Positive ΔS Research Articles

Determination of the Bioenergy Potential of Brazilian Pine-Fruit Shell via Pyrolysis Kinetics, Thermodynamic Study, and Evolved Gas Analysis

This work provides the first study about the evaluation of the bioenergy potential of lignocellulosic waste from Brazilian pine-fruit shell (Araucaria angustifolia). Physicochemical characterization, evolved gas from pyrolysis, and kinetic and thermodynamic studies were performed. A thermogravimetric analyzer was used for the pyrolysis experiments, where the runs were performed under an inert atmosphere of nitrogen at temperatures ranging from room temperature to 850 °C at different low heating rates (5, 10, 20, and 30 °C min−1). The physicochemical characterization of Brazilian pine-fruit shell showed good applicability for the gasification process due to the high fixed carbon content. Similarly, the pyrolysis experiments and FTIR-evolved gas analysis indicate its great potential for use as a solid biofuel. The kinetic study showed that the Kissinger–Akahira–Sunose method (e = 0.07–0.11%) had a smaller relative error, when compared with the methods of Friedman (e = 5.12–28.89%), Flynn–Wall–Ozawa (e = 0.26–1.21%), and Starink (e = 0.17%), and it was comparable to the Vyazovkin method (e = 0.08–0.09%). Furthermore, the conversion rate curves obtained from kinetic parameters showed a satisfactory behavior, with a high regression coefficient (R2 ≥ 0.9165), thus demonstrating the great applicability of the parameters for the design and optimization of the thermochemical system. The endothermic and nonspontaneous process was observed, based on the positive ΔH, positive ΔG, and positive ΔS values of Brazilian pine-fruit shell. The pyrolysis of Brazilian pine-fruit shell has been identified as a viable alternative for bioenergy generation, acting as a solution for the final disposal of this agricultural waste biomass.

Adsorptive removal of Cr(VI) from simulated wastewater in MOF BUC-17 ultrafine powder

A water-stable metal-organic framework [Co3(tib)2(H2O)12](SO4)3 (BUC-17) with 2D graphene-like crystal structure was tuned to produce ultrafine powder via the introduction of absolute ethanol. The BUC-17 powders were further characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Zeta potential analysis. The BUC-17 ultrafine powder exhibited excellent adsorption performance toward Cr(VI) from simulated wastewater, in which the maximum uptake capacity is 121 mg g−1 at pH = 4. The kinetics fitting results revealed that the adsorption process is more suitably described by the pseudo-second-order kinetics model. The negative standard free energy change ΔG°, positive enthalpy change ΔH°, and positive entropy change ΔS° of the adsorption process implied that the adsorption between BUC-17 and Cr(VI) was spontaneous and endothermal. As well, the influencing factors of the adsorption process like pH, adsorbent dosage, initial concentration and foreign ions were investigated. Finally, the adsorption mechanism of BUC-17 toward Cr(VI) was proposed.

Coating of PET Textiles with Anionic Cyclodextrin Polymer for Paraquat Removal from Aqueous Solution

Polyethylene terephthalate (PET) textile was coated with anionic cyclodextrin polymer issued by crosslinking between β-CD (β-Cyclodextrin) and BTCA (1,2,3,4-butanetetracarboxylic acid) for paraquat (PQ) removal from aqueous solution. The polymer covering operated by the thermofixation method (170 oC and 30 minutes) comprised 23.52 % of weight gain, which was related to 0.76 mmol/g of ionic exchange capacity. The functionalized textile was also characterized by FTIR, SEM and TGA. Adsorption experiment was performed employing different parameters such as the pH of the solution, adsorption time, the initial concentration of paraquat and the adsorption temperature. The suitable pH was equal to 8 and the equilibrium time was 420 minutes. At 30 oC, the adsorption capacity of PQ was increased (5.0, 20.4, and 25.9 mg/g) when the initial concentration of paraquat was enhanced (10, 50, and 250 mg/l). Adsorption kinetics was appropriated to the pseudo-second-order model and adsorption isotherm was fitted to the Langmuir model. Thermodynamic parameters were studied at different temperatures (30, 40, and 50 oC), in which the negative ΔH displayed an exothermic adsorption process, the negative ΔG showed a spontaneous adsorption process and the positive ΔS revealed an enhanced disorder. Eventually, the recyclability of the modified textile in methanol reached 85 % after four reusability cycles.

