Apparent Association Constant Research Articles

Dynamics and Thermodynamics Study of the Adsorption of Lysozyme to 2D SnSe Nanoflake Surfaces with Associated Unfolding

AbstractIn this work, we explore the interaction as well as corona formation and interfacial phenomena of lysozyme (LYZ) and 2D SnSe nanoflake. The optical results indicate formation of ground state complex with an apparent association constant of 1.5 × 103 (mol)−1. The dynamics of the in situ interaction exhibited surface reorganization (t2 = 73.5 min) and unfolding, which were more prominent than binding (t1 = 3 min) for tryptophan residues of LYZ. The formations of soft and hard corona of the LYZ structure were clearly observed from HRTEM. The emission quenching exhibited tertiary deformation of the LYZ structure with positive cooperative binding (n > 1) and dynamics of quenching followed the sigmoidal Boltzmann nature of the energy transfer process. The ultrafast lifetime of the bioconjugate varied from 1.9–1.35 ns with a maximum 63% energy transfer efficiency within 300 min of the corona evolution. The decrement of α helix (58.87%–21.78%) and increment of β sheet (5.27%–23.98%) were observed after interaction, with the conversion of large amount of α helix into β sheet. The ITC results showed that the injection heat results followed two‐site binding model, and the exothermic binding was dominate over the interaction. The second phase binding is driven by larger positive entropy, where the first binding site has a stronger association.

New Ru(II)-p-cymene compounds bearing indomethacin and indomethacin-pyridineamide ligands: synthesis, characterization, computational studies and investigation of their interactions with the Human Serum Albumin

The research interest in ruthenium(II) organometallic compounds has continually increased due to their promising performance in biological assays targeting diverse medicinal applications. This work reports the interactions of the [Ru(II)-η6-p-cymene] framework with the indomethacin (HL1) drug and with its modified analogue indomethacin-pyridineamide (L2). Two new half-sandwich organometallics, [Ru(η6-p-cymene)(L1)Cl] (1) and [Ru(η6-p-cymene)(L2)Cl2] (2), were successfully synthesized and fully characterized by ESI-MS, NMR, ATR-FTIR and UV/VIS spectroscopy. The chemical and electronic structures were corroborated by DFT computational calculations. Compound 2 was chemically stable, keeping the L2 ligand in the coordination sphere in coordinating solvents, and was investigated for the interaction with the Human Serum Albumin (HSA) protein by fluorescence quenching studies. This compound acted as a quencher to the intrinsic protein fluorescence, mainly through binding-related quenching (static) mechanism, showing high value for the apparent association constant (Ka). The analysis of the thermodynamic parameters revealed spontaneous process, presenting low temperature dependence, with electrostatic forces playing a key role in the HSA-compound 2 adduct binding. Site competitive studies suggested that the organometallic 2 may compete for the binding site of ibuprofen (subdomain IIIA) and also for the binding site of Ru(III) naked ion (subdomain IB or IIA). Molecular docking simulations showed that both Ru(II)-arenes may interact through hydrogen bonding with arginine residues, and exhibit high affinity for the microenvironment located in subdomains IIIA and IB. These findings suggest that the HSA protein might be a potential carrier for these Ru(II)-organometallic compounds in the blood plasma.

Cerium Oxide Nanoparticle-Papain Enzyme Bioconjugate: Synthesis, Characterization and Optical Absorption Study for Biomedical Applications

Objective: Here, we synthesize cerium oxide nanoparticles, successfully characterize them, and study the interaction between enzymes and cerium oxide nanoparticles (CeO2 NPs) using a simple optical spectroscopic technique. Method: CeO2 was fabricated by chemical method. Characterizations were done using UV-Vis absorption spectra, FESEM images, and XRD data. For the biomolecular study, papain was used as a model enzyme. Two different concentrations of CeO2, namely a small concentration range (0.05810 to 0.40670 mM) and a large concentration range (0.58100 to 0.92961 mM), were taken to study the interaction with the papain. UV-Vis absorption spectra were recorded to study bioconjugate formation. Findings: The CeO2 NPs have an intense absorption peak at 352 nm and a band gap of 3.78 eV. The XRD pattern showed the unit cell is cubic with an average particle size of 41.36 nm. The binding of papain with CeO2 NPs resulted in a red shift in its absorption peak. The apparent association constant (Kapp) was calculated for the bioconjugate and found to be 0.747 × 103 M-1 for small concentrations and 0.278 × 103 M-1 for large concentrations. In large concentrations, aggregation occurs instead of corona formation. Novelty: To the best of our knowledge, this may be the first study of the interaction of papain enzyme with CeO2 NPs. This study contributes to the application of NPs in the field of biomedicine. Keywords: Cerium oxide, Papaya proteinase (PP), Absorption spectra, Bioconjugate, Association constant

