Backbone and side chain assignment strategies for multiply labeled membrane peptides and proteins in the solid state

We demonstrate that the SPECIFIC CP technique can be used to obtain heteronuclear correlation (HETCOR) spectra of peptide backbones with greater efficiency than conventional HETCOR methods. We show that similar design principles can be employed to achieve selective homonuclear polarization transfer mediated through dipolar or scalar couplings. Both approaches are demonstrated in a tripeptide with uniform 15N and 13C labeling, and with uniform 15N labeling and natural abundance 13C. In other applications, the high efficiency of the heteronuclear SPECIFIC CP transfer allows discrimination of single amide signals in the 248-residue membrane protein bacteriorhodopsin (bR). In particular, variations are detected in the ordering of the Ala81-Arg82 peptide bond among the photocycle intermediates of bR and SPECIFIC CP is used to correlate 15N and 13C signals from the three Val-Pro peptide bonds.     

Iodometric microdetermination of phosphorus,sulfur and chlorine in organic compounds

Summary A method is presented for the determination of phosphorus, sulfur and chlorine in organic compounds in absence of nitrogen by iodometry. The titration is carried out in 50% acetone solution in order to eliminate the interference of hydrogen peroxide.

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Zusammenfassung Ein Verfahren zur jodometrischen Bestimmung von Phosphor, Schwefel und Chlor in stickstofffreien organischen Verbindungen wurde vorgeschlagen. Die Titration erfolgt in 50% acetonischer Lösung, um die Störung durch Wasserstoffperoxid auszuschalten.

     

Ipertrofan Revisited—The Proposal of the Complete Stereochemistry of Mepartricin A and B

Being a methyl ester of partricin, the mepartricin complex is the active substance of a drug called Ipertrofan (Tricandil), which was proven to be useful in treatment of benign prostatic hyperplasia and chronic nonbacterial prostatitis/chronic pelvic pain syndrome. Nevertheless, no direct structural evidence on the stereochemistry of its components has been presented to date. In this contribution, we have conducted detailed, NMR-driven stereochemical studies on mepartricins A and B, aided by molecular dynamics simulations. The absolute configuration of all the stereogenic centers of mepartricin A and B was defined as 3R, 7R, 9R, 11S, 13S, 15R, 17S, 18R, 19S, 21R, 36S, 37R, and 38S, and proposed as 41R. The geometry of the heptaenic chromophore of both compounds has been established as 22E, 24E, 26E, 28Z, 30Z, 32E, and 34E. Our studies on mepartricin ultimately proved that partricins A and B are structurally identical to the previously described main components of the aureofacin complex: gedamycin and vacidin, respectively. The knowledge of the stereochemistry of this drug is a fundamental matter not only in terms of studies on its molecular mode of action, but also for potential derivatization, aiming at improvement of its pharmacological properties.     

Cocrystals “Divorce and Marriage”: When a Binary System Meets an Active Multifunctional Synthon in a Ball Mill

A well-defined and stable “AB” binary system in the presence of “C” a crystalline synthon ground in a ball mill undergoes selective transformation in the solid state according to the equation AB+C→AC+B. When the amount of C is increased two times then the equation AB+2C→AC+BC is valid. The other variants are more complex. The pathway BC+A is allowed and leads to the AC and B products. The pathway AC+B is not preferred, and no transformation is observed. These non-obvious correlations were observed for cocrystal of barbituric acid (BA):thiobarbituric acid (TBA) recently reported by Shemchuk et al. (Chem. Commun. 2016 , 52, 11815–11818) in the presence of 1-hydroxy-4,5-dimethyl-imidazole 3-oxide (HIMO). This synthon shows high affinity for the BA_0.5TBA_0.5 cocrystal as well for its individual components, BA and TBA. Single-quantum, double-quantum (SQ-DQ) 2D ¹H very fast MAS NMR with a spinning rate of 60 kHz was employed as a basic and most diagnostic tool for the study of cocrystals transformations. Analysis of the experimental data was supported by theoretical calculations, including computation of the stabilization energy, E^stab, defined as the energy difference between the energy of a co-crystal and the sum of the energies of particular components in the respective stoichiometric ratios. Two mechanisms of synthon replacement have been proposed. Pathway 1 assumes a concerted mechanism of substitution. In this approach, synthon attack is synchronized in time with the departure of one of the components of the binary system. Pathway 2 implies a non-concerted process, with an intermediate stage in which three separate components are present. Evidence suggesting a preference for Pathway 2 is shown.     

Local probing of magnetic films by optical excitation of magnetostatic waves

Excitation of volume and surface magnetostatic spin waves in ferrite garnet films by circularly polarized laser pulses utilizing to the inverse magnetooptical Faraday effect has been studied experimentally. The region of excitation of the magnetostatic spin waves is determined by the diameter of the laser beam (～10 μm). At the same time, the characteristic propagation length of the modes is 30 μm. A method of finding the local characteristics of a magnetic film, in particular, the cubic and uniaxial anisotropy constants, based on the analysis of the azimuthal-angle dependence of the spectrum of the magnetostatic spin waves has been proposed.     

Correlations of NBO energies of individual hydrogen bonds in nucleic acid base pairs with some QTAIM parameters

Structural Chemistry - DNA and RNA base-pairs are the most important systems containing multiple hydrogen bonds. Characterizing the energy of individual intermolecular interactions in such systems...