Investigation of ovotransferrin conformation and its complexation with sugar beet pectin

Small-angle X-ray scattering (SAXS) has been used to investigate the conformation of ovotransferrin (OVT) in solutions of different pH values. OVT was found to be a spherical molecule at native state in aqueous solutions. Radius of gyration (Rg) of OVT increased by 19.6% upon transition from native to molten globular state, and Rg increased by 9.7% as OVT further unfolded to an extended conformation. Besides, OVT showed concentration scaling behaviors at molten globule state. The information about size and shape was expected to facilitate the understanding of interactions between OVT and sugar beet pectin (SBP). Increasing OVT/SBP mixing ratios altered critical pH transition points in phase diagram, and the ratio of 5:1 was close to the saturated complexation ratio between OVT and SBP. Turbidities of OVT–SBP mixtures decreased significantly in the presence of sodium chloride, indicating the domination of electrostatic interaction in OVT–SBP complexation. Atomic force microscopy data illustrated the formation, association and dissociation processes of OVT–SBP complexes. Fluorescence spectroscopy results revealed that binding constant between OVT and SBP decreased with increasing temperature. The negative ΔH and positive ΔS further indicated the strong attractive interaction between OVT and SBP, which overcame the entropy contribution. Emulsions stabilized by OVT–SBP complexes had smaller droplet sizes and better physical stability than those stabilized by either pure OVT or SBP.

Rotating magnetocaloric effect and unusual magnetic features in metallic strongly anisotropic geometrically frustrated TmB4

We have investigated the rotating magnetocaloric effect (R-MCE) of TmB4 - an anisotropic magnetic system with geometrical frustration of Shastry-Sutherland type. The R-MCE was obtained from detailed temperature dependencies of heat capacity in various magnetic fields of a single crystalline sample for crystal axes orientations c || B and c ⊥ B. The received results exhibit rather complex distributions of positive and negative entropy ΔS(T, B) and temperature ΔT(T, B) differences below and above TN when the direction of the magnetic field changes between directions c || B and c ⊥ B. The calculated results were confirmed by direct R-MCE measurements which, moreover, show an interesting angular dependence of R-MCE in the ordered phase, which seems to be related with the change of the effective magnetic field along the c axis during sample rotation. Thus, our study presents a new type of magnetic refrigerant with a rather large R-MCE for low temperature magnetic refrigeration, and points to further interesting magnetic features in the ordered phase of this frustrated system.

Specific Supramolecular Interaction Regulated Entropically Favorable Assembly of Amphiphilic Macromolecules

This article reports molecular interaction driven aqueous assembly of supramolecularly engineered amphiphilic macromolecules to cylindrical structure. Each polymer contains a single hydrophobic trialkoxybenzamide-linked naphthalene diimide (NDI) chromophore at the chain terminal as the supramolecular structure directing unit (SSDU). Irrespective of the structure of the appended hydrophilic polymer, H-bonding promoted J-aggregation among the NDI chromophore leads to the formation of thermally stable spherical micelle (critical aggregation concentration: 0.01–0.03 mM) which reorganizes to cylindrical micelle after a few hours. The reorganization time can be regulated by pH in the case of the anionic polymer as it affects the dynamics. Isothermal titration calorimetric (ITC) studies reveal positive ΔS values for assembly of all the polymers, reflecting the self-assembly process is favored by the entropy factor similar to the elegant examples in the biological domain. In contrast, a small molecule analogue of...