Study on the interactions between RS-licarbazepine and carboxyethylated cyclodextrins by electrokinetic chromatography and nuclear magnetic resonance

The interactions taking place in the enantiomeric separation of the antiepileptic drug licarbazepine by Electrokinetic Chromatography (EKC) using two carboxyethylated cyclodextrins (CE-CDs) (CE-β-CD and CE-γ-CD) as chiral selectors were investigated in this work to intend to explain the reversal in the enantiomer migration order observed depending on the CD. As these CDs were mixtures of isomers, only apparent and estimated association constants for enantiomer-CD complexes could be calculated based on EKC experiments, which agreed with the enantiomer migration order observed for CE-β-CD but not for CE-γ-CD. Additional experiments were carried out using NMR to study the stoichiometry of the complexes formed, their association constants and their structure. The construction of the corresponding Job plots, from 1H NMR titrations and intermolecular NOE interactions (between some of licarbazepine’s aromatic protons and the internal hydrogens of both CDs), permitted to conclude in the formation of 1:1 inclusion complex in both cases. Also, Scott plots for both systems were obtained showing a linear behavior for CE-β-CD while a non-linear behavior was observed for CE-γ-CD. As a result, only the apparent association constant for licarbazepine-CE-β-CD complexes was calculated (0.309 mM−1) (no significant differences between licarbazepine enantiomers were found in all NMR experiments). The structure of the complexes originated by licarbazepine and both CE-CDs was investigated. Insertion complexes were formed in each case, although some structural differences were noticed as the licarbazepine-CE-γ-CD complex would be less tight than that with CE-β-CD. Molecular dynamics (MD) simulations corroborated the structures derived from NMR results.

Microwave assisted structural engineering on efficient eco-friendly natural dye alizarin for dye sensitized solar cells application

The novel eco-friendly natural dyes, (9E, 10E), − 9, 10-bis(2-(4-nitrobenzylidene) hydrazono) − 9,10 dihydroanthracene-1,2-diol (NHA) have been synthesised using the one-pot microwave-assisted solvent evaporation method, and physicochemical characterizations were carried out using 1H NMR, 13C NMR, GC-MS, and FT-IR data. The photophysical properties of NHA dye were determined using experimental and theoretical techniques. The Stoke’s shift shows a large bathochromic shift in polar solvents, which is due to the π→π * transition. The ground-state optimization of NHA dye was carried out using density functional theory (DFT) with the B3LYP/6–31 G level basis set. The HOMO-LUMO and energy band gap values computed from density functional theory and absorption threshold wavelengths are good agreement with each other. Further, the effect of TiO2 nanoparticles on NHA dye has been studied using spectroscopic and electrochemical techniques. It was observed that, NHA dye showed fluorescence quenching in the presence of TiO2 NPs, which is due to the photo induced electron transfer process. The apparent association constant of the interaction between NHA dye and TiO2 nanoparticles is also calculated using the Benesi-Hildebrand model. The Rehm-Weller relation infers that thermodynamically favourable electron transfer takes place between dyes and TiO2 NPs. Further, the solar cell was constructed using NHA dye as a sensitizer, and the photovoltaic conversion efficiency was found to be 1.16%.

