Microbial fermentation-based synthesis of nano-curcumin suggesting the role of pullulan in nano-formulation

Curcumin is a multitargeting nutraceutical with numerous health benefits, however, its efficacy is limited due to poor aqueous solubility and reduced bioavailability. While nano-formulation has emerged as an alternative to encounter such issues, it often involves use of toxic solvents. Microbial synthesis may be an innovative solution to address this lacuna. Present study, for the first time, reports exploitation of Aureobasidium pullulans RBF4A3 for production of nano-curcumin. For this purpose, Aureobasidium pullulans RBF4A3 was inoculated in YPD media along with curcumin (0.1 mg/mL) and incubated for 24 h, 48 h, and 72 h. Subsequently, residual sugar, biomass, EPS concentration, curcumin concentration, and curcumin nanoparticle size were measured. As a result, nano-curcumin with an average particle size of 31.63 nm and enhanced aqueous solubility was obtained after 72 h. Further, investigations suggested that pullulan, a reducing polysaccharide, played a significant role in curcumin nano-formulation. Pullulan-mediated nano-curcumin formulation, with an average particle size of 24 nm was achieved with conversion rate of around 59.19 %, suggesting improved aqueous solubility. Additionally, the anti-oxidant assay of the resulting nano-curcumin was around 53.7 % per μg. Moreover, kinetics and thermodynamic studies of pullulan-based nano-curcumin revealed that it followed first-order kinetics and was favored by elevated temperature for efficient bio-conversion. Also, various physico-chemical investigations like FT-IR, NMR, and XRD reveal that pullulan backbone remains intact while forming curcumin nanoparticle. This study may open up new avenues for synthesizing nano-polyphenols through a completely green and solvent free process with plausible diverse applications.

A Study on Thiol-Michael Addition to Semi-Synthetic Elastin-Hyaluronan Material for Electrospun Scaffolds

Thiol-Michael addition is a chemical reaction extensively used for conjugating peptides to polysaccharides with applications as biomaterials. In the present study, for designing a bioactive element in electrospun scaffolds as wound dressing material, a chemical strategy for the semi-synthesis of a hyaluronan-elastin conjugate containing an amide linker (ELAHA) was developed in the presence of tris(2-carboxyethyl)phosphine hydrochloride (TCEP ⋅ HCl). The bioconjugate was electrospun with poly-D,L-lactide (PDLLA), obtaining scaffolds with appealing characteristics in terms of morphology and cell viability of dermal fibroblast cells. For comprehending the factors influencing the efficiency of the bioconjugation reaction, thiolated amino acids were also investigated as nucleophiles toward hyaluronan decorated with Michael acceptors in the presence of TCEP ⋅ HCl through the evaluation of byproducts formation.

Solvents effect on dansyl cysteamine depletion and reactivity classification of skin sensitizers: Tackling the challenges using binary solvent systems

The high throughput method using dansyl cysteamine (HTS-DCYA™) is a sensitive and rapid in chemico approach to characterize skin sensitizers' thio-reactivity. The direct quantification of fluorescent hapten-DCYA adducts facilitates the rapid testing of pure chemicals as well as mixtures. Poor solubility in acetonitrile was occasionally observed and can represent a limitation. To enable the range of solvent options compatible with the testing, the effect of binary solvent systems on thio-reactivity and the HTS-DCYA classification was explored. The method's robustness was validated using five different solvent modifiers: water, DMSO, methanol, ethanol, and tetrahydrofuran. Some modifiers, viz., water and methanol, resulted in unexpected DCYA depletion, negatively affecting the thio-reactivity and classification of potential sensitizers. This undesirable, non-specific depletion was circumvented by optimizing the original HTS-DCYA™ method's workflow, resulting in a more robust and reliable thio-reactivity and hence classification with a binary solvent system. The results were validated for both pure compounds and plant extracts as examples of complex test samples. Based on the obtained results, the modified HTS-DCYA optimal conditions in the various solvent systems were established. Concentrations of modifiers up to 10% DMSO, 40% water, 40% EtOH, 60% MeOH, or 60% THF in acetonitrile were found acceptable for the modified protocol, with results comparable to the original method. The improved workflow with binary solvent systems provides significant advantages by expanding the applicability of the HTS-DCYA to a wider array of chemicals poorly soluble in acetonitrile.

