Monomer Concentrations in Simple Micellar Systems from NMR Relaxation Times and C‐13 Chemical Shifts

The monomer concentrations for some simple micelle forming surfactants, octylammonium bromide (OTAB), sodium decyl sulphate, SDecS, and dodecyltrimethylammonium bromide (DTAB) were determined from the NMR‐paramagnetic relaxation enhancement experiment (the NMR‐PRE) on the H‐1 spin lattice relaxation times for the surfactant protons as a function of the surfactant concentration above and below the critical micelle concentration (cmc region). The NMR relaxation times, determined from the inversion recovery method, decrease steadily as the concentration of the surfactant is increased above the cmc value. A second, simple method is proposed whereby the p‐value of the surfactant in the micelle is determined from the two‐site model via extrapolation of the observed relaxation rate data to high concentrations. The p‐values and the calculated monomer concentrations are compared to each other and to those obtained using the mass‐action and phase separation models. In most cases, the monomer concentrations appear to remain constant at concentrations up to 2–3 times the cmc value, and then begin to decrease thereafter. These results are compared to the trends in monomer concentrations obtained from other NMR experiments in other surfactant systems.     

1H and 13C NMR Spectroscopy of Stereoisomeric 2′-deoxy-C-Nucleosides

Eleven furanosyl 2′-deoxy-C-nucleosides with β-D-erythro, α-D-erythro and β-D-threo configurations have Been studied by IH and ¹³C NMR spectroscopy recorded in CDC1₃ and/or DMSO-d ₆. Results obtained indicate that each of the three stereoisomeric configurations studied are identifiable by ¹H and ¹³C NMR spectroscopy using a combination of coupling constant and chemical shift criteria.     

Time-resolved limited proteolysis of mitogen-activated protein kinase-activated protein kinase-2 determined by LC/MS only

Mass spectrometry has gained prominence in limited proteolysis studies largely due to its unparalleled precision in determining protein molecular mass. However, proteolytic fragments usually cannot be identified through direct mass measurement, since multiple subsequences of a protein can frequently be matched to observed masses of proteolytic fragments. Therefore, additional information from N-terminal sequencing is often needed. Here we demonstrate that mass spectrometry analysis of the time course of limited proteolysis reactions provides new information that is self-sufficient to identify all proteolytic fragments. The method uses a non-specific protease like subtilisin and exploits information contained in the time-resolved dataset such as: increased likelihood of identifying larger fragments generated during initial proteolysis solely by their masses, additivity of the masses of two mutually exclusive sequence regions that generate the full-length molecule (or an already assigned subfragment), and analyses of the proteolytic subfragment patterns that are facilitated by having established the initial cleavage sites. We show that the identities of the observed proteolytic fragments can be determined by LC/MS alone because enough constraints exist in the time-resolved dataset. For a medium-sized protein, it takes about 8 h to complete the study, a significant improvement over the traditional SDS-PAGE and N-terminal sequencing method, which usually takes several days. We illustrate this method with application to the catalytic domain of mitogen-activated protein kinase-activated protein kinase-2, and compare the results with N-terminal sequencing data and the known X-ray crystal structure.     

Unexpected catalytic reactions of silyl-protected enol diazoacetates with nitrile oxides that form 5-arylaminofuran-2(3H)-one-4-carboxylates

Silyl-protected enol diazoacetates undergo dirhodium(II)-catalyzed reactions with nitrile oxides to form acid-labile ketenimines via dipolar cycloaddition of nitrile oxides to a donor/acceptor cyclopropene and Lossen rearrangement of the dipolar adduct; acid catalysis converts the ketenimine to the furan product.     

Gel-induced selective crystallization of polymorphs

Although nanoporous materials have been explored for controlling crystallization of polymorphs in recent years, polymorphism in confined environments is still poorly understood, particularly from a kinetic perspective, and the role of the local structure of the substrate has largely been neglected. Herein, we report the use of a novel material, polymer microgels with tunable microstructure, for controlling polymorph crystallization from solution and for investigating systematically the effects of nanoconfinement and interfacial interactions on polymorphic outcomes. We show that the polymer microgels can improve polymorph selectivity significantly. The polymorphic outcomes correlate strongly with the gel-induced nucleation kinetics and are very sensitive to both the polymer microstructure and the chemical composition. Further mechanistic investigations suggest that the nucleation-templating effect and the spatial confinement imposed by the polymer network may be central to achieving polymorph selectivity. We demonstrate polymer microgels as promising materials for controlling crystal polymorphism. Moreover, our results help advance the fundamental understanding of polymorph crystallization at complex interfaces, particularly in confined environments.     

Synthesis, structure and thermal analysis of the gallium complex of 1,4,7,10-tetraazacyclo-dodecane-N,N′,N″,N-tetraacetic acid (DOTA)

As part of our efforts to synthesize gallium bioconjugates based on folic acid and thiamin we have utilized 1,4,7,10-tetraazacyclo-dodecane-N,N′,N″,N-tetraacetic acid (DOTA) as the chelating ligand for gallium(III). The reaction of gallium chloride with DOTA at room temperature in aqueous solution affords the complex [Ga(HDOTA) · 5.5H₂O] (1), which is characterized by single-crystal X-ray spectroscopy, electrospray mass spectrometry and thermal gravimetric analysis. Gallium displays 6-coordinate distorted octahedral geometry within a puckered macrocyclic DOTA framework. The equatorial plane comprises two nitrogens of the DOTA ring and two oxygens from two of the four pendant carboxylic acid groups. The axial positions are comprised of the remaining two transannular nitrogens of the DOTA ring.     