An efficient fluorescence reversible regulation strategy with single labelled oligonucleotide HEX-OND successively triggered by Hg(II) and Cysteine: The application and mechanism

An efficient fluorescence reversible regulation system with HEX-OND was developed. Then the application potential was explored in probing Hg(II) & Cysteine (Cys) in real samples and the thermodynamic mechanism was further investigated by precise theory analysis combining multiple spectroscopic methods. The results showed that only mere disturbances were observed among 15 and 11 kinds of other substances for the optimal system in detecting Hg(II) & Cys, respectively; The linear ranges of quantification were identified as 1.0 ∼ 14.0 and 2.0 ∼ 20.0 (×10-8 mol/L) with LODs of 8.75 and 14.09 (×10-9 mol/L) for Hg(II) and Cys, respectively; no significant deviations were found in the quantification results of Hg(II) in three traditional Chinese herbs and Cys in two samples between the well-understood methods with ours respectively, showing excellent selectivity, sensitivity, and tremendous application feasibility. The detailed mechanism was further verified as that the introduced Hg(II) forced HEX-OND to transform into the Hairpin structure with the apparent equilibrium association constant of 6.02 ± 0.62 × 1010 L/mol in the bimolecular ratio, leading to the equimolar quencher, consecutive two guanine bases ((G)2), approaching and spontaneously static-quenching the reporter HEX (hexachlorofluorescein) (equilibrium constant, 8.75 ± 1.97 × 107 L/mol) in the Photo-induced Electron Transfer (PET) way that was driven by the Electrostatic Interaction. The additional Cys destructed the equimolar Hairpin structure with the apparent equilibrium constant of 8.87 ± 2.47 × 105 L/mol through breaking one of the formed T-Hg(II)-T mismatches by association with the involved Hg(II), occasioning (G)2 apart from HEX and consequently the fluorescence recovery.

Photoinduced electron transfer across the polymer-capped CsPbBr3 interface in a polar medium.

In-situ polymer capping of cesium lead bromide (CsPbBr3) nanocrystals with polymethyl acrylate is an effective approach to improve the colloidal stability in the polar medium and thus extends their use in photocatalysis. The photoinduced electron transfer properties of polymethyl acrylate (PMA)-capped CsPbBr3 nanocrystals have been probed using surface-bound viologen molecules with different alkyl chains as electron acceptors. The apparent association constant (Kapp) obtained for the binding of viologen molecules with PMA-capped CsPbBr3 was 2.3 × 107 M-1, which is an order of magnitude greater than that obtained with oleic acid/oleylamine-capped CsPbBr3. Although the length of the alkyl chain of the viologen molecule did not show any impact on the electron transfer rate constant, it influenced the charge separation efficiency and net electron transfer quantum yield. Viologen moieties with a shorter alkyl chain length exhibited a charge separation efficiency of 72% compared with 50% for the longer chain alkyl chain length viologens. Implications of polymer-capped CsPbBr3 perovskite nanocrystals for carrying out photocatalytic reduction in the polar medium are discussed.

How Pendant Groups Dictate Energy and Electron Transfer in Perovskite–Rhodamine Light Harvesting Assemblies

Energy and electron transfer processes allow for efficient manipulation of excited states within light harvesting assemblies for photocatalytic and optoelectronic applications. We have now successfully probed the influence of acceptor pendant group functionalization on the energy and electron transfer between CsPbBr3 perovskite nanocrystals and three rhodamine-based acceptor molecules. The three acceptors─rhodamine B (RhB), rhodamine isothiocyanate (RhB-NCS), and rose Bengal (RoseB)─contain an increasing degree of pendant group functionalization that affects their native excited state properties. When interacting with CsPbBr3 as an energy donor, photoluminescence excitation spectroscopy reveals that singlet energy transfer occurs with all three acceptors. However, the acceptor functionalization directly influences several key parameters that dictate the excited state interactions. For example, RoseB binds to the nanocrystal surface with an apparent association constant (Kapp = 9.4 × 106 M-1) 200 times greater than RhB (Kapp = 0.05 × 106 M-1), thus influencing the rate of energy transfer. Femtosecond transient absorption reveals the observed rate constant of singlet energy transfer (kEnT) is an order-of-magnitude greater for RoseB (kEnT = 1 × 1011 s-1) than for RhB and RhB-NCS. In addition to energy transfer, each acceptor had a subpopulation of molecules (∼30%) that underwent electron transfer as a competing pathway. Thus, the structural influence of acceptor moieties must be considered for both excited state energy and electron transfer in nanocrystal-molecular hybrids. The competition between electron and energy transfer further highlights the complexity of excited state interactions in nanocrystal-molecular complexes and the need for careful spectroscopic analysis to elucidate competitive pathways.

Synthetic Receptors with Micromolar Affinity for Chloride in Water.