Spectroscopic and molecular docking studies of the binding of the angiotensin II receptor blockers (ARBs) azilsartan, eprosartan and olmesartan to bovine serum albumin

Angiotensin receptor blockers (ARBs) represent a group of widely used therapeutic agents for the effective control of hypertension and other cardiovascular problems. Herein, the interactions of three important members of the ARBs (azilsartan, eprosartan and olmesartan) with bovine serum albumin (BSA) have been explored employing a set of simple spectroscopic approaches complemented with molecular docking studies. Steady state fluorescence emission results demonstrated the ARBs-induced quenching of the intrinsic fluorescence of BSA, which turned out to be a result of the formation of non-fluorescent complexes. The determined Stern-Volmer and binding constants were in the 104 magnitude, which were declined with the increase in temperature that primarily indicated static type of quenching. Subsequent analysis of the fluorescence data to explore the thermodynamic characteristics of these interactions showed spontaneous reactions with negative ΔH◦ values and positive ΔS◦ values, which suggest the involvement of electrostatic binding forces along with hydrogen bonding. Competitive binding studies were conducted with the aid of known BSA site markers to find the binding region of BSA for the investigated ligands. This was further supported by molecular docking simulations that ascertained the efficient binding of the three ligands to Sudlow site I (subdomain IIA) in the BSA structure.

Synthesis, characterization, DNA/HSA interactions and in vitro cytotoxic activities of two novel water-soluble copper(II) complexes with 1,3,5-triazine derivative ligand and amino acids

Two water-soluble copper(II) complexes of 6-(pyrazin-2-yl)-1,3,5-triazine-2,4-diamine (pzta) and amino acids, [Cu(pzta)(L-ArgH)(H2O)](ClO4)2 (1) and [Cu(pzta)(L-Met)(H2O)]ClO4·3H2O (2) (L-ArgH: protonated L-Argininate; L-Met: L-Methioninate), were synthesized and characterized. The determined X-ray crystallographic structures of 1 and 2 exhibited distorted square-pyramidal coordination geometries. Their binding properties toward calf thymus DNA (CT-DNA) and human serum protein (HSA) were measured by spectroscopic (UV–Vis, fluorescence, circular dichroism (CD)), calorimetric (isothermal titration calorimetry (ITC)) and molecular docking technology. DNA binding experiments showed that the complexes bound to DNA through a groove binding mode, the positive ΔH and ΔS values indicated that the hydrophobic interaction was the main force in the binding between the complexes and DNA. Besides, the complexes caused the fluorescence quenching of HSA through a static quenching procedure, changed the secondary structure and microenvironment of the Trp-214 residue, and preferably bound to subdomain IIA of HSA driven by hydrophobic and hydrogen-bond interactions. These results were further verified by the molecular docking technology. Furthermore, the in vitro cytotoxicities of the complexes against three human carcinoma cell lines (A549, PC-3 and HeLa) were evaluated, which confirmed that the complexation improved the anticancer activity of the pzta ligand significantly.

Selective removal of mercury(II) from water using a 2,2-dithiodisalicylic acid-functionalized graphene oxide nanocomposite: Kinetic, thermodynamic, and reusability studies

2,2-Dithiodisalicylic acid-functionalized magnetic graphene oxide (Fe3O4@DTSA_GO) was used for the selective removal of Hg(II) from aqueous solution. The equilibrium was reached after 10 min and the maximum Hg(II) adsorption capacity of the Fe3O4@DTSA_GO nanocomposite was 283.5 mg g−1. The Hg(II) adsorption ability increased with the pH value of the aqueous solution. Isotherm and kinetic studies revealed the Sips isotherm model and pseudo-second kinetic model to provide the best fit to the experimental results. The positive value of ΔH° indicated endothermic interactions between Hg(II) and Fe3O4@DTSA_GO, while the negative ΔG° revealed a spontaneous reaction and the positive ΔS° an increase of the randomness at the solid–solute interface during the adsorption process. The selective removal of Hg(II) by the nanocomposite confirmed the presence of higher-affinity binding sites for Hg(II) than for Cd(II), Co(II), Zn(II), and Ni(II) ions. Furthermore, the Fe3O4@DTSA_GO nanocomposite exhibited excellent preferential adsorption for Hg(II) spiked in drinking water samples. EDTA 0.01 N was found to be an efficient elution agent for nanocomposite regeneration, with which over 84% of the adsorbed Hg(II) was recovered after five adsorption/desorption cycles.

