Measuring fast hydrogen exchange rates by NMR spectroscopy

We introduce a method to measure hydrogen exchange rates based on the observation of the coherence of a neighboring spin S such as (15)N that has a scalar coupling J(IS) to the exchanging proton I. The decay of S(x) coherence under a Carr-Purcell-Meiboom-Gill (CPMG) multiple echo train is recorded in the presence and absence of proton decoupling. This method allows one to extract proton exchange rates up to 10(5)s(-1). We could extend the pH range for the study of the indole proton in tryptophan, allowing the determination of the exchange constants of the cationic, zwitterionic, and anionic forms of tryptophan.     

A law of the iterated logarithm for the product limit estimator with doubly censored data

We establish the law of the iterated logarithm for the product limit estimator, when the data are subject to double censoring. This investigation extends the results available for the model for singly censored data.     

Indirect controllability of locally coupled systems under geometric conditions

We consider systems of two wave/heat/Schrödinger-type equations coupled by a zero order term, only one of them being controlled. We prove an internal and a boundary null-controllability result in any space dimension, provided that both the coupling and the control regions satisfy the Geometric Control Condition. This includes several examples in which these two regions have an empty intersection.     

Computational investigation of enol/keto chloramphenicol with β-cyclodextrin

PM3 and ONIOM2 were carried out to investigate the structures and properties for the inclusion complexes of chloramphenicol tautomers into β-cyclodextrin (at 1:1 stoichiometry). Two possible orientations into host cavity were considered for both enol and keto chloramphenicol. The PM3 results gives that B orientation is more preferred for enol and keto form, the preference is of 0.63 and 1.67 kcal/mol respectively. This preference is greater in the case of ONIOM2 calculations. Finally, the chemical shifts (ppm) of free and complexed chloramphenicol were calculated at B3LYP/6-31G(d) by (GIAO method) and compared with experimental data taken from the literature.     

On the Configurational Stability of Chiral,Nonracemic Fluoro‐ and Iodo‐[D1]Methyllithiums

Enantiomerically pure fluoro‐[D₁]methyllithium and iodo‐[D₁]methyllithiums of up to 92 % ee were generated by transmetalation of the corresponding stannanes with MeLi in THF at various temperatures. The intermediate halo‐[D₁]methyllithiums were trapped with benzaldehyde or acetophenone already present in excess in the reaction mixture to either give halohydrins or to disintegrate to carbene. The fluoro‐[D₁]methyllithiums were found to be microscopically configurationally stable within the tested range of ?95 to 0 °C, but chemically only stable at temperatures below ?95 °C due to a rapidly increasing portion disintegrating to carbene. The iodo‐[D₁]methyllithiums were configurationally labile relative to the rate of addition to PhCHO at all temperatures tested (?95 to ?30 °C). Disintegration to carbene interfered as well.     

Molecular docking study on β-cyclodextrin Interactions of metobromuron and [3-(p-bromophenyl)-1-methoxy-1-methylurea]

The inclusion process involving β-cyclodextrin (β-cyclodextrin-CD) and phenylurea herbicide metobromuron (MB) has been investigated by using the MM+, PM3, B3LYP, HF, ONIOM2 and NBO methods. The binding and complexation energies for both orientations considered in this research are reported. The geometry of the most stable complex shows that the aromatic ring is deeply self-included inside the hydrophobic cavity of β-CD also an intermolecular hydrogen bond is established between host and guest molecules. This suggests that hydrophobic effect and hydrogen bond play an important role in the complexation process. The statistical thermodynamic calculations by PM3 demonstrate that 1:1?MB/β-CD complex is favored by a negative enthalpy change. Moreover, NBO calculations proved also that are a very useful means to quantify the interaction energies of the hydrogen bonds.     

Solvent effects in the geometric reorganization of an oxo-molybdenum(V) system

We have previously postulated a serine gated electron transfer hypothesis (Inorg. Chem, 2002, 41, 1281-1291) to possibly be involved in gating electron transfer between the Mo(V) and Mo(IV) states. In this study we explored the effect of solvent dielectric upon the rate and mechanism of isomerization of an oxo-Mo(V) core in attempt to understand the effect of solvent polarity to the isomerization reaction. To this end, the data suggests that there may be significant entropic contributions to the reorganization of metal center as a function of the local dielectric constant. Furthermore, we note that there is a change in the observed rate as well as the mechanism of the geometric rearrangement when it is examined in polar and non-polar environments. More specifically, in low dielectric media, the reaction proceeds either via a fast dissociation which is then followed by a twist mechanism or by a dissociation that is synchronized with the twist mechanism.     

Quantum Secret Sharing with Error Correction

We investigate in this work a quantum error correction on a five-qubits graph state used for secret sharing through five noisy channels. We describe the procedure for the five, seven and nine qubits codes. It is known that the three codes always allow error recovery if only one among the sent qubits is disturbed in the transmitting channel. However, if two qubits and more are disturbed, then the correction will depend on the used code. We compare in this paper the three codes by computing the average fidelity between the sent secret and that measured by the receivers. We will treat the case where, at most, two qubits are affected in each one of five depolarizing channels.     

Synthesis, characterization, electrochemistry, electronic structure, and isomerization of mononuclear oxo-molybdenum(V) complexes: the serine gate hypothesis in the function of DMSO reductases

Crystal structures of DMSO reductases isolated from two different sources and the crystal structure of related trimethylamine-N-oxide reductase indicate that the angle between the terminal oxo atom on the molybdenum and the serinato oxygen varies significantly. To understand the significance of this angular variation, we have synthesized two isomeric compounds of the heteroscorpionato ligand (L1OH) (cis- and trans-(L1O)Mo(V)OCl(2)), where the phenolic oxygen mimics the serinato oxygen donor. Density functional and semiempirical calculations indicate that the trans isomer is more stable than the cis. The lower stability of the cis isomer can be attributed to two factors. First, a strong antibonding interaction between the phenolic oxygen with molybdenum d(xy) orbital raises the energy of this orbital. Second, the strong trans influence of the terminal oxo group in the trans isomer places the phenol ring, and hence the bulky tertiary butyl group, in a less sterically hindered position. In solution, the cis isomer spontaneously converts to the thermodynamically favorable trans isomer. This geometric transformation follows a first-order process, with an enthalpy of activation of 20 kcal/mol and an entropy of activation of -9 cal/mol K. Computational analysis at the semiempirical level supports a twist mechanism as the most favorable pathway for the geometric transformation. The twist mechanism is further supported by detailed mass spectral data collected in the presence of excess tetraalkylammonium salts. Both the cis and trans isomers exhibit well-defined one-electron couples due to the reduction of molybdenum(V) to molybdenum(IV), with the cis isomer being more difficult to reduce. Both isomers also exhibit oxidative couples because of the oxidation of molybdenum(V) to molybdenum(VI), with the cis isomer being easier to oxidize. This electrochemical behavior is consistent with a higher-energy redox orbital in the cis isomer, which has been observed computationally. Collectively, this investigation demonstrates that by changing the O(t)-Mo-O(p) angle, the reduction potential can be modulated. This geometrically controlled modulation may play a gating role in the electron-transfer process during the regeneration steps in the catalytic cycle.