A water-soluble coordination cage was obtained by reaction of Pd(NO3 )2 with a 1,3-di(pyridin-3-yl)benzene ligand featuring a short PEG chain. The cavity of the metal-organic cage contains one nitrate anion, which is readily replaced by chloride. The apparent association constant for chloride binding in buffered aqueous solution is Ka =1.8(±0.1)×105 M-1 . This value is significantly higher than what has been reported for other macrocyclic chloride receptors. The heavier halides Br- and I- compete with binding or self-assembly, but the receptor displays very good selectivity over common anions such as phosphate, acetate, carbonate, and sulfate. A further increase of the chloride binding affinity by a factor of 3 was achieved using a fluorinated dipyridyl ligand.

Synthetic Receptors with Micromolar Affinity for Chloride in Water

AbstractA water‐soluble coordination cage was obtained by reaction of Pd(NO3)2with a 1,3‐di(pyridin‐3‐yl)benzene ligand featuring a short PEG chain. The cavity of the metal‐organic cage contains one nitrate anion, which is readily replaced by chloride. The apparent association constant for chloride binding in buffered aqueous solution isKa=1.8(±0.1)×105 M−1. This value is significantly higher than what has been reported for other macrocyclic chloride receptors. The heavier halides Br−and I−compete with binding or self‐assembly, but the receptor displays very good selectivity over common anions such as phosphate, acetate, carbonate, and sulfate. A further increase of the chloride binding affinity by a factor of 3 was achieved using a fluorinated dipyridyl ligand.

Tetracationic-to-dianionic tetraamino-dihydroxy-tetraoxacalix[4]arene: A paraquat receptor for all seasons

Tetraamino-dihydroxy-oxacalix[4]arenes 1·H64+·4Cl– and 2·H64+·4Cl– were synthesized in two steps from off-the-shelf components, and subjected to in-depth investigations on their molecular and supramolecular properties. UV–Vis titrations with aqueous NaOH provided six discrete protonation constants along with the distribution diagram for each of the oxacalixarenes, while a 1H NMR titration of 1·H64+·4Cl– with NaOD(aq), combined with DFT (B3LYP/6–311++G(d,p)) calculations allowed to shed light on the conformational preferences of macrocycle 1·Hn(n–2)+ in the different protonation states. 1H NMR titrations with N,N′-dimethyl-4,4′-bipyridinium (paraquat) dichloride gave, at different pH values, apparent association constants (Kapp) for the formation of the host–guest complexes. Kapp values, combined with the protonation constants, in turn provided the stability constant for the individual complexes between paraquat and 1·Hn(n–2)+ in the different states of ionization, and allowed to identify 1·H– as the most efficient paraquat receptor.

FRET pumping of rhodamine-based probe in light-harvesting nanoparticles for highly sensitive detection of Cu2+

In this work we presented novel strategy for increasing the performance of popular fluorescent probes on the basis of rhodamine-lactam platform. This strategy is based on the incorporation of probe molecules into the light-harvesting nanoparticles to pump modulated optical signal by Förster resonant energy transfer. Using the commercially available Cu2+ probe as a reference chemical, we have developed an efficient approach to significantly improve its sensing performance. Within obtained nanoparticles coumarin-30 nanoantenna absorbs excitation light and pumps incorporated sensing molecules providing bright fluorescence to a small number of emitters, while changing the probe-analyte equilibrium from liquid-liquid to solid-liquid significantly increased the apparent association constant, which together provided a ∼100-fold decrease in the detection limit. The developed nanoprobe allows highly sensitive detection of Cu2+ ions in aqueous media without organic co-solvents usually required for dissolution of the probe, and demonstrate compatibility with inexpensive fluorometers and the ability to detect low concentrations with the naked eye.

Presence of Circulatory Autoantibodies Against ROS-Modified Histone H1 Protein in Lymphoma Patients.

