Atomic charges in molecules defined by molecular real space partition into atomic subspaces.

Atomic charge (AC), which is the charge distribution of a molecule, is an important property that is closely associated with structures, reactivities, and intra- and inter-molecular interactions among molecules. Several theoretical models or methods can be used to obtain the magnitudes of AC with different characteristics. These models can be classified into fuzzy-atoms models and models partitioning a molecule into individual atoms with sharp boundaries. The first category includes Mulliken, natural population analysis (NPA), Hirshfeld, Merz-Kollman-Singh (MK), CHELPG, the electronegativity equalization method (EEM), the atom-bond electronegativity equalization method (ABEEM), and atomic polar tensor (APT). The second category is derived from quantum chemical topology (QCT) and includes the quantum theory of atoms in molecules (QTAIM) and QCT analysis based on the potential acting on one electron in a molecule (PAEMQCT). Herein, after giving a bird's-eye view of the population methods of the first category, we specifically describe some features of the second category. We only present the basic framework of QCT for obtaining ACs from QTAIM and PAEMQCT and show their important characteristics. QCT establishes the basis of the following chemical concept: a molecule is spatially partitioned into individual atoms with sharp boundaries. The ACs from QTAIM are close to the atomic valence in chemistry, and ACs from PAEMQCT may be practically suitable for modeling intra- and inter-molecular interactions.

Development of a QM/MM(ABEEM) method combined with a polarizable force field to investigate the excision reaction mechanism of damaged thymine.

This paper focuses on the development of a quantum mechanics/molecular mechanics method using the ABEEM polarizable force field (QM/MM(ABEEM) method) to investigate the excision reaction mechanism of damaged thymine. This method does not simply combine the QM method with the polarizable force field. A valence electronegativity piecewise function with the distance between atoms as a variable is introduced to describe the atomic partial charges, and changes greatly during the reaction process. At the same time, the charge transfer effect is treated using the condition of local charge conservation. Compared with the traditional QM/MM method, the QM/MM(ABEEM) method can more accurately simulate the polarization effect and charge transfer effect in the reaction process. Focusing on the controversial problems of the excision of damaged bases, six reaction pathways were designed for monofunctional and difunctional deglycosylation of neutral bases and protonated bases. The results show that the QM/MM(ABEEM) method accurately simulates the polarization effect, charge transfer effect, activation energy and other properties of the reaction process. The process in which the active residue Asp activates the nucleophile H2O to attack the protonated base is the preferred path. The average activation energy and free activation energy of the protonated base are 7.00-14.00 kcal mol-1 lower than that of the neutral base. The study in this paper is helpful to understand the mechanism of repair enzymes in repairing bases.

Development of a QM/MM(ABEEM) method for the deprotonation of neutral and cation radicals in the G-tetrad and GGX(8-oxo-G) tetrad.

The rapid deprotonation of G˙+ in the DNA strand impedes positive charge (hole) transfer, whereas the slow deprotonation rate of G˙+ in the G-tetrad makes it a more suitable carrier for hole conduction. The QM/MM(ABEEM) combined method, which involves the integration of QM and the ABEEM polarizable force field (ABEEM PFF), was developed to investigate the deprotonation of neutral and cation free radicals in the G-tetrad and GGX(8-oxo-G) tetrad (xanthine and 8-oxoguanine dual substituted G-tetrad). By incorporating valence-state electronegativity piecewise functions χ*(r) and implementing charge local conservation conditions, QM/MM(ABEEM) possesses the advantage of accurately simulating charge transfer and polarization effect during deprotonation. The activation energy calculated by the QM method of X˙ is the lowest among other bases in the GGX(8-oxo-G) tetrad, which is supported by the computation of the average electronegativity calculated by ABEEM PFF. By utilizing QM/MM(ABEEM) with a two-way free energy perturbation method, the deprotonation activation energy of X˙ in the GGX(8-oxo-G) tetrad is determined to be 33.0 ± 2.1 kJ mol-1, while that of G˙+ in the G-tetrad is 20.7 ± 0.6 kJ mol-1, consistent with the experimental measurement of 20 ± 1.0 kJ mol-1. These results manifest that X˙ in the GGX(8-oxo-G) tetrad exhibits a slower deprotonation rate than G˙+ in the G-tetrad, suggesting that the GGX(8-oxo-G) tetrad may serve as a more favorable hole transport carrier. Furthermore, the unequal average electronegativities of bases in the GGX(8-oxo-G) tetrad impede the deprotonation rate. This study provides a potential foundation for investigating the microscopic mechanism of DNA electronic devices.