Determining Michael Acceptor Reactivity from Kinetic, Mechanistic, and Computational Analysis for the Base-catalyzed Thiol-Michael Reaction

A combined experimental and computational study of the reactivities of seven commonly used Michael acceptors paired with two thiols within the framework of photobase-catalyzed thiol-Michael reactions is reported. The rate coefficients of the propagation (kP), reverse propagation (k-P), chain-transfer (kCT), and overall reaction (koverall) were experimentally determined and compared with the well-accepted electrophilicity parameters of Mayr and Parr, and DFT-calculated energetics. Both Mayr's and Parr's electrophilicity parameters predict the reactivities of these structurally varying vinyl functional groups well, covering a range of overall reaction rate coefficients from 0.5 to 6.2 s-1. To gain insight into the individual steps, the relative energies have been calculated using DFT for each of the stationary points along this step-growth reaction between ethanethiol and the seven alkenes. The free energies of the individual steps reveal the underlying factors that control the reaction barriers for propagation and chain transfer. Both the propagation and chain transfer steps are under kinetic control. These results serve as a useful guide for Michael acceptor selection to design and predict thiol-Michael-based materials with appropriate kinetic and material properties.

Mitigating the Undesirable Chemical Reaction between Organic Molecules for Highly Efficient Flexible Organic Photovoltaics

Organic photovoltaics (OPVs) with nonfullerene acceptors (NFAs) feature excellent device performance and device stability. However, they are facing problems when the amine-rich polyelectrolytes are used as cathode interfacial layers. In this work, a small molecule, ethanedithiol (EDT) at the polyethyleneimine ethoxylated (PEIE)/active layer interface is inserted for mitigating the undesirable reaction between amine-rich groups and electron-acceptor moieties in NFA. The main role of EDT is to passivate the PEIE surface and prevent electron flow to NFA and the unwanted reaction can be mitigated. It improves the performance of OPV devices by reducing the work function, decreasing trap-assisted recombination, and improving electron-mobility. As a result, the flexible device with the PEIE interfacial layer with a power conversion efficiency (PCE) of 7.20% can be improved to 10.11% after the inclusion of EDT. Moreover, EDT-modified device can retain 98.18% after it is bent for 200 cycles and can maintain 80.83% of its initial PCE under continuous light illuminated in ambient conditions without any encapsulation. Based on these findings, the proposed strategy constitutes a crucial step toward highly efficient flexible OPVs.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Byproducts formed During Thiol-Acrylate Reactions Promoted by Nucleophilic Aprotic Amines: Persistent or Reactive?

The nucleophile-initiated mechanism of thiol-Michael reactions naturally leads to the formation of undesired nucleophile byproducts. Three aza-Michael compounds representing nucleophile byproducts of thiol-acrylate reactions initiated by 4-dimethylaminopyridine (DMAP), 1-methylimidazole (MIM), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) have been synthesized and their reactivity in the presence of thiolate has been investigated. Spectroscopic analysis shows that each nucleophile byproduct reacts with thiolate to produce a desired thiol-acrylate product along with liberated aprotic amines DMAP, MIM, or DBU, thus demonstrating that these byproducts are reactive rather than persistent. Density functional theoretical computations support experimental observations and predict that a β-elimination mechanism is favored for converting each nucleophile byproduct into a desired thiol-acrylate product, though an S_N 2 process can be competitive (i. e. within <2.5 kcal/mol) in less polar solvents.

Decomplexation as a rate limitation in the thiol-Michael addition of N-acrylamides

The thiol-Michael addition is a popular, selective, high-yield "click" reaction utilized for applications ranging from small-molecule synthesis to polymer or surface modification. Here, we combined experimental and quantum mechanical modeling approaches using density functional theory (DFT) to examine the thiol-Michael reaction of N-allyl-N-acrylamide monomers used to prepare sequence-defined oligothioetheramides (oligoTEAs). Experimentally, the reaction was evaluated with two fluorous tagged thiols and several monomers at room temperature (22 °C and 40 °C). Using the Eyring equation, the activation energies (enthalpies) were calculated, observing a wide range of energy barriers ranging from 28 kJ mol-1 to 108 kJ mol-1 within the same alkene class. Computationally, DFT coupled with the Nudged Elastic Band method was used to calculate the entire reaction coordinate of each monomer reaction using the B97-D3 functional and a hybrid implicit-explicit methanol solvation approach. The thiol-Michael reaction is traditionally rate-limited by the propagation or chain-transfer steps. However, our test case with N-acrylamides and fluorous thiols revealed experimental and computational data produced satisfactory agreement only when we considered a previously unconsidered step that we termed "product decomplexation", which occurs as the product physically dissociates from other co-reactants after chain transfer. Five monomers were investigated to support this finding, capturing a range of functional groups varying in alkyl chain length (methyl to hexyl) and aromaticity (benzyl and ethylenephenyl). Increased substrate alkyl chain length increased activation energy, explained by the inductive effect. Aromatic ring-stacking configurations significantly impacted the activation energy and contributed to improved molecular packing density. Hydrogen-bonding between reactants increased the activation energy emphasizing the rate-limitation of the product decomplexation. Our findings begin to describe a new structure-kinetic relationship for thiol-Michael acceptors to enable further design of reactive monomers for synthetic polymers and biomaterials.