Lymphoma is a chronic inflammatory disease in which the immune system is highly affected. Increased oxidative stress is one of the common conditions of cancer and affects macromolecules. Histone modifications affect the chromatin structure and functions. In this study, histone H1 (His-H1) protein was modified by reactive oxygen species (ROS), and structural and chemical changes were studied. Hodgkin lymphoma (HL) and non-Hodgkin lymphoma (NHL) patients were selected, and oxidative stress markers, inflammatory cytokines, and serum autoantibodies were analyzed using biochemical and immunological assays. Furthermore, the formation of antigen-antibody immune complexes was assessed by the Langmuir plot. ROS-modified His-H1 (ROS-His-H1) showed substantial structural perturbation in protein (UV-hyperchromicity and increased intrinsic fluorescence) compared to the native His-H1 protein. A possible explanation for the changes is suggested by the exposure of the aromatic chromophore to the solvent. In-depth structural analysis by circular dichroism (CD) exhibited major changes in α-helix (−21.43%) and turns (+33%), reflecting changes in the secondary structure of histone H1 protein after ROS exposure. ELISA and competitive ELISA findings revealed high recognitions of serum autoantibodies to ROS-His-H1 from NHL, followed by HL subjects. Healthy controls showed negligible binding. Non-modified His-H1 did not show any binding with serum samples from either cohort. High apparent association constants (ACCs) were calculated for ROS-His-H1 using purified IgGs from NHL (1.46 × 10–6 M) compared to HL (1.33 × 10–6 M) patients. Non-modified His-H1 exhibited a hundred times less ACC for NHL (2.38 × 10–8 M) and HL (2.46 × 10–8 M) patients. Thus, ROS modifications of histone H1 cause structural changes and expose cryptic neo-epitopes on the protein against which autoantibodies were generated. These perturbations might affect the histone DNA interaction dynamics and potentially be correlated with gene dysregulation. These subtle molecular changes with an immune imbalance might further aggravate the disease.

Strontium Binding to α-Parvalbumin, a Canonical Calcium-Binding Protein of the “EF-Hand” Family

Strontium salts are used for treatment of osteoporosis and bone cancer, but their impact on calcium-mediated physiological processes remains obscure. To explore Sr2+ interference with Ca2+ binding to proteins of the EF-hand family, we studied Sr2+/Ca2+ interaction with a canonical EF-hand protein, α-parvalbumin (α-PA). Evaluation of the equilibrium metal association constants for the active Ca2+ binding sites of recombinant human α-PA (‘CD’ and ‘EF’ sites) from fluorimetric titration experiments and isothermal titration calorimetry data gave 4 × 109 M−1 and 4 × 109 M−1 for Ca2+, and 2 × 107 M−1 and 2 × 106 M−1 for Sr2+. Inactivation of the EF site by homologous substitution of the Ca2+-coordinating Glu in position 12 of the EF-loop by Gln decreased Ca2+/Sr2+ affinity of the protein by an order of magnitude, whereas the analogous inactivation of the CD site induced much deeper suppression of the Ca2+/Sr2+ affinity. These results suggest that Sr2+ and Ca2+ bind to CD/EF sites of α-PA and the Ca2+/Sr2+ binding are sequential processes with the CD site being occupied first. Spectrofluorimetric Sr2+ titration of the Ca2+-loaded α-PA revealed presence of secondary Sr2+ binding site(s) with an apparent equilibrium association constant of 4 × 105 M−1. Fourier-transform infrared spectroscopy data evidence that Ca2+/Sr2+-loaded forms of α-PA exhibit similar states of their COO− groups. Near-UV circular dichroism (CD) data show that Ca2+/Sr2+ binding to α-PA induce similar changes in symmetry of microenvironment of its Phe residues. Far-UV CD experiments reveal that Ca2+/Sr2+ binding are accompanied by nearly identical changes in secondary structure of α-PA. Meanwhile, scanning calorimetry measurements show markedly lower Sr2+-induced increase in stability of tertiary structure of α-PA, compared to the Ca2+-induced effect. Theoretical modeling using Density Functional Theory computations with Polarizable Continuum Model calculations confirms that Ca2+-binding sites of α-PA are well protected against exchange of Ca2+ for Sr2+ regardless of coordination number of Sr2+, solvent exposure or rigidity of sites. The latter appears to be a key determinant of the Ca2+/Sr2+ selectivity. Overall, despite lowered affinity of α-PA to Sr2+, the latter competes with Ca2+ for the same EF-hands and induces similar structural rearrangements. The presence of a secondary Sr2+ binding site(s) could be a factor contributing to Sr2+ impact on the functional activity of proteins.