Development of QM/MM (ABEEM polarizable force field) method to simulate the excision reaction mechanism of damaged cytosine.

DNA damages are regarded as having harmful effects on cell. The base excision repair mechanism combats these effects by removing damaged bases. The deglycosylation mechanism of excising damaged bases by DNA glycosylase and the state of the leaving base have been controversial. The enzymatic reaction of DNA glycosylase to remove the damaged bases involves not only the formation and breaking of chemical bonds, but also complex polarization effect and charge transfer, which cannot be accurately simulated by the QM/MM method combined with the fixed charge force field. This work has developed the ABEEM fluctuating polarizable force field combining with the QM method, that is (QM/MM[ABEEM]), to accurately simulate the proton transfer, charge transfer and the charge distribution. The piecewise function is used as the valence-state electronegativity in the QM/MM (ABEEM) to realize the accurate fitting of the charge distribution in reaction. And the charge transfer is accurately simulated by the local charge conservation conditions. Four deglycosylation mechanisms including the monofunctional and difunctional mechanisms of four neutral and protonated cytosine derivatives are explored. It is confirmed that the monofunctional mechanism of Asp-activated nucleophile water is a better deglycosylation mechanism and the base is protonated before the reaction occurs. Protonization of the base reduced the activation energy by 10.00-17.00 kcal/mol. Asp provides the necessary charge for the reaction, and DNA glycosylase preferentially cleaves ɛC. This work provides a theoretical basis for the research of excising damaged bases by DNA glycosylase.

Energetics and J-coupling constants for Ala, Gly, and Val peptides demonstrated using ABEEM polarizable force field in vacuo and an aqueous solution.

The development of an atom-bond electronegativity equalisation method at the σπ-level (ABEEM) polarisable force field (PFF) for peptides is presented. ABEEM PFF utilises a fluctuating charge model to explicitly describe the polarisation effects in an extensive environment. The partial charge of any individual site changes in response to changes in its surroundings. The peptide parameters are derived from ab initio methods in vacuum using a consistent and automatic protocol. By including explicit σ- and π-bond sites and lone pair sites, the anisotropy around an atom has been characterised. The fluctuating charge at each site ensures the distinction between the intrinsic behaviour observed among the various conformations of peptides, as corroborated by the agreement between quantum mechanics (QM) and ABEEM PFF concerning the calculated energy order, charge distribution, locations of minima, and potential energy surface (PES) in vacuo. The energy barriers in the PES have been clearly described using ABEEM PFF, in which a good charge distribution plays a vital role. Molecular dynamic simulations have been performed for short peptides in explicit ABEEM 7P-water boxes to examine their conformational properties in solution. The J-coupling constants obtained using ABEEM PFF are consistent with the experimental nuclear magnetic resonance (NMR) spectra and the influence of the chain length and temperature also investigated. The results demonstrate that the ABEEM PFF method is capable of locating conformations and describing the energetics of peptides with high accuracy and efficiency both in vacuo and an aqueous solution.

Systematic Parameterization and Simulation of Boronic Acid-ß-Lactamase Aqueous Solution in Developing the ABEEMσπ Polarizable Force Field.

