Everything you wanted to know about phase and reference frequency in one- and two-dimensional NMR spectroscopy

The fundamental concept of phase discussed in this tutorial aimed at providing students with an explanation of the delays and processing parameters they may find in nuclear magnetic resonance (NMR) pulse programs. We consider the phase of radio-frequency pulses, receiver, and magnetization and how all these parameters are related to phases and offsets of signals in spectra. The impact of the off-resonance effect on the phase of the magnetization is discussed before presenting an overview of how adjustment of the time reference of the free induction decay avoids first-order correction of the phase of spectra. The main objective of this tutorial is to show how the relative phase of a pulse and the receiver can be used to change the reference frequency along direct and indirect dimensions of NMR experiments. Unusual of phase incrementation with non-90° angles will be illustrated on one- and two-dimensional NMR spectra.     

Enantiomeric Cyclic Peptides with Different Caco‐2 Permeability Suggest Carrier‐Mediated Transport

Recently, oral absorption of cyclic hexapeptides was improved by N‐methylation of their backbone amides. However, the number and position of N‐methylations or of solvent exposed NHs did not correlate to intestinal permeability, measured in a Caco‐2 model. In this study, we investigate enantiomeric pairs of three polar and two lipophilic peptides to demonstrate the participation of carrier‐mediated transporters. As expected, all the enantiomeric peptides exhibited identical lipophilicity (logD_7.4) and passive transcellular permeability determined by the parallel artificial membrane permeability assay (PAMPA). However, the enantiomeric polar peptides exhibited different Caco‐2 permeability (P_app) in both directions a–b and b–a. The same trend was observed for one of the lipophilic peptide, whereas the second lipophilic enantiomer pair showed identical Caco‐2 permeability (within the errors). These findings provide the first evidence for the involvement of carrier‐mediated transport for peptides, especially for those of polar nature.     

Investigation of surface and thermogravimetric characteristics of carbon-coated iron nanopowder

Demand for high-density press and sinter components is increasing day by day. Of the different ways to improve the sinter density, the addition of nanopowder to the conventional micrometer-sized metal powder is an effective solution. The present investigation is aimed at studying the surface chemistry of iron nanopowder coated with graphitic carbon, which is intended to be mixed with the conventional iron powder. For this purpose, iron nanopowder in the size range of 30 nm to submicron (less than 1 micron) was investigated using thermogravimetry at different temperatures: 400°C, 600°C, 800°C, 1000°C, and 1350°C. The X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and high-resolution scanning electron microscopy (HR-SEM) were used for characterizing the powder as well as samples sintered at different temperatures. The presence of iron, oxygen, carbon, chromium, and zinc were observed on the surface of the nanopowder. Iron was present in oxide state, although a small metallic iron peak at 707 eV was also observed in the XPS spectra obtained from the surface indicating the oxide scale to be maximum of about 5 nm in thickness. For the sample treated at 600°C, presence of manganese was observed on the surface. Thermogravimetry results showed a two-step mass loss with a total mass loss of 4 wt.% when heated to 1350°C where the first step corresponds to the surface oxide reduction.     

QSAR modelling and structural aspects concerning synthetic heterocycles with larvicidal activity against Aedes aegypti

Structural Chemistry - In this paper, we present a quantitative structure–activity relationships modelling for two series of heterocyclic synthetic compounds with larvicidal activity against...     

Synthesis of 4-polyfluoroalkyl-1,3-dithiolanes via [3+2] cycloaddition of thiocarbonyl ylide to polyfluoroalkanethioamides

Chemistry of Heterocyclic Compounds - New 4-polyfluoroalkyl-1,3-dithiolanes were synthesized by reaction of polyfluoroalkanethioamides with thiocarbonyl ylide that was generated in situ by...     

Nickel‐Catalyzed Barton Decarboxylation and Giese Reactions: A Practical Take on Classic Transforms

Two named reactions of fundamental importance and paramount utility in organic synthesis have been reinvestigated, the Barton decarboxylation and Giese radical conjugate addition. N ‐hydroxyphthalimide (NHPI) based redox‐active esters were found to be convenient starting materials for simple, thermal, Ni‐catalyzed radical formation and subsequent trapping with either a hydrogen atom source (PhSiH₃) or an electron‐deficient olefin. These reactions feature operational simplicity, inexpensive reagents, and enhanced scope as evidenced by examples in the realm of peptide chemistry.     

Effector Regulated Catalytic Cyclization of Alkynoic Acids Using Pt2L4 Cages

Metabolic pathways are highly regulated by effector molecules that influences the rate of enzymatic reactions. Inspired by the catalytic regulation found in living cells, we report a Pt₂L₄ cage of which the activity can be controlled by effectors that bind inside the cage. The cage shows catalytic activity in the lactonization of alkynoic acids, with the reaction rates dependent on the effector guest bound in the cage. Some effector guests enhance the rate of the lactonization by up to 19-fold, whereas one decreases it by 5-fold. When mixtures of specific substrates are used, both starting materials and products act as guests for the Pt₂L₄ cage, enhancing its catalytic activity for one substrate while reducing its activity for the other. The reported regulatory behavior obtained by the addition of effector molecules paves the way to the development of more complex, metabolic-like catalyst systems.