High removal performance of a magnetic FPA90-Cl anion resin for bromate and coexisting precursors: kinetics, thermodynamics, and equilibrium studies

In this study, the FPA90-Cl resin was magnetized with supported Fe3O4 particles using a chemical co-precipitation method and its removal performance of bromate and coexisting precursors was explored. The magnetized FPA90-Cl resin was structurally characterized by SEM, FT-IR, and XRD. The effects of the initial concentrations, temperature, and resin dosage on bromate and bromide ion removal in drinking water were investigated using batch experiments. The magnetized FPA90-Cl resin exhibited a high removal efficiency for bromate and bromide ions at three initial concentrations, and the residual bromate concentrations were under the maximum contaminant level (MCL) of 10μgL-1 after 80min. The adsorption data of bromate and bromide ion could be well described by a pseudo-first-order kinetic model (R2˃0.98). The bromate removal alone was further studied by varying the initial solution pH, temperature, and competitive anions. The results showed that the magnetized FPA90-Cl resin could be used over a wide pH range (4.0-9.0). The maximum sorption capacity of the magnetized FPA90-Cl resin for bromate reached 132.83mgg-1 at 298K. The Freundlich and Redlich-Peterson isotherm models fit the bromate adsorption equilibrium better (R2˃0.99) than the Langmuir isotherm model (R2˃0.98). The thermodynamic analysis showed that the bromate adsorption process was endothermic. The negative ΔG and positive ΔS indicated that the process was spontaneous and that randomness increased after adsorption, respectively. The competition of coexisting anions with bromate was in the order of SO42-> CO32-> Cl-> NO3-> HCO3-> PO43-. Additionally, the magnetized FPA90-Cl resin could maintain a high bromate and bromide ion adsorption capacity after five cycles of regeneration by a 0.1M NaCl solution. Graphical abstract ᅟ.

Removal of nickel ions from aqueous solutions by manganese dioxide derived from groundwater treatment sludge

Abstract In this study, manganese dioxide (MnO2) derived from groundwater treatment sludge was used in the adsorption of nickel (Ni(II)) ions from aqueous solutions. The synthetic MnO2 was prepared via permanganate reduction using manganese extracted from the sludge by reductive acid leaching and hydroxide precipitation. Scanning electron microscopy images showed aggregated micron-sized MnO2 particles. Fourier transform infrared analysis of MnO2 revealed functional groups at 3,396 cm−1, 1,630 cm−1, 1,427 cm−1 and 468 cm−1 bands. Zeta potential measurements at the pH range of 2–8 confirmed the net negative surface charge of MnO2 particles. Moreover, Ni(II) adsorption by MnO2 was best described by the Langmuir isotherm model, as indicated by the high values for the coefficients of determination (R2> 0.9703). The separation factor (RL) for the range of pH values and initial Ni(II) concentrations considered in this study indicated that Ni(II) adsorption by MnO2 was favorable. The kinetic data of Ni(II) adsorption by MnO2 at pH 6.5 and initial Ni(II) concentrations from 10 to 200 mg L−1 conformed to the pseudo-second order adsorption kinetic model, with R2 > 0.9997. Chemisorption occurred through the complexation of Ni(II) ions with available MnO2 functional groups. The thermodynamic study at temperatures of 298.15, 308.15 and 318.15 K revealed that Ni(II) adsorption by MnO2 was spontaneous and thermodynamically favorable for initial Ni(II) concentrations ranging from 50 to 200 mg L−1. Overall, Ni(II) adsorption by MnO2 was endothermic, as indicated by the positive ΔH° values. In addition, MnO2 had good affinity towards the Ni(II) ions, as shown by the positive ΔS° values at all Ni(II) concentrations. Simple cost analysis revealed that the MnO2 production from groundwater sludge was economically viable and may be scaled up for commercial applications.

