Enzyme Promiscuity as a Remedy for the Common Problems with Knoevenagel Condensation

A new protocol based on lipase-catalyzed tandem reaction toward α,β-enones/enoesters is presented. For the synthesis of the desired products the tandem process based on enzyme-catalyzed hydrolysis and Knoevenagel reaction starting from enol acetates and aldehyde is developed. The relevant impact of the reaction conditions including organic solvent, enzyme type, and temperature on the course of the reaction was revealed. It was shown that controllable release of the active methylene compound from the corresponding enol carboxylate ensured by enzymatic reaction diminishes significantly the formation of the unwanted co-products. Furthermore, this protocol was extended by including a second tandem chemoenzymatic transformation engaging various aldehyde precursors. After a careful optimization of the reaction conditions, the target products were obtained with yields up to 86 % and with excellent E/Z-selectivity.     

Chemoselective Derivatization of Folded Synthetic Insulin Variants with Potassium Acyltrifluoroborates (KATs)

Synthetic folded insulin variants containing an ornithine‐hydroxylamine residue are readily modified in aqueous buffers by amide‐forming ligations with potassium acyltrifluoroborates (KATs). The synthetic insulin analogs were prepared by Fmoc‐SPPS, α‐ketoacid‐hydroxylamine (KAHA) ligation, and a prosthetic C‐peptide that delivers the correct disulfide pattern and allows facile incorporation at the B0 position of Glargine M2 of a new ornithine hydroxylamine protected with a photolabile group. The folded insulin is readily modified by photo‐deprotection followed by amide‐forming KAT ligation to give insulin variants labeled with dyes, lipids, and PEGs, as well as the formation of a covalent dimer.     

Dielectric relaxation spectroscopy of aqueous micellar electrolyte solutions: A novel application to infer Dukhin number and zeta potential of a micelle

The complex permittivities of aqueous SDS solutions, with and without the addition of sodium chloride (NaCl), are measured in the frequency range from 200 MHz to 14 GHz. The SDS concentrations are chosen such that the SDS molecules aggregate to micelles. In this frequency range, the measured spectra allow for the identification of two different relaxation processes. That is, the relaxation of the water molecules at frequencies above 1 GHz and the micellar relaxation at frequencies lower than 1 GHz. It is found that the addition of NaCl to the system mostly affects the micellar relaxation process. In detail, the time constant as well as the amplitude of the relaxation decrease by adding NaCl. These effects are attributed to the change in the solution conductivity that changes the properties of the micelle's electrical double layer. We also extract the Dukhin number of the micelles as a function of surfactant and electrolyte content from the measurements. The Dukhin number is a dimensionless group that describes the influence of the surface conductivity on a phenomena. A regression between Dukhin numbers and free sodium ions is found so that all data collapses on a single curve independent of the surfactant concentration. The surface conductivity is a manifestation of the electrical double layer and we use the Bikerman equation to infer the zeta potential of the micelles. Comparison to literature data shows very good agreement and proves that dielectric relaxation spectroscopy can be engaged to infer the zeta potential of micelles. Abbreviations: CMC critical micelle concentration, DRS dielectric relaxation spectroscopy, EDL electrical double layer     

A Crystallographic Charge Density Study of the Partial Covalent Nature of Strong N⋅⋅⋅Br Halogen Bonds

The covalent nature of strong N?Br???N halogen bonds in a cocrystal ( 2 ) of N‐bromosuccinimide ( NBS ) with 3,5‐dimethylpyridine ( lut ) was determined from X‐ray charge density studies and compared to a weak N?Br???O halogen bond in pure crystalline NBS ( 1 ) and a covalent bond in bis(3‐methylpyridine)bromonium cation (in its perchlorate salt ( 3 ). In 2 , the donor N?Br bond is elongated by 0.0954 Å, while the Br???acceptor distance of 2.3194(4) is 1.08 Å shorter than the sum of the van der Waals radii. A maximum electron density of 0.38 e Å^?3 along the Br???N halogen bond indicates a considerable covalent contribution to the total interaction. This value is intermediate to 0.067 e Å^?3 for the Br???O contact in 1 , and approximately 0.7 e Å^?3 in both N?Br bonds of the bromonium cation in 3 . A calculation of the natural bond order charges of the contact atoms, and the σ*(N1?Br) population of NBS as a function of distance between NBS and lut , have shown that charge transfer becomes significant at a Br???N distance below about 3 Å.     

