Flexible NHC-aryloxido aluminum complex and its zwitterionic imidazolium aluminate precursor in ring-opening polymerization of ε-caprolactone.

Anionic donor-functionalized NHC (N-heterocyclic carbene) complexes of Al are rare. We report one such case here, an NHC-aryloxido AlMe2 complex [Al(L)Me2] (2), following a stepwise synthesis from the proligand [HO-4,6-tBu2-C6H2-2-CH2{CH(NCHCHNAr)}]Br [LH2Br; Ar = 2,6-iPr2-C6H3 (Dipp)] and AlMe3via the zwitterionic intermediate [Al(LH)Me2Br] (1). The ligand's flexibility in 2 is evident from the conformational fluxionality revealed by VT-1H NMR spectroscopic analysis. The ∠O-Al-C (ca. 100.5°) bite angle is also wider than the ∠O-Ti-C (ca. 80.6°) as seen in our recently reported Ti complex [Ti(L)(NMe2)2Br]. DFT analysis showed that the CNHC-Al bond is significantly ionic, as is the CNHC-Ti bond. Both 1 and 2 are active in the ring-opening polymerization (ROP) of ε-caprolactone (CL). 2, similar to [Ti(L)(NMe2)2Br], exhibits bifunctional MLC-type monomer activation, but only at an elevated temperature. However, the 2/BnOH combination is catalytically active at room temperature, likely through a zwitterionic [Al(LH)Me2(OBn)]. The 1/BnOH combination follows a similar mechanism but surprisingly at a faster rate.

A Heteronuclear NMR Study of Aqueous Lithium Salt Solutions of l-Alanine: Revealing Solute Hydrophobic Association through the NMR B' Coefficient.

In the present study, a set of heteronuclear NMR approaches has been adopted to investigate the solution behavior of a small hydrophobic solute l-alanine in the presence of lithium (Li) salts. The presence of salts plays a major role in determining the structure and solvation of biomolecules. It therefore evokes interest to understand the effect of Li salts on amino acids (alanine), the building block of biomolecules. The ionic solute dynamics in the present case has been probed using 1H, 7Li, and 13C nuclei available in the aqueous Li salt solution of l-alanine. Nuclear longitudinal spin relaxation of alanine protons was examined at a variable concentration range of three lithium salts, i.e., LiCl, Li2SO4 and LiClO4, to introduce the NMR B' coefficient for each salt defining ionic solute/solvent interaction in the solution. Analysis of the active relaxation mechanism of 7Li spin-lattice relaxation further revealed the presence of alanine in the solvation shell of Li ion depending on the anionic counterpart.

Investigation of Adsorption Behavior of Anticancer Drug on Zinc Oxide Nanoparticles: A Solid State NMR and Cyclic Voltammetry (CV) Analysis.

The present study aims to comprehend the adsorption behavior of a set of anticancer drugs namely 5-fluorouracil (5-FU), doxorubicin and daunorubicin on ZnO nanoparticles (ZnO NPs) proposed as drug delivery systems employing solid state (ss) NMR, FTIR and Cyclic Voltammetry (CV) analysis. FTIR and 1H MAS ssNMR data recorded for bare ZnO nanoparticle confirmed the presence of adsorbed -OH groups on the surface. 13C CP-MAS NMR spectra recorded for free and ZnO surface adsorbed drug samples exhibited considerable line broadening and chemical shift changes that complemented our earlier report on UV-DRS and XRD data of surface adsorption in case of 5-FU. Moreover, a remarkable enhancement of 13C signal intensity in case of loaded 5-FU was observed. This clearly indicated rigid nature of the drug on the surface allowing efficient transfer of 1H polarization from the hetero nitrogen of 5-FU to ZnO to form surface hydroxyl (-OH) groups and the same has been observed in the quantum chemical calculations. To further analyze the motional dynamics of the surface adsorbed 5-FU, longitudinal relaxation times (T1) were quantified employing Torchia method that revealed significant enhancement of 13C relaxation rate of adsorbed 5-FU. The enhanced rate suggested an effective role of quadrupolar contribution from 67Zn to the 13C relaxation mechanism of ZnO_5-FU. The heterogeneous rate constant (khet), average free energy of activation (∆G≠) and point of zero charge (PZC) measured for free and drug loaded ZnO NPs samples using CV further support the SS-NMR results.