Direct evidences for the groove binding of the Clomifene to double stranded DNA

It has been reported that the antiestrogen Tamoxifen induces liver tumors in rats and genotoxic effects in vitro through DNA interaction. So, it can be proposed that its structural analogue, Clomifene, also can bind to DNA. To test this hypothesis, the DNA binding properties of Clomifene have been studied by absorption spectroscopy, fluorescence spectroscopy, cellular uptake, cell viability, cell proliferation and molecular modeling techniques. Evidences are provided that Clomifene could interact with DNA via minor groove interaction mode. The negative ΔG value implied that the interaction occurred between DNA and Clomifene spontaneously. Also, the positive ΔH and positive ΔS values indicated that the binding of Clomifene with DNA is mainly entropy driven and the enthalpy is unfavorable parameter. This also suggests that the hydrophobic interaction plays a major role in the binding with overall binding constant of K=5.645×107M−1 at 298K. From the results of docking, it can be concluded that Hydrogen bonds is also one of the most important interactions. The increase in entropy of system after binding might be due to the destruction of the DNA structure.

Adsorption of Cd(II) from Aqueous Solutions by a Novel Layered Double Hydroxide FeMnMg-LDH

The layered double hydroxides (LDHs) with a hydrotalcite-like structure are believed to possess great potentials as environmental remediation materials including removal of heavy metals from aqueous solutions by adsorption. A new LDH was synthesized with Mg2+ as the structure-stabilizing ion (FeMnMg-LDH) based on a co-precipitation method, which showed promised adsorption capacity for Pb. Its adsorption characteristics for Cd2+, an environmental active element relative to Pb, were examined in this paper. The results showed that adsorption equilibria were well described by Langmuir isotherm and the adsorption kinetics well followed a pseudo-second-order kinetic model. The maximum Cd2+ adsorption capacity of FeMnMg-LDH was about 59.99 mg/g at 25 °C, which is significantly higher than that of other similar kinds of absorbents. The high Cd2+ removal efficiency could maintain at a wide pH range due to its buffering capacity. The coexisting cations competed with Cd2+ adsorption on the FeMnMg-LDH with a sequence of Cu2+ > Pb2+ > Mg2+ > Ca2+ when coexisting ions were added in the adsorption system separately. The positive value of ΔH° (14.016 kJ/mol) suggested that the adsorption process is endothermic while the positive ΔS° value (0.08 kJ/mol K) revealed that the randomness increased at the solid-solution interface during the adsorption process. FeMnMg-LDH removes Cd2+ from aqueous solution mainly by surface adsorption, surface-induced precipitation, and ion exchange. The FeMnMg-LDH has been further proved to be a good absorbent for the removal of heavy metals from aqueous solution.

Investigation of pH effect on cationic solute binding to keratin and partition to hair.

In the process of hair treatment, various cationic actives contained in hair care products can be absorbed into hair fibre to modulate the physicochemical properties of hair such as colour, strength, style and volume. There have been very limited studies on the binding and partition properties of hair care actives to hair. This study aimed to investigate the pH effects on cationic solute absorption into hair and binding to keratin. The keratin binding and hair partition properties of three cationic solutes (theophylline, nortriptyline and amitriptyline) have been measured at different pH using fluorescence spectroscopy and equilibrium absorption experiment. The binding constants, thermodynamic parameters and hair-water partition coefficients determined at different pH were compared and analysed. Increasing the pH from 2.0 to 6.0 resulted in the net charge of hair keratin changed from positive to negative. As a consequence, the binding constants of the three cationic solutes with keratin increased with the increasing pH. This correlated with the variation of the electrostatic interaction between cationic solutes and keratin from repulsion to attraction. The positive ΔH and ΔS values indicated that hydrophobic interaction also played a major role in the binding of the three cationic solutes to keratin. There was a good correlation between solutes binding to keratin and hair-water partition of solutes. It appears that solute binding to hair keratin is driven first by hydrophobic interaction and then by electrostatic interaction. The fitted thermodynamic parameters suggested that hydrophobic interaction dominates for the binding of the three cationic solutes to keratin. That binding of cationic solutes to keratin correlates with the partition of the solutes to hair could provide theoretical guidance for further developing mathematical models of hair partition and penetration properties.