Supramolecular Assemblies Constructed from Cucurbit[8]uril and N‐Alkyl Carboxymethylbenzotriazole through Host‐Guest Interactions

AbstractDifferent supramolecular self‐assembly between 1‐dodecyl‐3‐carboxymethylbenzotriazole (Lc12), 1‐octyl‐3‐carboxymethylbenzotriazole (Lc8) and 1‐butyl‐3‐carboxymethylbenzotriazole (Lc4) in the solid state and in aqueous solution has been investigated. For Lc12@CB[8] system, 1 : 1 host‐guest complexes were formed in aqueous solution and 2 : 2 host‐guest complexes were formed in the solid state, and the dodecyl chains adopted contorted conformations when bound to CB[8] molecules. For Lc8@CB[8] system, 1 : 1 host‐guest complexes were formed both in aqueous solution and in solid state, and the alkyl chain and benzotriazole moiety of guest Lc8 were both squeezed into the cavity of the CB[8]. The Lc4@CB[8] system was similar to Lc8@CB[8], 1 : 1 host‐guest complexes were formed in aqueous solution, the alkyl chain and benzotriazole moiety of guest Lc4 were encapsulated into the cavity of the CB[8]. Isothermal titrations in water afforded high apparent association constants for complexes Lc12@CB[8], Lc8@CB[8]and Lc4@CB[8].

Optical characterization and tunable antibacterial properties of gold nanoparticles with common proteins

The interaction of three proteins, viz. Bovine Serum Albumin (BSA), Human Serum Albumin (HSA) and Hen Egg White Lysozyme (HEWL) with gold nanoparticles (GNPs) is investigated using surface plasmon resonance (SPR) spectroscopy, fluorescence spectroscopy and circular dichroism (CD). Size and morphology of the samples was established using Transmission Electron Microscopy (TEM) and stability studies was established using zeta potential analysis. The stability of protein-GNP complex was found to be greater than that of individual protein as well as individual GNPs. Also HEWL-GNP complex was more stable compared to the other protein complexes. Absorbance of proteins increases with increase in gold nanoparticle concentration due to the extension of peptide strands of protein and decrease in hydrophobicity of gold nanoparticles. A ground state complex is also formed which is evident from the moderate shift observed in the absorbance peaks. Apparent association constant was also determined from the absorption spectra and was found to be maximum for HEWL and minimum for HSA. Gold nanoparticles were found to act as quenchers and reduced the protein fluorescence intensity. Binding constant and number of binding sites were found to be maximum for HEWL and minimum for HSA. The temperature dependent fluorescence studies were also performed to calculate the thermodynamic parameters and to determine the nature of interaction between the proteins and gold nanoparticles. The circular dichroism studies elucidate the reason behind the maximum binding for HEWL and minimum binding for HSA. TGA analysis determined the thermal stability of the samples. Fluorescence lifetime studies indicate static quenching of proteins. Antibacterial activity of protein-gold nanoparticles was studied against four pathogens, viz. Bacillus pumilus, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. HEWL exhibits a tunable antimicrobial activity against Pseudomonas aeruginosa due to the maximum binding of HEWL with gold nanoparticles. The study proposes a novel method for adjusting the antibacterial activity of HEWL against Pseudomonas aeruginosa when the resistance of this pathogen is a major issue in the chemotherapy of many infectious diseases. Thus the combination therapy of protein-gold nanoparticles could prove to be a new approach in medical field in the near future.

The apparent formation constants of asiatic acid and its derivatives existing in Centella asiatica with cyclodextrins by HPLC

The retention of triterpenoids in the C18 column and apparent formation constants of triterpenoids with cyclodextrins (CDs) were studied using HPLC. As the effective mobile phase additives, CDs can improve the separation of triterpenoid glycosides (asiaticoside B, madecassoside and asiaticoside) and reduce the retention of triterpenoids, and it is attributed to that triterpenoids forms a 1:1 inclusion complexes with CDs. The apparent formation constants of these complexes depend on the structure of triterpenoids, as well as on the substituent group and the hydrophobic cavity size of CDs. Triterpenoid glycosides had less apparent formation constants with HP-β-CD and Glu-β-CD, which might be related that hydroxypropyl group and glucose group in the structure of HP-β-CD and Glu-β-CD were unfaver to formation of the inclusion complexes. Asiaticoside B had larger apparent formation constants with CDs than madecassoside and asiaticoside. Larger apparent association constants of triterpenoid acids (madecassic acid and asiatic acid) were obtained with γ-CD than β-CD and its derivatives, which are related that triterpenoid acids fit well into the γ-CD cavity, and form relatively stable inclusion complexes. The ∆G (25 °C), ∆H and ∆S reveal that the inclusion processes between triterpenoids and CDs were not spontaneous, exothermic, and enthalpically driven. In addition, the usefulness of CDs for simultaneous analysis of five triterpenoids in HPLC and drug delivery vehicles was described.

