Imidazole-imidazole hydrogen bonding in the pH sensing Histidine sidechains of Influenza A M2

ABSTRACTThe arrangement of histidine sidechains in influenza A M2 tetramer determines their pKa values, which define pH controlled proton conduction critical to the virus lifecycle. Both water associated and hydrogen bonded Imidazole–Imidazolium histidine quaternary structures have been proposed, based on crystal structures, and NMR chemical shifts, respectively. Here we show, using the conduction domain construct of M2 in lipid bilayers, that the imidazole rings are hydrogen bonded even at a pH of 7.8 in the neutral charge state.An intermolecular 8.9 ± 0.3 Hz 2hJNN hydrogen bond is observed between H37 Nε and Nδ recorded in a fully protonated sample with 100 kHz magic-angle spinning. This interaction could not be detected in the drug-bound sample.

The fine structure of the neutral nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is a widely utilized system due to its useful quantum properties. Almost all research focuses on the negative charge state (NV−) and comparatively little is understood about the neutral charge state (NV0). This is surprising as the charge state often fluctuates between NV0 and NV− during measurements. There are potentially under-utilized technical applications that could take advantage of NV0, either by improving the performance of NV0 or utilizing NV− directly. However, the fine structure of NV0 has not been observed. Here, we rectify this lack of knowledge by performing magnetic circular dichroism measurements that quantitatively determine the fine structure of NV0. The observed behavior is accurately described by spin-Hamiltonians in the ground and excited states with the ground state yielding a spin-orbit coupling of λ = 2.24 ± 0.05 GHz and a orbital g-factor of 0.0186 ± 0.0005. The reasons why this fine structure has not been previously measured are discussed and strain-broadening is concluded to be the likely reason.

Effect of the Fermi level position on the annealing characteristic of interstitial carbon defect in silicon

We present experimental results showing that the migration ability of interstitial carbon atom (Сi) in silicon depends noticeably on its charge state. The experimental results were obtained from the analysis of deep level transient spectra in n+–p diodes subjected to irradiation with 4–6 MeV electrons or α-particles at T < 273 k and subsequent heat-treatments in the temperature range 280–330 k under reverse bias and without it. It has been found that in the positive charge state the Сi migration energy is 0.88 ± 0.02 eV, while in the neutral charge state it is lowered down to 0.73–0.74 eV.

On concentration dependence of arsenic diffusivity in silicon

An analysis of the equations used for modeling thermal arsenic diffusion in silicon has been carried out. It was shown that for arsenic diffusion governed by the vacancy-impurity pairs and the pairs formed due to interaction of impurity atoms with silicon self-interstitials in a neutral charge state, the doping process can be described by the Fick’s second law equation with a single effective diffusion coefficient which takes into account two impurity flows arising due to interaction of arsenic atoms with vacancies and silicon self-interstitials, respectively. Arsenic concentration profiles calculated with the use of the effective diffusivity agree well with experimental data if the maximal impurity concentration is near the intrinsic carrier concentration. On the other hand, for higher impurity concentrations a certain deviation in the local regions of arsenic distribution is observed. The difference from the experiment can occur due to the incorrect use of effective diffusivity for the description of two different impurity flows or due to the formation of nonuniform distributions of neutral vacancies and neutral self-interstitials in heavily doped silicon layers. We also suppose that the migration of nonequilibrium arsenic interstitial atoms makes a significant contribution to the formation of a low concentration region on thermal arsenic diffusion.

Electron Paramagnetic Resonance Studies of Nb in 6H-SiC

Defects in unintentionally Nb-doped 6H-SiC grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). An EPR spectrum with a hyperfine (hf) structure consisting of ten equal-intensity lines was observed. The hf structure is identified to be due to the hf interaction between an electron spin S=1/2 and a nuclear spin of 93Nb. The hf interaction due to the interaction three nearest Si neighbors was also observed, suggesting the involvement of the C vacancy (VC) in the defect. Only one EPR spectrum was observed in 6H-SiC polytype. The obtained spin-Hamiltonian parameters are similar to that of the Nb-related EPR defect in 4H-SiC, suggesting that the EPR center in 6H-SiC is the NbSiVC complex in the neutral charge state, NbSiVC0. Photoexcitation EPR experiments suggest that the single negative charge state of the NbSiVC complex is located at ~1.3 eV below the conduction band minimum.

Identification of Niobium in 4H-SiC by EPR and Ab Initio Studies

In unintentionally Nb-doped 4H-SiC grown by high-temperature chemical vapor deposition (HTCVD), an electron paramagnetic resonance (EPR) center with C1h symmetry and an electron spin S=1/2 was observed. The spectrum shows a hyperfine structure consisting of ten equal-intensity hyperfine (hf) lines which is identified as due to the hf interaction between the electron spin and the nuclear spin of 93Nb. An additional hf structure due to the interaction with two equivalent Si neighbors was also observed. Ab initio supercell calculations of Nb in 4H-SiC suggest that Nb may form complex with a C-vacancy (VC) resulting in an asymmetric split-vacancy (ASV) defect, NbSi-VC. Combining results from EPR and supercell calculations, we assign the observed Nb-related EPR center to the hexagonal-hexagonal configuration of the AVS defect in the neutral charge state, (NbSi-VC)0.

First-principles Study of Nitrogen Vacancies in GdN

ABSTRACTThe electronic structure of nitrogen vacancies in gadolinium nitride are studied using the full-potential linearized muffin-tin orbital method in the local spin density approximation with Hubbard U corrections (LSDA+U). The vacancy is found to have two localized defect levels in the gap, one of each spin. The third electron of each vacancy in the neutral state dopes the conduction band. The single positive state is found to be the ground state for Fermi levels located anywhere within the band gap. The vacancy has a net magnetic moment of 1 μB in the neutral charge state. The presence of the vacancy is found to increase the average exchange interactions between Gd atoms and hence the Curie temperature but only by about a factor 2 compared to GdN without vacancies.

Ab-Initio Modeling of Arsenic Pile-Up and Deactivation at the Si/SiO2 Interface

ABSTRACTTwo recent papers by Pei et al. and Steen et al. have shown that the observed pile-up of arsenic at Si/SiO2 interfaces surprisingly does not seem to involve point defects as a major factor, causes local distortions that strain the Si in the pile-up region locally, and that the segregated arsenic atoms are deep donors. In this paper, we use ab-initio modeling to study possible configurations for high As concentrations that may fulfill these criteria. We find for a simple model structure that As nearest neighbors become stable in Si in the vicinity of the interface. We also have stud-ied dopant deactivation using bulk-Si models. Even without invoking point defects explicitly and starting from a purely substitutional arrangement, we find that the energetically most favorable configurations are most stable in the neutral charge state, indicating that high enough concentra-tions of arsenic atoms make them electrically inactive and hence result in dopant dose loss.

Manganese Impurity in Boron Nitride and Gallium Nitride

We carried out a theoretical investigation on the properties of manganese impurity centers in cubic boron and gallium nitrides. The calculations were performed using the all electron spin-polarized full-potential linearized augmented plane wave methodology. Our results indicate that manganese in boron nitride, in a neutral charge state, is energetically more favorable in a divacancy site as compared to a substitutional cation site. We present the results on stability, spin states, impurity magnetic moment, hyperfine parameters, and formation and transition energies of manganese at the divacancy site in several charge states.