Preferential solvation of carbohydrates in water-trifluoroethanol mixtures: a solvent detected heteronuclear NMR approach.

The present study aims to establish a simple approach involving multi-field multinuclear longitudinal relaxation (R1) analysis of the solvents to decipher solute-solvent interactions during the solvation of model carbohydrates in aqueous trifluoroethanol (TFE) co-solvent systems (TFE:D2O). The behavior of D2O and TFE is monitored around ß-CD (ß-cyclodextrin) and glucose through R1D (2H) and R1F (19F), respectively. Correlation times (τc) are estimated for D2O and TFE for various % (v/v) compositions of TFE:D2O mixtures. The differential trends of the R1 or τc ratio for D2O and TFE (in the presence and absence of carbohydrates) revealed that both ß-CD and glucose undergo selective solvation by TFE in comparison to D2O. Owing to its encapsulation properties, ß-CD exhibited a comparatively higher tendency to undergo solvation by TFE than glucose. The maximum transfer of solute bound water to bulk solvent appears in the 20-30% (v/v) TFE range. The current approach emerges as being straightforward in contrast to traditional methods that primarily focus on solute behavior to unravel the preferential solvation dynamics.

Solution-state NMR evaluation of molecular interaction between monoaromatic carboxylic acids and dissolved humic acid.

Understanding the nature of interactions between the aromatic organic pollutants with dissolved humic acid (HA) is fundamental for the prediction of their environmental fate and subsequent development of efficient remediation methods. The present study employs solution-state 1H/19F NMR methods to investigate the non-covalent interaction between aqueous peat humic acid (Aldrich HA) and monoaromatic carboxylic acids (CA), viz., 2, 6 diflourobenzoic acid (DFBA) and its non-fluorinated analog, benzoic acid (BA). NMR self-diffusion measurement of HA protons confirmed micellar nature indicating possibility of encapsulation of small molecules through host-guest interaction. 19F-1H and 1H-1H saturation transfer difference (STD) experiments reveal the mode of insertion of CA into HA superstructure. The strength of interaction has been evaluated by analyzing T1/T2 relaxation times and self-diffusion coefficients of CA as a function of HA concentration. Association constants extracted for CA-HA complexes from NMR diffusion experiments reflected that the association between DFBA-HA (2.34 mM-1) is significantly higher than that of BA-HA (0.97 mM-1). The experimental outcome reiterated that substitution of -H with halogen atoms (-F in specific) to aromatic ring plays a dominant role in modulating the strength of association and mode of insertion of organic pollutants into HA superstructure. The present study emphasizes that AHA can be a potential remediating agent for organic contaminants due to its superior binding affinity compared to less humified extracted HA (EHA) from Karwar, Rajasthan, India.

Analyzing organophosphate pesticide-serum albumin binding interaction: a combined STD NMR and molecular docking study.

In Vitro analysis of the interaction of organophosphate pesticides (OP) with bovine serum albumin (BSA) is crucial to understand their potential effects at the molecular level. In this context, we have employed Saturation Transfer Difference (STD) NMR experiments in conjunction with molecular docking studies to unravel the binding interaction of the OP chlorpyrifos (CPF), diazinon (DZN) and parathion (PA) in solution. The relative STD (%) suggested the detailed epitope mapping of these OP with BSA while the concentration-dependent STD NMR studies were performed to obtain the complex dissociation constant (KD) of the OP-BSA complexes; KD=1.81 × 10-4 M, 1.30 × 10-3 M and 1.11 × 10-3 M for CPF, DZN and PA were extracted respectively. Similar binding modes were identified for all the three OP using STD site-marker experiment. ITC experiments were performed as a complementary method that revealed a high binding affinity of OP-BSA complexes through non-covalent interaction. Molecular docking confirmed the possible interacting chemical groups of OP-BSA complexes. These significant results furnish valuable information about the toxicity risk of OP to proteins.Communicated by Ramaswamy H. Sarma.