Boronic acid, an inhibitor of ß-lactamase, has begun to be applied to the treatment of biological infections and tumors. Scientists are working to develop new and more effective boronic acid. Molecular dynamics (MD) simulation provides a powerful auxiliary tool for drug design. However, the current force fields have no boron-related parameters. In this work, an atom-bond electronegativity equalization method at the σπ level (ABEEMσπ) polarizable force field (ABEEMσπ PFF) of boronic acid and ß-lactamase has been developed to determine the potential functions and parameters. The interaction between boron and serine in ß-lactamase is regarded as a bonded mode. The interaction between them is simulated by the Morse potential energy function, which is close to the experimental change of the stretching potential energy in a large range. The potential energy surfaces of the bond length, bond angle, and dihedral angle of boronic acid-ß-lactamase have the same stability point and change trend as M06-2X/6-311G**. For 47 boronic acid-ß-lactamase training molecules, the linear correlation coefficient (R) of the charge distribution between the ABEEMσπ PFF and HF/STO-3G is greater than 0.96. Attributed to the fact that the charge distribution of the ABEEMσπ PFF can fluctuate with the change of geometry and environment, the polarization effect and charge-transfer effect are well reflected. The binding ability of different boronic acids with the same ß-lactamase is different. A total of 10 boronic acid-ß-lactamase model molecules and 10 boronic acid-ß-lactamase and water complexes are simulated. The order of binding energy of five large model molecules calculated by the ABEEMσπ PFF is consistent with that of the MP2 method. The binding energies of boronic acid-ß-lactamase and water complexes are close to those of the MP2 method. The results of MD simulation of five aqueous boronic acid-ß-lactamase complexes in the NVT ensemble verify the rationality of boron-related parameters of the ABEEMσπ PFF, which have a good application prospect. This study lays a solid theoretical foundation for further study of the inhibition of boronic acid on ß-lactamase.

Partitioning a Molecule into the Atomic Basins and the Resultant Atomic Charges from Quantum Chemical Topology Analysis of the Kohn-Sham Potential.

Quantum chemical topology (QCT) solidifies the chemical basic concepts demonstrating how a molecular system is intrinsically partitioned into its components and what the interaction lines between them are. Here, QCT analysis using a Kohn-Sham one-electron potential (KSpot) in KS equation as a scalar function is initiated and explored, showing KSpot and its resultant electron force lines have novel spatial features which reveal that an atom in a molecule is a spatial basin governed by its nucleus as a 3D-attractor that terminates all the electron force lines defined by the negative gradient of KSpot and that a chemical bond line is just a minimum path of KSpot for the electron motion. Particularly, the atomic charges from this KSpot QCT analysis are moderate and good, having much lower dependence on basis sets chosen for computation. This may provide a platform for the study of molecular structures and properties, intra- and intermolecular electrostatic interaction, energy decomposition, and construction of force field.

ABEEM/MM OH- Models for OH-(H2O)n Clusters and Aqueous OH-: Structures, Charge Distributions, and Binding Energies.

Based on the atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM), two fluctuating charge models of OH--water system were proposed. The difference between these two models is whether there is charge transfer between OH- and its first-shell water molecules. The structures, charge distributions, charge transfer, and binding energies of the OH-(H2O)n (n = 1-8, 10, 15, 23) clusters were studied by these two ABEEM/MM models, the OPLS/AA force field, the OPLS-SMOOTH/AA force field, and the QM methods. The results demonstrate that two ABEEM/MM models can search out all stable structures just as the QM methods, and the structures and charge distributions agree well with those from the QM calculations. The structures, the charge transfer, and the strength of hydrogen bonds in the first hydration shell are closely related to the coordination number of OH-. Molecular dynamics simulations on the aqueous OH- solution are performed at 298 and 278 K using ABEEM/MM-I model. The MD results show that the populations of three-, four-, and five-coordinated OH- are 29.6%, 67.1%, and 3.4% at 298 K, respectively, and those of two-, three-, four-, and five-coordinated OH- are 10.8%, 44.9%, 39.2%, and 4.9% at 278 K, respectively; the average hydrogen bond lengths and the hydrogen bond angle in the first shell increase with the temperature decreasing.

Reaction mechanism of NO with hydrolysates of NAMI-A: an MD simulation by combining the QM/MM(ABEEM) with the MD-FEP method.