Chemoselective Synthesis of N-Terminal Cysteinyl Thioesters via β,γ-C,S Thiol-Michael Addition

Dehydroalanine (ΔAla) is a highly electrophilic residue that can react efficiently with sulfur nucleophiles to furnish cysteinyl analogues. Herein, we report an efficient synthesis of N-terminal cysteinyl thioesters, suitable for S, N-acyl transfer, based on β,γ-C,S thiol-Michael addition. Both ionic and radical-based methodologies were found to be efficient for this process.

Evaluating Nucleophile Byproduct Formation during Phosphine- and Amine-Promoted Thiol-Methyl Acrylate Reactions

The commonly accepted mechanism of nucleophile-initiated thiol-acrylate reactions requires the formation of undesired nucleophile byproducts. A systematic evaluation of the formation of such nucleophile byproducts has been carried out to understand the relationships between byproduct formation and nucleophile structure, stoichiometry, solvent, and reaction type. Three common nucleophiles for thiol-Michael reactions were investigated: dimethylphenylphosphine (DMPP), diethylamine (DEA), and hexylamine (HA). The formation of phosphonium ester and aza-Michael byproducts upon initiating a representative thiol-acrylate reaction between 1-hexanethiol and methyl acrylate at a range of initiator loading (0.01-10.0 equiv) and in different solvents (neat, DMSO, THF, and CHCl₃) was determined by ¹H NMR spectroscopy. The influence of reaction type was investigated by expanding from small molecule reactions to end group thiol-acrylate functionalization of PEG-diacrylate polymers and through investigations of polymer-polymer coupling reactions. Results indicate that the propensity of forming nucleophile byproducts varies with nucleophile type, solvent, and reaction type. Interestingly, for all but polymer-polymer ligation reactions, nucleophile byproduct formation is largely unobserved for nitrogen-centered nucleophiles DEA and HA and essentially nonexistent for the phorphorous-centered nucleophile DMPP. A rationale for the differences in nucleophile byproduct formation for DMPP, DEA, and HA is proposed and supported by experimental and computational analysis.

Hyperbranched Aliphatic Polyester via Cross-Metathesis Polymerization: Synthesis and Postpolymerization Modification

A novel postpolymerization modification methodology is demonstrated to achieve selective functionalization of hyperbranched polymer (HBP). Terminal and internal acrylates of HBP derived from cross-metathesis polymerization (CMP) are functionalized in a chemoselective fashion using the thiol-Michael chemistries. Model reactions between different thiols (benzyl mercaptan and methyl thioglycolate) and acrylates (n-hexyl acrylate and ethyl trans-2-decenoate) by using dimethylphenylphosphine or amylamine as the catalyst are investigated to optimize the modification protocol for HBP. High-molecular-weight HBP P0 is generated through CMP of AB₂ monomer 2, a compound containing one α-olefin and two acrylate metathetically polymerizable groups. CMP kinetics is monitored by NMR and gel permeation chromatography (GPC). Accordingly, microstructural analysis is conducted in detail, and CMP procedure is optimized. Postpolymerization modification of HBP P0 is performed via two distinguished strategies, namely one-step complete modification and sequential modification, to generate terminally and/or internally functionalized HBPs P1-P3 in a chemoselective fashion by using phosphine-initiated and/or base-catalyzed thiol-Michael chemistries. Finally, thermal stability and glass transition behaviors of HBPs P0-P3 are studied by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively.

Investigation and Demonstration of Catalyst/Initiator-Driven Selectivity in Thiol-Michael Reactions

Thiol-Michael "click" reactions are essential synthetic tools in the preparation of various materials including polymers, dendrimers, and other macromolecules. Despite increasing efforts to apply thiol-Michael chemistry in a controlled fashion, the selectivity of base- or nucleophile-promoted thiol-Michael reactions in complex mixtures of multiple thiols and/or acceptors remains largely unknown. Herein, we report a thorough fundamental study of the selectivity of thiol-Michael reactions through a series of 270 ternary reactions using ¹H NMR spectroscopy to quantify product selectivity. The varying influences of different catalysts/initiators are explored using ternary reactions between two Michael acceptors and a single thiol or between a single Michael acceptor and two thiols using three different catalysts/initiators (triethylamine, DBU, and dimethylphenylphosphine) in chloroform. The results from the ternary reactions provide a platform from which sequential quaternary, one-pot quaternary, and sequential senary thiol-Michael reactions were designed and their selectivities quantified. These results provide insights into the design of selective thiol-Michael reactions that can be used for the synthesis and functionalization of multicomponent polymers and further informs how catalyst/initiator choice influences the reactivity between a given thiol and Michael acceptor.