Molecular dynamics of hydrogen bonds in protein-D2O: the solvent isotope effect

We suggest that the H-bond in proteins not only mirrors the motion of hydrogen in its own atomistic setting but also finds its origin in the collective environment of the hydrogen bond in a global lattice of surrounding H2O molecules. This water lattice is being perturbed in its optimal entropic configuration by the motion of the H-bond. Furthermore, bonding interaction with the lattice drop the H-bond energy from some 5 kcal/mol for the pure protein in the absence of H2O, to some 1.6 kcal/mol in the presence of the H2O medium. This low value here is determined in a computer experiment involving MD calculations and is a value close to the generally accepted value for biological systems. In accordance with these computer experiments under ambient conditions, the H-bond energy is seriously depressed, hence confirming the subtle effect of the H2O medium directly interacting with the H-bond and permitting a strong fluxional behavior. Furthermore, water produces a very large change in the entropy of activation due to the hydrogen bond breakage, which affects the rate by as much as 2 orders of magnitude. We also observe that there is an entire ensemble of H-bond structures, rather than a single transition state, all of which contribute to this H-bond. Here the model is tested by changing to D2O as the surrounding medium resulting in a substantial solvent isotope effect. This demonstrates the important influence of the environment on the individual hydrogen bond.     

Bioactive Components from the Mycelium of Antrodia Salmonea

Three new compounds, 2,4‐dimethoxy‐6‐methylbenzene‐1,3‐diol ( 1 ), salmoquinone ( 2 ), and 3‐(4‐hydroxyphenyl)‐4‐isobutyl‐1H‐pyrrole‐2,5‐dione ( 3 ), together with six known compounds, 2‐methoxy‐6‐methyl‐p‐benzoquinone ( 4 ), 2,3‐dimethoxy‐5‐methyl‐p‐benzoquinone ( 5 ), 2‐hydroxy‐5‐methoxy‐3‐methyl‐p‐benzoquinone ( 6 ), eburcoic acid ( 7 ), fomefficinic acid C ( 8 ), and a pyrroledione ( 9 ), were isolated from the mycelium of Antrodia salmonea. The structures of these compounds were elucidated by spectrometric analyses including IR, NMR, and MS. Among these compounds, 4 and 5 exhibited cytotoxicity against KB, HepG2, and H2058 cell lines.     

Amberlyst‐15‐Catalyzed Novel Synthesis of Quinoline Derivatives in Ionic Liquid

In recent years, ionic liquids have attracted much attention as useful synthetic solvents. Compared with classical molecular solvents, the ionic liquids are environmentally benign reaction media. A variety of quinoline derivatives have been synthesized under ionic liquid conditions using Amberlyst‐15 as catalyst.     

Briarenolides A-C, briarane diterpenoids from the gorgonian coral Briareum sp.

Chemical investigation on the gorgonian coral Briareum sp. has led to the isolation of six oxygenated briaran diterpenes 1-6, including three new compounds briarenolides A-C (1-3). The structures of 1-3 were determined on the basis of extensive spectroscopic analysis and by comparison of their spectral data with those of related metabolites. Among these metabolites, 1 and 2 are rarely found 9-ketobriaranes. Also, 1 is the first briarane derivative possessing a 20-hydroxy group.     

Xeniaphyllane-derived terpenoids from the formosan soft coral Sinularia gibberosa

New xeniaphyllane-derived metabolites (1-7) were isolated from the EtOAc extract of the Formosan soft coral Sinularia gibberosa. The structures and relative configurations of these compounds were elucidated on the basis of extensive spectroscopic analysis (including 2D NMR) and by comparison of their spectral data with those of related compounds. In vitro cytotoxic evaluation of the above metabolites towards a limited panel of cancer cell lines is also described.     

