Time-resolved CIDNP: an NMR way to determine the EPR parameters of elusive radicals

Chemically Induced Dynamic Nuclear Polarization (CIDNP) of the diamagnetic products of radical reactions is exploited for the purpose of determination of the hyperfine coupling constants (HFCCs) of the radical intermediates. A simple proportionality relation between geminate CIDNP of a nucleus and its HFCC at the radical stage is established. The applicability range of this relation is determined: the relation is fulfilled in the case of a large difference in g-factor between the radicals involved and for the situation where the number of magnetic nuclei in the system is sufficiently large. The validity of the relation was confirmed by CIDNP experiments on radical pairs with precisely known HFCCs. Using the proportionality relation we were able to measure the HFCCs in various short-lived radicals of the amino acids histidine and tryptophan and of the S-N-centered cyclic radical of methionine derived from the methionine-glycine dipeptide in aqueous solution.     

Violating the Aufbau principle: an efficient way to improve the stability of luminescent radicals

<正>The Aufbau principle,also known as"building-up"principle,is a method of explaining the arrangements of electrons within atoms of different chemical elements.The principle was initially introduced by Danish physicist Niels Bohr and the basic statements include"Electrons will fill orbitals in increasing order of energy,and each orbital can contain a maximum of two electrons,and these must have opposite     

Mechanistic insight into the cleavage of an aromatic C-C bond by tungsten

The pathway for the cleavage of an aromatic C-C bond in quinoxaline by a tungsten(II) complex [W(PMe(3))(4)(η(2)-CH(2)PMe(2))H] is explored by performing detailed DFT calculations. The real active complex was found to be [W(PMe(3))(2)(η(2)-CH(2)PMe(2))H] rather than [W(PMe(3))(4)]. The key step in the whole reaction is the reductive elimination of two hydrides that are located originally on quinoxaline (see scheme).     

Recovery by means of electrodialysis of an aromatic amino acid from a solution with a high concentration of sulphates and phosphates

In the industrial synthesis of -α-p-hydroxyphenylglycine the separation of the amino acid is carried out by precipitation. During this process, a mother liquor is produced with a high salt content (phosphates and sulphates) and an amino acid concentration of approximately 0.12–0.15 M. The disposal of this mother liquor not only causes an environmental problem due its high salinity and COD (chemical oxygen demand) content but also an economic loss due to the high price of the unrecovered amino acid. To avoid this problem an electrodialytic process has been developed that allows the recovery of 85% of the amino acid in the form of a low salinity stream with a salt content 70% lower than that of the initial mother liquor. This low salinity stream can be incorporated into the main process and in this way the amino acid can be recovered.     

Intramolecular homolytic substitution at the sulfur atom: an alternative way to generate phosphorus- and sulfur-centered radicals

Two efficient procedures involving tin hydride or thiophenol-mediated intramolecular homolytic substitution at the sulfur atom are reported. They lead to the generation of varied P(V)-centered radicals from the corresponding aryl or alkyne thiophosphorus substrates. The radical formed can be trapped by an olefin via an intermolecular addition, leading to the construction of C-P bonds. Thiophosphination of triple bonds was also achieved using a radical cycloisomerization process. Extension of the methodology to sulfur-containing species was examined.     

Scavenging with TEMPO* to identify peptide- and protein-based radicals by mass spectrometry: advantages of spin scavenging over spin trapping