Assessment of the Role of 2,2,2-Trifluoroethanol Solvent Dynamics in Inducing Conformational Transitions in Melittin: An Approach with Solvent 19F Low-Field NMR Relaxation and Overhauser Dynamic Nuclear Polarization Studies.

2,2,2-Trifluoroethanol (TFE) is one of the fluoroalcohols that have been known to induce and stabilize an open helical structure in many proteins and peptides. The current study has benchmarked low-field 19F NMR relaxation and 19F Overhauser dynamic nuclear polarization (ODNP) by providing a brief account of TFE solvent dynamics in a model melittin (MLT, an antimicrobial peptide) solution with a TFE-D2O cosolvent mixture at pH 7.4. Further, this approach has been employed to reveal the solvation of MLT by TFE in a nonbuffered solution with pH 2.8 for the first time. The structural transition of MLT has been elucidated via solvent dynamics by measuring the 19F TFE relaxation rates at 0.34 T for various TFE-D2O compositions in the absence (bulk TFE) and in the presence of MLT at both the pH values. A complementary initial record of circular dichroism experiments on these aqueous MLT solutions with TFE as the cosolvent at two different pH conditions demonstrated the structural transition from a random coil to a helical or from a folded helical to an open helical structure. The molecular correlation time derived from the corresponding relaxation rates shows that TFE resides on the MLT surface in both pH conditions. However, the trends in the variation of molecular correlation time ratio as a function of TFE concentration represent that the mechanism and the extent to which TFE affects the MLT structural integrity are different at different pH values. The extraction of the DNP coupling parameter from steady-state 19F ODNP experiments performed in the presence of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl at 0.34 T revealed changes in the solvation dynamics of TFE concomitant with the MLT structural transition. In summary, 19F relaxation and ODNP measurements made at a low field have allowed direct monitoring of TFE dynamics during the MLT structural transition in terms of preferential solvation. The choice of experiments performed at a moderately low field (0.34 T) enabled us to exploit on the one hand almost 1200-fold mitigation of the strong contribution of 19F chemical shift anisotropy at 11.76 T, whereas on the other hand, the ODNP experiment offered a window for probing molecular dynamics on timescales of the order of 10-1000 ps.

In vitro interaction of organophosphate metabolites with bovine serum albumin: A comparative 1H NMR, fluorescence and molecular docking analysis.

Since the exposure of organophosphate pesticides are known to cause severe health consequences, it is important to understand the molecular interaction of these pesticides metabolites with vital biomolecules, especially with the proteins. Here, considering bovine serum albumin (BSA) as a model protein, we have examined its interaction with two selected organophosphate metabolites, 3,5,6-trichloro-2-pyridinol (TCPy) and paraoxon methyl (PM). TCPy and PM are resultant metabolites of two most widely used organophosphate pesticides chlorpyrifos and parathion respectively. 1H NMR line broadening, selective spin-lattice relaxation rate measurements, saturation transfer difference (STD) NMR of both TCPy and PM were carried out in the presence and absence of BSA. The obtained values of the affinity index (A), binding constants (Ka) and thermodynamic parameters indicated strong organophosphates-BSA interaction. Further, fluorescence quenching data on TCPy-BSA and PM-BSA interactions strongly supported the NMR results, besides providing the stoichiometry of these complexes. Molecular docking analysis unraveled viable, strong hydrogen bonds and electrostatic interactions in TCPy-BSA and PM-BSA complexes. This study also revealed substantial time-dependent changes in the 1H NMR intensity of PM in the presence of BSA, which suggests faster degradation of PM with increasing protein concentration during protein-metabolite interactions. The hydrolysis is attributed to the esterase-like action of BSA. The result provides key insights into the direct interaction of the organophosphate metabolites with a biologically important carrier protein, serum albumin.

Ag(I) and Au(III) Mercaptobenzothiazole complexes induced apoptotic cell death.