Enthalpy-driven micellization of oligocarbonate-fluorene end-functionalized Poly(ethylene glycol)

A fluorescent pyrene probe method was applied to measure the critical micelle concentration (CMC) of oligocarbonate-fluorene end-functionalized poly(ethylene glycol) (FmE445Fm) triblock copolymers in water. The CMC decreases with lower temperature and higher values of the hydrophobic block length, m. When analyzed by a closed-assembly micelle model, the estimated energetic parameters find a negative ΔH°mic and small positive ΔS°mic suggestive of enthalpy-driven micellization, which differs from entropy-driven oxyethylene/oxybutylene triblock copolymers and octaethylene glycol-n-alkyl ethers. The enthalpy-driven micellization of FmE445Fm may result from the limited hydration of individual hydrophobic F blocks that leads to few hydrogen-bonded waters released during F block association. The π-π stacking oligocarbonate-fluorene system also observed enthalpy-entropy compensation when compared to a series of published data on diblock and triblock copolymer systems. An anomalously low partition equilibrium constant for m = 15.3 implies a tightly-packed core that excludes pyrene intercalation into the fluorene core. This is discussed along with the possible limited applicability to estimate the CMC and potential model drug molecule insertions into the intercalated micelle core.

Kinetic Study and Mechanism Hydrolysis of 4‐Bromo‐3,5 dimethylphenyl N‐methylcarbamate in Aqueous Media

ABSTRACTDegradation via hydrolysis is among the main transformation pathways and particularly for N‐methylcarbamates. Carbamate pesticide hydrolysis is known to proceed through alkaline catalysis, with reaction of the hydroxide ion with the carbonyl function or with abstraction of hydrogen in the α position with respect to the carbonyl. This reaction leads to the formation of methylamine and corresponding phenol. In this respect, the reaction kinetics of 4‐bromo‐3,5‐dimethylphenyl N‐methylcarbamate (BDMC) hydrolysis have been investigated in alkaline solution using a spectrophotometric technique and reversed phase liquid chromatography. The kinetic constants were determined following a proposed pseudo–first‐order kinetic model. The positive activation entropy ΔS≠ = +35.73 J mol−1 K−1 and the absence of general base catalysis indicated an unimolecular elimination conjugate base (E1cB) hydrolytic mechanism involving the formation of methyl isocyanate. This result was confirmed by the fact that BDMC fits well into brönsted and Hammett lines, obtained for a series of substituted N‐methylcarbamate whose decomposition in aqueous media was established to follow an E1cB mechanism.

Synthesis of hexagonal mesoporous MgAl LDH nanoplatelets adsorbent for the effective adsorption of Brilliant Yellow

Hexagonal mesoporous MgAl layered double hydroxide (LDH) was synthesized through a simple hydrothermal method and was employed without calcination as adsorbent in the removal of the direct anionic azo dye, Brilliant Yellow (BY). The adsorbent was characterized by XRD, FT-IR, SEM and BET surface area. Hexagonal platelets of MgAl LDH were fabricated with nanometric building blocks. Batch experiments were carried out for study effects of contact time, amount of adsorbent, solution pH and solution temperature on the removal efficiency of BY. The adsorption of BY onto the MgAl LDH was pH dependent and the highest value of adsorption capacity was observed at pH=6. The equilibrium adsorption data were obtained using the Langmuir, Freundlich and Temkin isotherms. The results indicated that the experimental adsorption data were fitted to Langmuir and Freundlich models. The maximum adsorption capacity of BY onto the MgAl LDH was found to be 115mg/g. The kinetic data of adsorption were evaluated by pseudo-first order and pseudo-second order, which described well by pseudo-second order model. The negative values of ΔG° at all temperature indicated that the adsorption process was spontaneous. The values of ΔH° and ΔS° were calculated 19.3kJ/mol and 80.5J/mol·K, respectively. The positive value of ΔH° shows that the adsorption was endothermic, while positive ΔS° value reflects increased disorder at the solid–solution interface during the adsorption.