Nitrosylation reaction mechanisms of the hydrolysates of NAMI-A and hydrolysis reactions of ruthenium nitrosyl complexes were investigated in the triplet state and the singlet state. Activation free energies were calculated by combining the QM/MM(ABEEM) method with free energy perturbation theory, and the explicit solvent environment was simulated by an ABEEMσπ polarizable force field. Our results demonstrate that nitrosylation reactions of the hydrolysates of NAMI-A occur in both the triplet and the singlet states. The Ru-N-O angle of the triplet ruthenium nitrosyl complexes is in the range of 132.0°-138.2°. However, all the ruthenium nitrosyl complexes at the singlet state show an almost linear Ru-N-O angle. The nitrosylation reaction happens prior to the hydrolysis reaction for the first-step hydrolysates. The activation free energies of the nitrosylation reactions show that the H2 O-NO exchange reaction of [RuCl4 (Im)(H2 O)] in the singlet spin sate is the most likely one. Comparing with the activation free energies of the hydrolysis reactions of the ruthenium nitrosyl complexes, the results indicate that the rate of the DMSO-H2 O exchange reaction of [RuCl3 (NO)(Im)(DMSO)] is faster than that of [RuCl3 (H2 O)(Im)(DMSO)] in both the triplet spin state and the singlet spin state. © 2018 Wiley Periodicals, Inc.

Theoretical reflections on the structural polymorphism of the oxygen-evolving complex in the S2 state and the correlations to substrate water exchange and water oxidation mechanism in photosynthesis.

The structural polymorphism of the oxygen-evolving complex is of great significance to photosynthetic water oxidation. Employing density functional theory calculations, we have made further advisement on the interconversion mechanism of O5 transfer in the S2 state, mainly focusing on the potentiality of multi-state reactivity and spin transitions. Then, O5 protonation is proven impossible in S2 for irreversibility of the interconversion, which serves as an auxiliary judgment for the protonation state of O5 in S1. Besides, the structural polymorphism could also be archived by alternative mechanisms involving Mn3 ligand exchange, one of which with Mn3(III) makes sense to substrate water exchange in S2, although being irresponsible for the derivations of the observed EPR signals. During the water exchange, high-spin states would prevail to facilitate electron transfer between the ferromagnetically coupled Mn centers. In addition, water exchange in S1 could account for the closed-cubane structure as the initial form entering S2 at cryogenic temperatures. With regard to water oxidation, the structural flexibility and variability in both S2 and S3 guarantee smooth W2-O5 coupling in S4, according to the substrate assignments from water exchange kinetics. Within this theoretical framework, the new XFEL findings on S1-S3 can be readily rationalized. Finally, an alternative mechanistic scenario for OO bond formation with ·OH radical near O4 is presented, followed by water binding to the pivot Mn4(III) from O4 side during S4-S0. This may diversify the substrate sources combined with the Ca channel in water delivery for the forthcoming S-cycle.

The open-cubane oxo-oxyl coupling mechanism dominates photosynthetic oxygen evolution: a comprehensive DFT investigation on O-O bond formation in the S4 state.

The dioxygen formation mechanism of biological water oxidation in nature has long been the focus of argument; many diverse mechanistic hypotheses have been proposed. Based on a recent breakthrough in the resolution of the electronic and structural properties of the oxygen-evolving complex in the S3 state, our density functional theory (DFT) calculations reveal that the open-cubane oxo-oxyl coupling mechanism, whose substrates preferably originate from W2 and O5 in the S2 state, emerges as the best candidate for O-O bond formation in the S4 state. This is justified by the overwhelming energetic superiority of this mechanism over alternative mechanisms in both the isomeric open and closed-cubane forms of the Mn4CaO5 cluster; spin-dependent reactivity rooted in variable magnetic couplings was found to play an essential role. Importantly, this oxygen evolution mechanism is supported by the recent discovery of femtosecond X-ray free electron lasers (XFEL), and the origin of the observed structural changes from the S1 to S3 state has been analyzed. In this view, we corroborate the proposed water binding mechanism during S2-S3 transition and correlate the theoretical models with experimental findings from aspects of substrate selectivity according to water exchange kinetics. This theoretical consequence for native metalloenzymes may serve as a significant guide for improving the design and synthesis of biomimetic materials in the field of photocatalytic water splitting.