The detection and characterization of radicals in biomolecules are challenging due to their high reactivity and low concentration. Mass spectrometry (MS) provides a tool for the unambiguous identification of protein-based radicals by exploiting their reactivity with suitable reagents. To date, protein-radical detection by MS has been modeled after electron paramagnetic resonance experiments, in which diamagnetic spin traps, such as 3,5-dibromo-4-nitrosobenzene sulfonic acid, convert unstable radicals to more stable spin adducts. Since MS detects mass changes, and not unpaired spins, conversion of radicals to stable diamagnetic adducts is more desirable. The use of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO(*)) in the MS identification of protein-based radicals was explored here to establish whether scavenging via radical combination would give rise to TEMPO adducts that were stable to MS analysis. The horseradish peroxidase/H(2)O(2) reaction was used to generate radicals in derivatives of tyrosine, tryptophan, and phenylalanine as models of protein-based radicals. TEMPO(*) was added as a radical scavenger, and the products were analyzed by electrospray ionization (ESI) MS. Dramatically higher mass-adduct yields were obtained using radical scavenging vs radical trapping, which greatly enhanced the sensitivity of radical detection. The efficiency of TEMPO(*) in protein radical scavenging was examined in horse heart myoglobin and cytochrome c peroxidase (CCP) from Saccharomyces cerevisiae. On H(2)O(2) binding to their ferric hemes, two oxidizing equivalents are transferred to the proteins as an Fe(IV)=O species and a polypeptide-based radical. In addition, CCP has been shown to reduce up to 10 equiv of H(2)O(2) using endogenous donors, thereby generating as many as 20 radicals on its polypeptide. Following myoglobin and CCP incubation with a 10-fold molar excess of H(2)O(2) and TEMPO(*), matrix-assisted laser desorption ionization (MALDI) time-of-flight analysis of the tryptic peptides derived from the proteins revealed 1 and 9 TEMPO adducts of myoglobin and CCP, respectively. Given the high scavenging efficiency of TEMPO(*) and the stability of TEMPO-labeled peptides in ESI and MALDI sources, scavenging by stable nitroxide radicals coupled with MS analysis should provide sensitive and powerful technology for the characterization of protein-based radicals.     

Addition of carbon-centered radicals to C60. Determination of the rate constants by the spin trap method

The rate constants of addition of the^.CMe₃,^.CH₂Me,^.CH₂(CH₂)₃Me,^.CH₂Ph,^.CH₂CH=CH₂, and^.CH(Me)Et radicals to fullerene C₆₀ were determined by the method of competitive addition of free radicals to spin traps. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp 2369–2372, December, 1999.     

Paving the way to conformationally unravel complex glycopeptide antibiotics by means of Raman optical activity

It is crucial for fundamental physical chemistry techniques to find their application in tackling real-world challenges. Hitherto, Raman optical activity (ROA) spectroscopy is one of the examples where a promising future within the pharmaceutical sector is foreseen, but has not yet been established. Namely, the technique is believed to be able to contribute in investigating the conformational behaviour of drug candidates. We, herein, strive towards the alignment of the ROA analysis outcome and the pharmaceutical expectations by proposing a fresh strategy that ensures a more complete, reliable, and transferable ROA study. The strategy consists of the treatment of the conformational space by means of a principal component analysis (PCA) and a clustering algorithm, succeeded by a thorough ROA spectral analysis and a novel way of estimating the contributions of the different chemical fragments to the total ROA spectral intensities. Here, vancomycin, an antibiotic glycopeptide, has been treated; it is the first antibiotic glycopeptide studied by means of ROA and is a challenging compound in ROA terms. By applying our approach we discover that ROA is capable of independently identifying the correct conformation of vancomycin in aqueous solution. In addition, we have a clear idea of what ROA can and cannot tell us regarding glycopeptides. Finally, the glycopeptide class turns out to be a spectroscopically curious case, as its spectral responses are unlike the typical ROA spectral responses of peptides and carbohydrates. This preludes future ROA studies of this intriguing molecular class.

Raman optical activity tackles the complex conformational space of glycopeptide antibiotics.     

The desalination of a mixed solution of an amino acid and an inorganic salt by means of electrodialysis with charge-mosaic membranes

A new electrodialysis with charge-mosaic membranes was proposed to achieve efficient desalination of a mixed solution of an amino acid and an inorganic salt. For such a mixed solution, the conventional electrodialytic desalination with both cation-and anion-exchange membranes had resulted in a considerable loss of the amino acid through the membranes. In this method, however, the amino acid in the desalination channel of the electrodialyzer migrates away from the membranes so that the permeation loss of the amino acid through the membrane can be prevented.

Batchwise desalination experiments by this method were carried out with a glutamic acid or arginine solution including NaCl under the condition of constant electric current density. Similar experiments by the conventional method were also carried out. As a result of comparing both methods, the amino acid loss in this method became much smaller than that in the conventional one. It was confirmed that this method was very useful for the desalination of an amino acid solution. The effects of operating conditions on the desalination process are also discussed.     

Transition-metal-mediated reduction and reversible double-cyclization of cyanuric triazide to an asymmetric bitetrazolate involving cleavage of the six-membered aromatic ring

Cyanuric triazide reacts with several transition metal precursors, extruding one equivalent of N₂ and reducing the putative diazidotriazeneylnitrene species by two electrons, which rearranges to N-(1′H-[1,5′-bitetrazol]-5-yl)methanediiminate (biTzI²⁻) dianionic ligand, which ligates the metal and dimerizes, and is isolated from pyridine as [M(biTzI)]₂Py₆ (M = Mn, Fe, Zn, Cu, Ni). Reagent scope, product analysis, and quantum chemical calculations were combined to elucidate the mechanism of formation as a two-electron reduction preceding ligand rearrangement.

