Intrinsic and environmental modulation of stereoselective aldol organocatalyzed reactions by proline-containing lipopeptides.

Proline, along with its derivatives, has been employed as an efficient organocatalyst for aldol reactions, with the ability to promote the creation of stereoselective C-C bonds. Even though the Houk-List transition state model is able to explain the stereoselectivity observed when proline is used as a catalyst, few studies investigate the role of microheterogeneous media in modulating the reaction outcome. In this work, molecular dynamics and electronic structure calculations were used to investigate the aldol reaction in the condensed phase. Our research focused on a lipopeptide compound incorporating the proline residue within the sequence PRWG-(C18H37), where P represents l-proline, R stands for l-arginine, W for l-tryptophan, and G for l-glycine. This sequence is covalently bonded to a hydrophobic segment formed by a long aliphatic chain, acting as an organocatalyst in an aqueous solution. This catalytic phase utilizes the complex chemical environment of the solution to achieve high selectivity. Our findings indicate a Houk-List-like mechanism, in which the amide acts as an H-bond donor, complemented by a mechanism in which the counter ion, trifluoracetic acid (TFA), acts as a proton shuttle. Both mechanisms demonstrated low energy barriers-12.23 and 1.42 kcal mol-1 for the (S,R) stereoisomer formation, computed using DLPNO-CCSD with def2-TZVPP basis set. Further, to explore the catalytic effect of the PRWG-(C18H37) lipopeptide in water, molecular dynamics simulations were conducted. It was observed that the micellar phase significantly enhances stereospecific encounters, favouring the experimentally observed ratio of (SR/SS) isomers, in contrast to reactions in a pure cyclohexanone medium. By quantifying the effects enabled by the supramolecular assembly, we were able to shed light on the factors that modify and enhance the stereoslectivity of the reaction.

Synergistic Interaction of Hyperbranched Polyglycerols and Cetyltrimethylammonium Bromide for Oil/Water Interfacial Tension Reduction: A Molecular Dynamics Study.

Applying surfactants to reduce the interfacial tension (IFT) on water/oil interfaces is a proven technique. The search for new surfactants and delivery strategies is an ongoing research area with applications in many fields such as drug delivery through nanoemulsions and enhanced oil recovery. Experimentally, the combination of hyperbranched polyglycerol (HPG) with cetyltrimethylammonium bromide (CTAB) substantially reduced the observed IFT of oil/water interface, 0.9 mN/m, while HPG alone was 5.80 mN/m and CTAB alone IFT was 8.08 mN/m. Previous simulations in an aqueous solution showed that HPG is a surfactant carrier. Complementarily, in this work, we performed classical molecular dynamics simulations on combinations of CTAB and HPG with one aliphatic chain to investigate further the interaction of this pair in oil interfaces and propose the mechanism of IFT decrease. Basically, from our results, one can observe that the IFT reduction comes from a combination of effects that have not been observed for other dual systems: (i) Due to the CTAB-HPG strong interaction, a weakening of their specific and isolated interactions with the water and oil phases occurs. (ii) Aggregates enlarge the interfacial area, turning it into a less ordered interface. (iii) The spread of individual molecules charge profiles leads to the much lower interfacial tension observed with the CTAB+HPG systems.

Diving into the optoelectronic properties of Cu(II) and Zn(II) curcumin complexes: a DFT and wavefunction benchmark.

CONTEXT: Curcumin is a popular food additive around the world whose medicinal properties have been known since ancient times. The literature has recently highlighted several biological properties, but besides the health-related usages, its natural yellowish color may also be helpful for light-harvesting applications. This research aims to close a knowledge gap regarding the photophysical description of curcumin and its metallic complexes. METHODS: We conducted benchmark experiments comparing NEVPT calculations with several DFT functionals (B3LYP, M06-L, M06-2X, CAM-B3LYP, and ωB97X-D) for describing the UV spectra of curcumin and its metallo-derivative, curcumin-copper(II). Once we determined the most suitable functional, we performed tests with different basis sets and conditions, such as solvation and redox state, to identify their impact on excited state properties. These results are also reported for the curcumin-zinc(II) derivative. We found that the accuracy of DFT functionals depends strongly on the nature of curcumin's excitations. Intra-ligand transitions dominate the absorption spectra of the complexes. Curcumin absorption is marginally affected by solvation and chelation, but when combined with redox processes, they may result in significant modifications. This is because copper cation changes its coordination geometry in response to redox conditions, changing the spectrum. We found that, compared to a NEVPT reference, B3LYP is the best functional for a general description of the compounds, despite not being appropriate for charge transfer transitions. M06-L was the best for LMCT transitions. However, compared with NEVPT2 and PNO-LCCSD(T)-F12 results, no functional achieved acceptable accuracy for MLCT transitions.

Addressing the Environment Electrostatic Effect on Ballistic Electron Transport in Large Systems: A QM/MM-NEGF Approach.

The effects of the environment in nanoscopic materials can play a crucial role in device design. Particularly in biosensors, where the system is usually embedded in a solution, water and ions have to be taken into consideration in atomistic simulations of electronic transport for a realistic description of the system. In this work, we present a methodology that combines quantum mechanics/molecular mechanics methods (QM/MM) with the nonequilibrium Green's function framework to simulate the electronic transport properties of nanoscopic devices in the presence of solvents. As a case in point, we present further results for DNA translocation through a graphene nanopore. In particular, we take a closer look into general assumptions in a previous work. For this sake, we consider larger QM regions that include the first two solvation shells and investigate the effects of adding extra k-points to the NEGF calculations. The transverse conductance is then calculated in a prototype sequencing device in order to highlight the effects of the solvent.

The interaction of an azo compound with a surfactant and ion pair adsorption to solid phases.

The adsorption of SPADNS (trisodium salt of 2-(p-sulfophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid) onto resins XAD 2, XAD 7 and silica gel was studied in the presence and in the absence of the cationic surfactant CTAB (cetyl trimethylammonium bromide). At a ratio of 2.5 CTAB to 1 SPADNS, the surfactant caused a marked increase in SPADNS adsorption. The experimental results for adsorption versus time were applied on the basis of three kinetic models (pseudo-first-order Lagergren, pseudo-second-order, and intraparticle diffusion). The interaction between CTAB and SPADNS was investigated using spectrophotometric, conductometric, and computational techniques. Theoretical results point to the formation of an ion pair between CTAB and SPADNS that influences the solution spectra, in agreement with conductometric and spectrophotometric data.