2-Mercaptobenzothiazole (MBT) complexes of Ag(I) and Au(III) were synthesized by wet chemical method. The structural, optical, 1HNMR, ICP - MS and electrochemical studies of the complexes were carried out. The TUNEL assay studies of Ag(I)MBT and Au(III)MBT complexes on A549 cell line indicated induced apoptosis in the cells. TUNEL assay showed 60% cell viability for Ag(I)MBT whereas 80% for Au(III)MBT. Thus Ag(I)MBT can induce cell apoptosis in cells at a higher rate than Au(III)MBT. Therefore these complexes studied here can be a viable option as anti - proliferating agent.

Supplementation of biotin to sperm preparation medium enhances fertilizing ability of spermatozoa and improves preimplantation embryo development.

PURPOSE: Motility of spermatozoa helps not only in planning the type of infertility treatment but also directly reflects the success rate in assisted reproductive technology (ART). Previously, biotin, a water-soluble vitamin, has been shown to increase the motility and longevity of cryopreserved human spermatozoa. The present study was designed to understand the molecular basis of the beneficial effects of presence of biotin in sperm wash medium on early embryo development. METHODS: The effect biotin supplementation to sperm wash medium on the sperm parameters were assessed in swim-up fraction of normozoospermic and asthenozoospermic ejaculates collected from infertile men. Fertilization and early embryo development was studied using Swiss albino mice. RESULTS: Even though both biotin and pentoxifylline (PTX) enhanced the motility of spermatozoa from normozoospermic and asthenozoospermic samples, biotin group exhibited higher in vitro survival. Using mouse model, we observed that presence of biotin or PTX in sperm wash medium improved the fertilization rate and blastocyst rate compared to control. Blastocysts from these groups had significantly higher total cell number (P < 0.01) and lower apoptotic index. In silico target prediction revealed that GTPase HRas (HRas), tyrosine-protein phosphatase nonreceptor type 1 (PTP1B), and glucokinase are the probable targets for biotin. Solution-state Nuclear Magnetic Resonance (NMR) studies confirmed that biotin interacts both with human HRas and PTP1B. CONCLUSION: Our results indicate that presence of biotin in sperm wash medium can improve the fertilization potential and preimplantation embryo development and can be considered as a safe alternate to PTX.

Solution dynamics of 5-fluorouracil entrapped in poly lactic-co-glycolic acid (PLGA) microsphere-A study with 1D selective NMR methods.

In this report, our main focus is to introduce a set of one-dimensional (1D) NMR methods based on chemical shift, relaxation, and magnetization transfer, namely, NOE and chemical exchange involving selective pulse excitation to study the solution dynamics of drug in free and encapsulated state within polymeric microsphere. In this regard 5-fluorouracil (5-FU) loaded poly lactic-co-glycolic acid (PLGA) microspheres are prepared as model system via standard water-in-oil-in-water emulsification method. One-dimensional 1 H and 19 F nuclear magnetic resonance (NMR) spectra of 5-FU in presence of PLGA microspheres presented a significant change in linewidth and relaxation rates compared with free 5-FU confirming encapsulation. Furthermore, loss of coupling pattern in 1 H and 19 F NMR of PLGA encapsulated 5-FU as compared with free 5-FU suggests an enhanced -NH and -H2 O protons exchange dynamics in the interior of the microsphere indicating hydrated microsphere cavity. Quantification of exchange dynamics in case of free and PLGA-encapsulated 5-FU was attempted employing 1D selective NOESY and 1D multiply selective inversion recovery experiments. Analysis of the exchange rates confirmed existence of more than one kind of water population within the cavity as mentioned in an earlier solid state NMR report.

Binding Interaction of Organofluorine-Serum Albumin: A Comparative Ligand-Detected 19F NMR Analysis.