Cyanuric triazide reacts with transition metal precursors, extruding N₂ and reducing the ligand by two electrons, which breaks an aromatic ring and rearranges to a bitetrazolylmethanediiminate (biTzI²⁻) ligand, forming two new aromatic rings.     

Catalysis by amino acid-derived tetracoordinate complexes: enantioselective addition of dialkylzincs to aliphatic and aromatic aldehydes

Me(2)Zn and Et(2)Zn added to aromatic and aliphatic aldehydes in the presence of 3 mol % of 2. (S)-1-Phenylethanol (91% ee) and (S)-1-phenylpropanol (86% ee) were synthesized from benzaldehyde and (S)-1-furan-2-yl-1-propanol (86% ee) from 2-furaldehyde. Nonanal and 3-phenylpropanal provided (S)-3-undecanol (96% ee) and (S)-1-phenyl-3-pentanol (94% ee). A solid-phase variant was effective with reduced ee's (e.g., 86% ee --> 79% ee) for (S)-1-phenylpropanol.     

Cyclopolycondensations. VII. Preparation of aromatic poly(ureido acids) by the low-temperature solution polymerization and cyclodehydration to fully aromatic polyquinazolinediones

Fully aromatic polyquinazolinediones (IV) of high molecular weight were obtained by thermal cyclodehydration of aromatic poly(uredio acids) (III) prepared by the polyaddition reaction of 4,4′-diaminobiphenyl-3,3′-dicarboxylic acid (I) with aromatic diisocyanates (II). From the kinetic study of reactions of model systems (anthranilic acid with phenyl isocyanate) in the presence of a variety of basic catalysts, it was established that tertiary amines had the highest catalytic activity for the formation of ureido linkage. The optimum polymerization conditions were determined by the study of reaction variables such as monomer concentration, polymerization temperature, monomer ratio, and catalyst concentration. The effect of polarity and purity of organic solvents and reactants was also studied.     

Electron paramagnetic resonance spin trapping of glutathiyl radicals by PBN in the presence of cyclodextrins and by PBN attached to beta-cyclodextrin

The reaction of the glutathiyl radical (GS*) with a widely used spin trap N- tert-butyl-alpha-phenylnitrone (PBN) has been studied in the presence of various methylated beta-cyclodextrins and with PBN covalently bound to dimethylated beta-cyclodextrin (PBN-DIMEB) and permethylated beta-cyclodextrin (PBN-TRIMEB). Scavenging rate constants for GS* by PBN were obtained in the presence of randomly methylated cyclodextrin (RAMEB) and PBN-TRIMEB and found to be close to the rate constant previously measured for PBN. RAMEB and 2,6-di- O-Me-beta-cyclodextrin (DIMEB) were found to be the most efficient in the increasing PBN/GS* lifetime, by a factor of 5.5 for RAMEB and 6.8 for DIMEB compared with the lifetime of PBN/GS*. It is concluded that the presence or "attachment of" beta-cyclodextrins does not influence the scavenging rate constant of GS* but it does lead to stabilization of the spin adducts formed.     

The ionic liquid [bmim]Br as an alternative medium for the catalytic cleavage of aromatic C-F and C-Cl bonds

The potential of [bmim]Br as an alternative to aprotic dipolar solvents in nickel-catalyzed hydrodehalogenation reactions is demonstrated. Hydrodechlorination of pentafluorochlorobenzene proceeds under the action of zinc in aqueous [bmim]Br. Under the above conditions aromatic C-F bonds also undergo slow cleavage. The reaction is significantly accelerated in the presence of nickel complexes with 2,2′-bipyridine or 1,10-phenanthroline. In the case of pentafluoroacetanilide highly regioselective ortho-hydrodefluorination leading to the formation of 3,4,5-trifluoroacetanilide is observed.     

Magnetic resonance parameters and structure of substituted stannyloxyphenoxyl radicals: Investigation by means of ESR in the 2-mm band

Institute of Chemical Physics, Russian Academy of Sciences. Institute of Heteroorganic Compounds, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 5, pp. 69–77, September–October, 1992.