Hydrogen-bond tuning of ferromagnetic interactions: synthesis, structure and magnetic properties of polynuclear copper(II) complexes incorporating p-block oxo-anions

The reaction of copper(II) hydroxide with 2,2'-bipyridine (bipy) (1 : 1) in alkaline aqueous solution (pH 14) at room temperature affords the alternating carbonate/hydroxo-bridged copper(II) polymeric chain compound {[Cu3(bipy)3(mu-OH)2(mu-CO3)2].11H2O}n, 1, as determined by single-crystal X-ray diffraction. The structure of 1 is built up from two similar centro-symmetric dinuclear [(bipy)Cu(mu-OH)]2 cores which link together via bridging carbonate groups to mononuclear [(bipy)Cu] fragments to form the chain. Interdigitation of adjacent chains through pi-pi interactions, which involve each bipy ligand, forms sheets that are separated by the water molecules of crystallisation. Variable-temperature magnetic susceptibility measurements have shown that 1 behaves as an isolated spin doublet with two non-interacting spin triplets with the magnetic coupling through the bis-mu-hydroxo bridges being strongly ferromagnetic in nature. The hydrothermal reaction of copper(II) hydroxide, bipy and ammonium hydrogenphosphate (pH 8) yields a dinuclear copper(II) complex of formula {[Cu2(bipy)2(mu-OH)2(HPO4)(H2O)].4H2O}, 2, as determined by single-crystal X-ray diffraction. The structure of consists of an asymmetric dinuclear bis-mu-hydroxo copper(II) core with a monodentate hydrogenphosphate dianion coordinating to one copper(II) atom (above) and a water molecule to the other (below). Intra- and inter-molecular hydrogen bond interactions involving the hydrogenphosphate, water molecules and bis-mu-hydroxo groups link adjacent dinuclear fragments into chains, which interdigitate to form sheets that are separated by the water molecules of crystallisation. The investigation of the magnetic properties of 2 showed that the strength of the ferromagnetic interaction through the bis-mu-hydroxo bridges is influenced by the significant out-of-plane displacement of the hydrogen atom of the hydroxo groups, brought about through hydrogen bonding to the hydrogenphosphate ligand, and yielded the strongest ferromagnetic coupling yet reported for the bis-mu-hydroxo copper(II) core.     

Solvent effects on the structural and formyl substrate reactivity properties of a nitrogen/sulfur-ligated zinc hydroxide complex

The solution structural and formyl substrate reactivity properties of a nitrogen/sulfur-ligated zinc hydroxide complex, [(bmnpaZn)2(mu-OH)2](ClO4)2 (1, bmnpa = N,N-bis-2-(methylthio)ethyl-N-((6-neopentylamino-2-pyridyl)methyl)amine), in acetonitrile and methanol are reported. In CH3CN, 1 has a binuclear cation [(bmnpaZn)2(mu-OH)2]2+ that is stabilized by secondary hydrogen bonding and CH/pi interactions involving the bmnpa chelate ligand. In CH3OH, 1 undergoes reaction with solvent to yield a zinc methoxide species, as determined by 1H NMR and electrospray mass spectral analysis. Treatment of 1 with methyl formate in CH3CN results in stoichiometric hydrolysis of the formyl ester to produce [(bmnpa)Zn(O2CH)]ClO4 (2) and methanol. The formate complex was identified via independent synthesis and characterization (X-ray crystallography, 1H and 13C NMR, FTIR, LRFAB-MS, conductance, and elemental analysis). In the solid state, 2n has a formate-bridged coordination polymer-type structure. However, in CH3CN, 2 behaves as 1:1 electrolyte, indicating cleavage of the polymer structure into mononuclear [(bmnpa)Zn(O2CH)]ClO4 species. Treatment of 1 with a stoichiometric amount of formanilide in CH3CN for 48 h at 45 degrees C results in decomposition of the zinc hydroxide complex to yield the free bmnpa ligand and an inorganic solid, presumably a zinc hydroxide or oxide species. Treatment of 1 with a stoichiometric amount of ethyl formate in CD3OD results in rapid, quantitative transesterification of the formyl carboxylate ester. A control reaction indicates that this transesterification reaction does not occur on the same time scale in the absence of the catalyst. Treatment of 1 with an excess of ethyl formate in CD3OD results in catalytic formyl carboxylate ester transesterification, with approximately 1000 turnovers in 60 min at 22(1) degrees C. Treatment of a CD3OD solution of 1 (0.5 equiv) with formanilide (1 equiv) results in the formation of aniline, d3-methyl formate, and the zinc formate complex 2. While aniline is produced stoichiometrically, the yield of d3-methyl formate varied from 30 to 50%, and the yield of 2 varied from 50 to 70% in repetitive experiments. Formation of both d3-methyl formate and 2 indicates that both methanolysis and hydrolysis reactions take place.