Spectroscopic and thermodynamic studies of light harvesting perylenediimide derivative - zinc porphyrin complex in aqueous media

Self-assembly of perylene derivative such as N,N′-bis(2(trimethylammonium iodide) ethylene)perylene-3,4,9,10-tetracarboxyldiimide (TAIPDI) can produce one-dimensional form (1D) in an aqueous media. The ability of one-dimensional TAIPDI to form light harvesting complex with water-soluble zinc porphyrin (ZnTPPS4) via the π-π and electrostatic interactions has been described. Owing to electronic interactions between the π-systems, the complex formation is accompanied by pronounced absorption spectral changes in the UV/Vis absorption bands. The formation constant of the ZnTPPS4-TAIPDI complex has been determined as 2.60×104M−1 suggests a moderately stable complex. The steady-state fluorescence measurements exhibited fluorescence quenching of both the singlet TAIPDI and ZnTPPS4 because of the electron transfer process from the electron-donating ZnTPPS4 to the electron-accepting TAIPDI. Based on the picosecond time-resolved fluorescence, the rate and quantum yield of the electron transfer were found to be 2.47×1010s−1 and 0.99, respectively, indicating fast and efficient electron transfer. The thermodynamic parameters of the complex formation have been determined from the stopped-flow measurements. The interaction between ZnTPPS4 and TAIPDI occurs in two steps, a fast and reversible step followed by a slow and irreversible one. The activation parameters for the complex formation (ΔH#=22±5kJmol−1 and ΔS#=−123±18JK−1mol−1), (ΔH#=133±4kJmol−1 and ΔS#=167±13Jmol−1K−1) were determined from variable temperature studies for the “on” and the “off” of the first step and ΔS#=246±37.89Jmol−1K−1 and ΔH#=130±11kJmol−1 for the second step. The negative and positive ΔS# values found for the interaction reactions are consistent with an associative interaction for the first step followed by dissociative mechanism for both the “off” and the second step.

A light harvesting perylene derivative - zinc phthalocyanine complex in water: spectroscopic and thermodynamic studies.

A perylene derivative, namely N,N'-bis(2(trimethylammonium iodide)ethylene)perylene-3,4,9,10-tetracarboxyldiimide (TAIPDI) forms nanoscale columnar stacks in water that have been characterized by using optical absorption and emission measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM). This behaviour was compared with that of unstacked TAIPDI in methanol. Assembly formation between the one-dimensional TAIPDI stacks and zinc phthalocyanine tetrasulphonic groups (ZnPcS4) via strong π-π and ionic interactions has been described in an aqueous medium. The formation constant of the supramolecular dyad has been determined as 2.94 × 104 M-1 from both the absorption and fluorescence measurements. Upon addition of ZnPcS4, the fluorescence quenching of the singlet-excited state of TAIPDI was observed because of the electron transfer process from ZnPcS4 to TAIPDI via the singlet-excited states of ZnPcS4 and TAIPDI entities. The electrochemical studies supported the electron transfer pathways via the singlet states of ZnPcS4 and TAIPDI. The thermodynamic parameters of the supramolecular complex have been determined from stopped-flow measurements. The interaction between ZnPcS4 and TAIPDI occurs in two steps, where the rate constant of the second step with TAIPDI (207 ± 8 M-1 s-1) is much slower than the first one (3515 ± 101 M-1 s-1). Activation parameters for the complex formation (ΔH# = 76 ± 11 kJ mol-1 and ΔS# = 83 ± 37 J K-1 mol-1, and ΔH# = 221 ± 15 kJ mol-1 and ΔS# = 540 ± 50 J K-1 mol-1) were determined from variable temperature studies for the first and second steps, respectively. The significantly positive ΔS# values found for both steps of the interaction reactions are consistent with a dissociative mechanism.

Entropy Drives the Formation of Salt Bridges in the Protein GB3

AbstractSalt bridges are very common in proteins. But what drives the formation of protein salt bridges is not clear. In this work, we determined the strength of four salt bridges in the protein GB3 by measuring the ΔpKa values of the basic residues that constitute the salt bridges with a highly accurate NMR titration method at different temperatures. The results show that the ΔpKa values increase with temperature, thus indicating that the salt bridges are stronger at higher temperatures. Fitting of ΔpKa values to the van't Hoff equation yields positive ΔH and ΔS values, thus indicating that entropy drives salt‐bridge formation. Molecular dynamics simulations show that the protein and solvent make opposite contributions to ΔH and ΔS. Specifically, the enthalpic gain contributed from the protein is more than offset by the enthalpic loss contributed from the solvent, whereas the entropic gain originates from the desolvation effect.