In the present study, we attempt to characterize fluorinated ligand-serum albumin interaction in solution by a set of one-dimensional 19F ligand-based experiments. In this regard, a model system diflunisal (DFL)-human serum albumin (HSA) has been chosen to benchmark the utility of 19F relaxation and diffusion-based experiments in deciphering ligand-protein interactions. Further, we extend the application of a similar set of 19F experiments to unravel the molecular interaction in an unexplored system of 2,6-difluorobenzoic acid (DFBA)-bovine serum albumin (BSA). Interaction analysis of DFBA-SA is of particular interest because DFBA is not only a stable metabolite of a number of pesticides but also used as the starting reagent of many fluorinated drugs. Observation of 19F-1H & 1H-1H saturation transfer difference effects confirmed binding of the ligands to SA. Further, these ligand-protein complexes were probed in terms of the dissociation constant ( KD), number of binding sites ( n), bound fraction of the ligand ( Pb), the complex lifetime (τres), and exchange rate ( Kex). Although Carr-Purcell-Meiboom-Gill (CPMG)-based transverse relaxation and diffusion analysis quantified the former three quantities, the latter two were determined by the constant time fast pulsing CPMG method. Additionally, 19F competition binding experiments performed with well-characterized BSA site markers and DFBA indicated nonspecific binding of DFBA to BSA, whereas similar measurements in the case of HSA with DFL and DFBA revealed superior binding interaction of DFL with SA.

Solvent-dependent binding interactions of the organophosphate pesticide, chlorpyrifos (CPF), and its metabolite, 3,5,6-trichloro-2-pyridinol (TCPy), with Bovine Serum Albumin (BSA): A comparative fluorescence quenching analysis.

Analysis of the interaction of pesticides and their metabolites with the cellular proteins has drawn considerable attention in past several years to understand the effect of pesticides on environment and mankind. In this study, we have investigated the binding interaction of Bovine Serum Albumin (BSA) with a widely used organophosphorous insecticide chlorpyrifos (CPF), and its stable metabolite, 3,5,6-trichloro-2-pyridinol (TCPy) to provide a comparative analysis of the two molecules by employing various spectroscopic techniques viz., UV-vis absorption, Circular Dichroism (CD), and Fluorescence spectroscopy. The fluorescence quenching studies of BSA emission in two different solvents viz., water and methanol in presence of CPF and TCPy have led to the revelation of several interesting facts about the pesticide-protein interaction. It has been found that both the molecules cause static quenching of BSA emission as seen from the Stern-Volmer constant (Ksv) irrespective of the solvent used for the analysis. While TCPy is a stronger quencher in water, it exhibits comparable quenching capacity with CPF in methanol. The solvent dependent differential binding interaction of the two molecules finally indicates possibility of diverse bio-distribution of the pesticides within human body. The UV-vis and CD spectra of BSA in presence of the test molecules have unravelled that the molecules formed ground state complex that are highly reversible in nature and have minimal effect on the protein secondary structure. Furthermore it is also understood that structural changes of BSA in presence of CPF is significantly higher compared to that in presence of TCPY.

Analysis of Molecular Interaction of Drugs within ß-Cyclodextrin Cavity by Solution-State NMR Relaxation.

The prime focus of the present study is to employ NMR relaxation measurement to address the intermolecular interactions, as well as motional dynamics, of drugs, viz., paracetamol and aspirin, encapsulated within the ß-cyclodextrin (ß-CD) cavity. In this report, we have attempted to demonstrate the applicability of nonselective (R1ns), selective (R1se), and bi-selective (R1bs) spin-lattice relaxation rates to infer dynamical parameters, for example, the molecular rotational correlation times (τc) and cross-relaxation rates (σij) of the encapsulated drugs. Molecular rotational correlation times of the free drugs were calculated using the selective relaxation rate in the fast molecular motion time regime (ωH2τc2 ≪ 1 and R1ns/R1se ≈ 1.500), whereas that of the 1:1 complexed drugs were found from the ratio of R1ns/R1se in the intermediate motion time regime (ωH2τc2 ∼ 1 and R1ns/R1se ≈ 1.054), and these values were compared with each other to confirm the formation of inclusion complexes. Furthermore, the cross-relaxation rates were used to evaluate the intermolecular proton distances. Also, density functional theory calculations were performed to determine the minimum energy geometry of the inclusion complexes and the results compared with those from experiments. The report, thus, presents the possibility of utilizing NMR relaxation data, a more cost-effective experiment, to calculate internuclear distances in the case of drug-supramolecule complexes that are generally obtained by extremely time consuming two-dimensional nuclear Overhauser enhancement-based methods. A plausible mode of insertion of the drug molecules into the ß-CD cavity has also been described based on experimental NMR relaxation data analysis.