperfused myocardial infarction. Finally, the possible mechanism of necrosis avidity was investigated by in vitro DNA binding and in vivo blocking experiments. The aggregation constants of four hypocrellins were all much smaller than that of hypericin, a most studied necrosis avid agent. The radiochemical purities of the four radiotracers after purification were all greater than 95%, and more than 90% of tracers remained intact after incubation in rat serum for 24h. Among the four tracers, [131I]HD exhibited the highest necrotic to viable tissue uptake ratio and the fastest blood clearance. The necrotic myocardium could be clearly visualized 4h after injection of [131I]HD by single-photon emission computed tomography/X-ray computed tomography (SPECT/CT). DNA binding studies suggested that HD could bind to DNA through intercalation. Blocking studies demonstrated that uptake of [131I]HD in necrotic muscle could be significantly blocked by excess unlabeled HD and ethidium bromide with 67 and 60% decline at 6h after coinjection, respectively. [131I]HD can be used to rapidly visualize necrotic myocardium. The necrosis avidity mechanism of [131I]HD may be attributed to its binding to the exposed DNA in necrotic tissues.

Aggregation and metal-complexation behaviour of THPP porphyrin in ethanol/water solutions as function of pH

The effect of pH change on 5,10,15,20-Tetrakis(4-hydroxyphenyl)-21H,23H-porphine (THPP) with its aggregation as function of water-ethanol mixture was studied with UV–vis, fluorescence, Raman and computational analysis. In neutral pH, THPP was present as free-base and, increasing the water amount, aggregation occurred with the formation of H- and J-aggregates. The aggregation constant and the concentration of dimers were calculated, other information about the dimer aggregation were evaluated by computational study.In acidic pH, by the insertions of two hydrogens in the porphyrin rings, the porphyrin changed its geometry with a ring deformation confirmed by red-shifted spectrum and quenching in fluorescence; at this low pH, increasing the water amount, the acidic form (THPPH2)2+ resulted more stable due to a polar environment with stronger interaction by hydrogen bonding.In basic pH, reached by NH4OH, THPP porphyrin was able to react with alkali metals in order to form sitting-atop complex (M2THPP) confirmed by the typical absorption spectrum of metallo-porphyrin, Raman spectroscopy and by computational analysis.

Optimal experiment design: Link between the concentration and the accuracy of estimation of aggregation parameters

The principal condition for optimal experiment design, required for getting reasonable error for equilibrium aggregation constant, K, determination is obtained. This condition states that the selected concentration range for performing titration experiment should be inversely proportional to the expected value of K. As a consequence, the choice of physico-chemical methods for determination of aggregation parameters must obey this condition.

H-aggregation of the amphiphilic dye TDPI: Photophysical, electrochemical, DFT and SAXS studies

A new amphiphilic dye 2,4,5-tris(1-dodecylpyridinium-4-yl)imidazol-1-ide tetrafluoroborate (TDPI) was synthesized and characterized by 1H and 13C NMR, ESI-TOF mass spectra and elemental analysis. The photophysical behavior of TDPI was investigated by means of UV–Vis and fluorescence spectroscopy techniques. The results show a solvent-dependent blue-shift absorption and emission which were assigned to an H-aggregation. The calculated dimerization aggregation constant is 10 times bigger in dichloromethane than in acetonitrile, which is probably due to stronger ionic pair stabilization in low dielectric media. The structural characteristics of the formed aggregate were also investigated by SAXS analysis, revealing a surface fractal structure for all solvents. DFT calculations were used to investigate the photophysical properties of the monomer and the aggregate dimer. The TD-DFT calculations successfully described the electronic spectra of the monomer, but failed to simulate the dimer spectra as expected for this type of system. However, the TD-DFT of the dimer corroborates the experimental observed blue-shift tendency.

Influence of the Debye length on the interaction of a small molecule-modified Au nanoparticle with a surface-bound bioreceptor

We report that a shorter Debye length and, as a consequence, decreased colloidal stability are required for the molecular interaction of folic acid-modified Au nanoparticles (Au NPs) to occur on a surface-bound receptor, human dihydrofolate reductase (hDHFR). The interaction measured using surface plasmon resonance (SPR) sensing was optimal in a phosphate buffer at pH 6 and ionic strength exceeding 300 mM. Under these conditions, the aggregation constant of the Au NPs was approximately 10(4) M(-1) s(-1) and the Debye length was below 1 nm, on the same length scale as the size of the folate anion (approximately 0.8 nm). Longer Debye lengths led to poorer SPR responses, revealing a reduced affinity of the folic acid-modified Au NPs for hDHFR. While high colloidal stability of Au NPs is desired in most applications, these conditions may hinder molecular interactions due to Debye lengths exceeding the size of the ligand and thus preventing close interactions with the surface-bound molecular receptor.