Ruthenium complexes with mono- or bis-heterocyclic chelates: DNA/BSA binding, antioxidant and anticancer studies

Deoxyribonucleic acid (DNA) and bovine serum albumin (BSA) binding interactions for a series of ruthenium heterocyclic complexes were monitored using ultraviolet-visible (UV-Vis) spectrophotometry, fluorescence emission spectroscopy and agarose gel electrophoresis. Investigations of the DNA interactions for the metal complexes revealed that they are groove-binders with intrinsic binding constants in the order of 104 – 107 M −1. Electronic spectrophotometric DNA titrations of the bis-heterocyclic metal complexes illustrated hypochromism of their intraligand electronic transitions and the presence of diffuse isosbestic points which are synonymous with homogeneous binding modes. Metal complexes with the mono-heterocyclic chelates also showed alterations in their intraligand transitions and changes in their metal-based electronic transitions which are suggestive of metal coordination to the CT-DNA structure. Using agarose gel electrophoresis assessments, Hoechst DNA binding competition studies corroborate that the metal complexes are DNA groove-binders. Optimal uptake of these metal complexes by BSA was observed based on their optimal apparent association and Stern-Volmer constants (K app and K SV > 104 M −1). Radical scavenging studies revealed that the metal complexes have high activities towards the neutralization of NO and DPPH radicals. Data attained from the BSA electronic spectrophotometric titrations for the majority of the metal complexes illustrated distinct hyperchromism accompanied with blue shifts which indicates unwinding of the protein strands. Predominately, the metal complexes showed moderate cytotoxicity against both triple-negative breast cancer and cervical cancer cell lines that was greater than that of 5-fluorouracil. Communicated by Ramaswamy H. Sarma

Insights into the binding mechanism of 2D copper-tetrakis-(4-carboxyphenyl)-porphyrin metal-organic framework nanosheets with Rhodamine B: Spectroscopic and thermodynamics studies

Metal-organic frameworks (MOFs) of copper-tetrakis-(4-carboxyphenyl)-porphyrin (CuTCPP) were prepared by a solvothermal route. The two-dimensional (2D) ultrathin nanosheets with larger specific surface area show higher adsorption capacity for rhodamine B (RhB), compared with the bulk MOF of CuTCPP. The binding mechanism of the 2D nanosheets with RhB was investigated based on spectroscopic methods. The results reveal that the fluorescence of RhB is efficiently quenched by 2D CuTCPP nanosheets, and the quenching mechanism is totally the static quenching by forming a ground state RhB-CuTCPP nanosheets complex with an apparent association constant of 3.953 × 103 M−1. The interaction between RhB and CuTCPP nanosheets leads to the H-aggregate of RhB in the complex. Higher-order aggregate may also be involved in the complexation. Thermodynamic studies demonstrate that RhB-CuTCPP nanosheets interaction is a spontaneous process and driven by electrostatic and hydrophobic forces. The 2D CuTCPP nanosheets can be regenerated from the complex and used repeatedly.

Stochastic or Not? Method To Predict and Quantify the Stochastic Effects on the Association Reaction Equilibria in Nanoscopic Systems.

The stochastic nature of chemical reaction and impact of the stochasticity on their evolution is soundly documented. Both theoretical predictions and emerging experimental evidence indicate the influence of stochastic effects on the equilibrium state of association reaction. In this work simple mathematical formulas are introduced to estimate these effects. First, the dependence of the ratio of observed reactants (apparent association constant, equivalent of macroscopic association constant in stochastic analysis) on the volume and the number of molecules of reagents is discussed and the limiting factors of this effect are shown. Next, the apparent association constant is approximated for nanoscale systems by closed-form formulas derived for this purpose. Finally, an estimation for the macroscopic constant value from the apparent one is provided and validated on the published experimental data. This work was inspired by chemical reactions occurring in biological compartments, but the results can be used for all systems belonging to the stochastic regime of chemical reactions.