Ethynylphosphonamidates for the Rapid and Cysteine‐Selective Generation of Efficacious Antibody–Drug Conjugates

Requirements for novel bioconjugation reactions for the synthesis of antibody–drug conjugates (ADCs) are exceptionally high, since conjugation selectivity as well as the stability and hydrophobicity of linkers and payloads drastically influence the performance and safety profile of the final product. We report Cys‐selective ethynylphosphonamidates as new reagents for the rapid generation of efficacious ADCs from native non‐engineered monoclonal antibodies through a simple one‐pot reduction and alkylation. Ethynylphosphonamidates can be easily substituted with hydrophilic residues, giving rise to electrophilic labeling reagents with tunable solubility properties. We demonstrate that ethynylphosphonamidate‐linked ADCs have excellent properties for next‐generation antibody therapeutics in terms of serum stability and in vivo antitumor activity.     

Adiabatic triplet state tautomerization of p-hydroxyacetophenone in aqueous solution

The primary photophysical processes of p-hydroxyacetophenone (HA) and the ensuing proton transfer reactions in aqueous solution were investigated by picosecond pump-probe spectroscopy and nanosecond laser flash photolysis. Previous studies have led to mutually inconsistent conclusions. The combined data allow us to rationalize the excited-state proton transfer processes of HA in terms of a comprehensive, well-established reaction scheme. Following fast and quantitative ISC to the triplet state, (3)HA*, adiabatic proton transfer through solvent water simultaneously forms both the anion, (3)A(-)*, and the quinoid triplet enol tautomer, (3)Q*. The latter subsequently equilibrates with its anion (3)A(-)*. Ionization and tautomerization are likely to compete with the desired release reactions of p-hydroxyphenacyl photoremovable protecting groups.     

Molecular simulation and macroscopic modeling of the diffusion of hydrogen, carbon monoxide and water in heavy n-alkane mixtures

The self-diffusion coefficient of hydrogen (H(2)), carbon monoxide (CO) and water (H(2)O) in n-alkanes was studied by molecular dynamics simulation. Diffusion in a few pure n-alkanes (namely n-C(8), n-C(20), n-C(64) and n-C(96)) was examined. In addition, binary n-C(12)-n-C(96) mixtures with various compositions as well as more realistic five- and six-n-alkane component mixtures were simulated. In all cases, the TraPPE united atom force field was used for the n-alkane molecules. The force field for the mixture of n-alkanes was initially validated against experimental density values and was shown to be accurate. Moreover, macroscopic correlations for predicting diffusion coefficient of H(2), CO and H(2)O in n-alkanes and mixtures of n-alkanes were developed. The functional form of the correlation was based on the rough hard sphere theory (RHS). The correlation was applied to simulation data and an absolute average deviation (AAD) of 5.8% for pure n-alkanes and 3.4% for n-alkane mixtures was obtained. Correlation parameters vary in a systematic way with carbon number and so they can be used to provide predictions in the absence of any experimental or molecular simulation data. Finally, in order to reduce the number of adjustable parameters, for the n-alkane mixtures the "pseudo-carbon number" approach was used. This approach resulted in relatively higher deviation from MD simulation data (AAD of 18.2%); however, it provides a convenient and fast method to predict diffusion coefficients. The correlations developed here are expected to be useful for engineering calculations related to the design of the Gas-to-Liquid process.