Methyl 3-methyl-2H-azirine-2-carboxylate photochemistry studied by matrix-isolation FTIR and DFT calculations

The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/B3LYP/6-311++G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C-O bond but differing in the arrangement around the C-C(alpha) bond. The two conformers were photoreactive upon in situ broadband UV excitation (lambda > 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C-C or C-N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C-N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C-C bond breakage. This behavior is attributed to the presence in the molecule of the electron-withdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T(1) triplet state and, in this way, favors the C-N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed.     

Sensory hybrid host materials for the selective chromo-fluorogenic detection of biogenic amines

Pyrylium-containing mesoporous materials have been used for the chromo-fluorogenic sensing of biogenic amines in an aqueous environment.     

Coordination polymers undergoing spin crossover and reversible ligand exchange in the solid

Here we report the synthesis and characterisation of a polymer made up of a system of parallel 2-D grids of Fe(II) ions linked by [Au(CN)2]- bridges and its transformation into a new system of three interpenetrated 3-D coordination open frameworks with the NbO topology. Reversibility of this crystal-to-crystal transformation is evidenced by X-ray crystallographic data and from their spin crossover properties.     

Deprotonated cytosine anions: a theoretical prediction of photoelectron spectra

Predictions on the photoelectron spectra of deprotonated cytosine anions (cytosinate, Cye(-)) have been made with ab initio electron propagator methods. Two imino-oxo forms are most stable, but four other isomers have energies within 10 kcal/mol. The first vertical electron detachment energies (VEDEs) for the three most stable Cye(-) isomers are approximately 3.4 eV. Imino-oxy VEDEs are about 0.3 eV smaller. For each anion, the lowest VEDE corresponds to a pi Dyson orbital. The order of higher final states is changed when relaxation and correlation effects are considered. Considerable mixing between lone-pair and bonding lobes occurs in the sigma Dyson orbitals.     

Unusual photochemical C-N bond cleavage in the novel methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate

The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions.     

Synthesis of new chiral amines with a cyclic 1,2-diacetal skeleton derived from (2R, 3R)-(+)-tartaric acid

The syntheses of new chiral cyclic 1,2-diacetals from (2R, 3R)-( )-tartaric acid are described. C(2)-symmetrical diamines were prepared via direct amidation of the tartrate or from the corresponding bismesylate via reaction with sodium azide. For C1-symmetrical compounds, the Appel reaction was used to form the key intermediate, a monochlorocarbinol, from the diol. Some of the new chiral compounds, produced in good to high yields, may be potentially useful as asymmetric organocatalysts or as nitrogen and sulfur chelating ligands for asymmetric metal catalyzed reactions. Thus, a bis-N-methyl-methanamine derivative, used in substoichiometric amounts, was found to catalyze the enantioselective addition of cyclohexanone to (E)-beta-nitrostyrene with high diastereoselectivity (syn / anti = 92:8), albeit giving moderate optical purity (syn: 30 %).     

Coherence and wavefront characterization of Si‐111 monochromators using double‐grating interferometry

A study of the coherence and wavefront properties of a pseudo‐channel‐cut monochromator in comparison with a double‐crystal monochromator is presented. Using a double‐grating interferometer designed for the hard X‐ray regime, the complex coherence factor was measured and the wavefront distortions at the sample position were analyzed. A transverse coherence length was found in the vertical direction that was a factor of two larger for the channel‐cut monochromator owing to its higher mechanical stability. The wavefront distortions after different optical elements in the beam, such as monochromators and mirrors, were also quantified. This work is particularly relevant for coherent diffraction imaging experiments with synchrotron sources.     

Spin Label Studies of the Hemoglobin–Membrane Interaction During Sickle Hemoglobin Polymerization

An enhanced hemoglobin–membrane association has been previously documented in sickle cell anemia. However, it is not known how this interaction is modified during the hemoglobin S polymerization process. In this work, we use a model of reconstituted erythrocytes from ghost membranes whose cytoskeleton proteins had been previously labeled with the 4-maleimido Tempo spin label, and that were subsequently resealed with hemoglobin S or A solutions. Using electron paramagnetic resonance spectroscopy, we studied the time dependence of the spectral W/S parameter, indicative of the conformational state of cytoskeleton proteins (mainly spectrin) under spontaneous deoxygenation, with the aim of detecting the eventual effects due to hemoglobin S polymerization. The differences observed in the temporal behavior of W/S in erythrocytes reconstituted with both hemoglobins were considered as experimental evidence of an increment in hemoglobin S–membrane interaction as a result of the polymerization process of hemoglobin S under spontaneous deoxygenation.