Theoretical studies of the [2 + 4] Diels-Alder cycloaddition reactions of alkene analogues of the group 13 elements with toluene

The potential energy surfaces for the cycloaddition reactions of formally double-bonded molecules containing group 13 elements have been studied using density functional theory (B3LYP/LANL2DZ). Five group 13 alkene analogues, ArX=XAr, where X = B, Al, Ga, In, and Tl, have been chosen as model reactants in this work. Our present theoretical work predicts that the smaller the singlet-triplet splitting in ArX=XAr, the lower the activation barrier and, in turn, the more rapid are its [4 + 2] cycloaddition reactions. Moreover, the theoretical investigations suggest that the relative dimeric reactivity decreases in the order B > Al > Ga > In > Tl. That is, the heavier the group 13 atom (X), the more stable is its dimetallene toward chemical reactions. In consequence, our results predict that the dimetallenes containing heavier group 13 elements (in particular, X = Ga, In, and Tl) should be stable and should be readily synthesized and isolated at room temperature. This is in good agreement with available experimental observations. Besides this, the singlet-triplet energy splitting of a dimetallene, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict its reactivity. The results obtained allow a number of predictions to be made.     

Charge‐transfer‐to‐solvent absorption spectra of I−(H2O)3–5 at a finite temperature via simulation

Ground‐state equilibrium Born–Oppenheimer molecular dynamics on I^?(H₂O)_3–5 clusters at ～200 K are performed to sample configurations for calculating the charge‐transfer‐to‐solvent (CTTS) absorption spectra for these clusters. When there are more water molecules in clusters, the calculated CTTS spectra are found to become more intense with the absorption maxima shifting to higher energies, which is in agreement with experimental results. In addition, compared with the findings for optimized structures, the absorption energies of the iodide 5p orbitals are red‐shifted at ～200 K because, on average, the distances between the iodide and the dangling hydrogen atoms are increased at finite temperatures which weakens the interactions between the iodide and water molecules in the clusters. Moreover, the number of ionic hydrogen bonds in the clusters are also reduced. However, it is found that all dangling hydrogen atoms must be considered to obtain a good correlation between the CTTS excitation energy and the average distance between the iodide and the dangling hydrogen atoms, which indicates the existence of the strong interactions of the CTTS electron with all of the dangling hydrogen atoms.     

Onset of Ferromagnetism in Low-Doped Ga1-xMnxAs

We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of Ga1-xMnxAs. We compare it with measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originate from Mn disorder and As antisite defects. The experimentally determined hopping energy is an excellent predictor of the Curie temperature, in agreement with the theory.     

Analytical models of lateral power devices with arbitrary vertical doping profiles in the drift region

By solving the 2D Poisson's equation, analytical models are proposed to calculate the surface potential and electric field distributions of lateral power devices with arbitrary vertical doping profiles. The vertical and the lateral breakdown voltages are formulized to quantify the breakdown characteristic in completely-depleted and partially-depleted cases. A new reduced surface field (RESURF) criterion which can be used in various drift doping profiles is further derived for obtaining the optimal trade-off between the breakdown voltage and the on-resistance. Based on these models and the numerical simulation, the electric field modulation mechanism and the breakdown characteristics of lateral power devices are investigated in detail for the uniform, linear, Gaussian, and some discrete doping profiles along the vertical direction in the drift region. Then, the mentioned vertical doping profiles of these devices with the same geometric parameters are optimized, and the results show that the optimal breakdown voltages and the effective drift doping concentrations of these devices are identical, which are equal to those of the uniform-doped device, respectively. The analytical results of these proposed models are in good agreement with the numerical results and the previous experimental results, confirming the validity of the models presented here.     

Potential energy surface of O2-(H2O) and factors controlling water-to-O2- binding motifs

The potential energy surface (PES) of O(2)(-)(H(2)O) is investigated by varying the interoxygen distance of O(2)(-) via ab initio calculations with a large basis set. Although two stationary points, C(s) and C(2v) conformers, are found along the interoxygen-distance coordinate, only the C(s) conformer is identified as a minimum-energy species. We find a critical distance, r(c), separating these two conformers in the PES. The C(s) conformer prevails at interoxygen distances of O(2)(-) that are less than r(c), while the C(2v) conformer dominates at the distances larger than r(c). The structural features of these two conformers are also discussed. Although the water deformation energy is shown to be the stabilization source responsible for the prevalence of the C(s) cluster conformer at the interoxygen distances of O(2)(-) less than r(c), the ionic hydrogen bonding is the major driving force for transformation of the water binding motif from C(s) to C(2v) when the interoxygen distance of O(2)(-) increases.