How does ammonia bind to the oxygen-evolving complex in the S2 state of photosynthetic water oxidation? Theoretical support and implications for the W1 substitution mechanism.

Ammonia as a water analogue can bind to the Mn4CaO5 cluster of the oxygen-evolving complex in concomitance with ligand substitution and underlying structural transformation. On account of current controversies of the binding site and the absence of the viewpoint of reactivity and mechanistic proofs, we have investigated three modes of NH3 binding based on our elaborations of the possible reaction mechanisms, in correspondence with experimental observation for the NH3-altered g ≈ 2.0 EPR multiline signal. Broken-symmetry density functional theory was employed to construct all the spin surfaces. As a result, we rule out the O5 substitution strategy owing to the impenetrable free energy barrier exceeding 30 kcal mol-1, and alternative routes to destroy the O5 bridge are also blocked. The W1 substitution mechanism is shown to be quite facile, with the barrier not above 11.4 kcal mol-1. For the Mn4 addition scheme, the 'redox switch mechanism' was not implemented by our model, and the effective ways found render 15-22 kcal mol-1 energetic disadvantage by contrast. Consequently, it is strongly in favor of the W1 substitution mechanism for its overwhelming superiority in reactivity, reaching a consensus with the new pulse EPR conclusion. Then, we point out that ammonia departure occurs in the S4' state, with the O-O bonding but unreleased molecular O2. In the meantime, we propose two alternative channels for water binding in the S0' state and expound the significance to substrate selectivity. Ultimately, implications for the mechanism of O-O bond formation are discussed and all the remaining options are listed for future explorations.

Polycyclic aromatic hydrocarbons in deposited bedroom dust collected from Xinxiang, a fast developing city in North China.

To investigate the contamination levels of polycyclic aromatic hydrocarbons (PAHs) in the indoor environment, 16 PAHs were measured in 20 deposited bedroom dust (DBD) samples collected from four residential areas in Xinxiang City. The total PAH concentrations (∑PAHs, defined as the sum of 16 PAHs) in 20 DBD samples ranged from 1.47 to 21.8 µg/g dry weight. PAH ratios indicated that main sources of PAHs in most DBD samples were coal, grasses, and wood combustion. Correlation analysis showed that there were no significant positive correlations between ∑PAH concentrations in DBD and the number of family members or duration of residence. Analysis using the t test revealed that tobacco smoke was a source of PAHs in DBD and that an unattached kitchen may be a significant influencing factor related to PAHs in DBD collected from residential areas in Xinxiang City. Our results showed that the kitchen exhaust fan or hood had no significant influence on total PAHs in DBD, and there was no significant difference in ∑PAH concentrations in DBD in homes which used liquefied petroleum gas and those which used natural gas.

Investigation of the distinction between van der Waals interaction and chemical bonding based on the PAEM-MO diagram.

In recent years, the basic problem of understanding chemical bonding, nonbonded, and/or van der Waals interactions has been intensively debated in terms of various theoretical methods. We propose and construct the potential acting on one electron in a molecule-molecular orbital (PAEM-MO) diagram, which draws the PAEM inserted the MO energy levels with their major atomic orbital components. PAEM-MO diagram is able to show clear distinction of chemical bonding from nonbonded and/or vdW interactions. The rule for this is as follows. Along the line connecting two atoms in a molecule or a complex, the existence of chemical bonding between these two atoms needs to satisfy two conditions: (a) a critical point of PAEM exists and (b) PAEM barrier between the two atoms is lower in energy than the occupied major valence-shell bonding MO which contains in-phase atomic components (positive overlap) of the two considered atoms. In contrast to the chemical bonding, for a nonbonded interaction or van der Waals interaction between two atoms, both conditions (a) and (b) do not be satisfied at the same time. This is demonstrated and discussed by various typical cases, particularly those related to helium atom and H-H bonding in phenanthrene. There are helium bonds in HHeF and HeBeO molecules, whereas no H-H bonding in phenanthrene. The validity and limitation for this rule is demonstrated through the investigations of the curves of the PAEM barrier top and MO energies versus the internuclear distances for He2 , H2 , and He2 (+) systems.

Valence state parameters of all transition metal atoms in metalloproteins--development of ABEEMσπ fluctuating charge force field.

To promote accuracy of the atom-bond electronegativity equalization method (ABEEMσπ) fluctuating charge polarizable force fields, and extend it to include all transition metal atoms, a new parameter, the reference charge is set up in the expression of the total energy potential function. We select over 700 model molecules most of which model metalloprotein molecules that come from Protein Data Bank. We set reference charges for different apparent valence states of transition metals and calibrate the parameters of reference charges, valence state electronegativities, and valence state hardnesses for ABEEMσπ through linear regression and least square method. These parameters can be used to calculate charge distributions of metalloproteins containing transition metal atoms (Sc-Zn, Y-Cd, and Lu-Hg). Compared the results of ABEEMσπ charge distributions with those obtained by ab initio method, the quite good linear correlations of the two kinds of charge distributions are shown. The reason why the STO-3G basis set in Mulliken population analysis for the parameter calibration is specially explained in detail. Furthermore, ABEEMσπ method can also quickly and quite accurately calculate dipole moments of molecules. Molecular dynamics optimizations of five metalloproteins as the examples show that their structures obtained by ABEEMσπ fluctuating charge polarizable force field are very close to the structures optimized by the ab initio MP2/6­311G method. This means that the ABEEMσπ/MM can now be applied to molecular dynamics simulations of systems that contain metalloproteins with good accuracy.

Theoretical exploration of the potential and force acting on one electron within a molecule.

The potential and force acting on one electron within a molecule (PAEM and FAEM) have been investigated and analyzed. The PAEM, defined as the interaction energy on one electron provided by all the nuclei and the remaining electrons in a molecule, can be precisely expressed and calculated by ab initio method and our in-house program. Although the analysis of the scalar function PAEM is similar to that of the molecular electron density in the Bader's AIM theory, the former is distinct from the latter mainly in three points: (a) The minus gradient of the PAEM is the force acting on one electron within a molecule (FAEM). (b) The bond center is defined in terms of the features of FAEM and PAEM between two bonded atoms, and it is a two-dimensional attractive center whereas a nucleus is a three-dimensional attractive source for electrons. We have calculated the physical quantities of one electron at the bond center, such as Dpb, the Hessian matrix, and its eigenvalues. Interestingly, it is found that the force constant and frequency of the electron interflow around the bond center are well correlated with those corresponding quantities for the nuclear vibration which relate to the bond strength, for some series of diatomic molecules. (c) The bond center locates at a different point from that of the critical point of the electron density in the Bader's AIM theory, which will lead to different partitioning of the molecular space into the atomic regions.

Origin of Enantioselectivity in Engineered Cytochrome c-Catalyzed Carbon-Radical FePP Hydrolysis Revealed Using QM/MM (ABEEM Polarizable Force Field) and MD Simulations.

The origin of highly efficient asymmetric aminohydroxylation of styrene catalyzed by engineered cytochrome c is investigated by the developed Atom-Bond Electronegativity Equalization Method polarizable force field (ABEEM PFF), which is a combined outcome of electronic and steric effects. Model molecules were used to establish the charge parameters of the ABEEM PFF, for which the bond-stretching and angle-bending parameters were obtained by using a combination of modified Seminario and scan methods. The interactions between carbon-radical Fe-porphyrin (FePP) and waters are simulated by molecular dynamics, which shows a clear preference for the pre-R over the pre-S. This preference is attributed to the hydrogen-bond between the mutated 100S and 101P residues as well as van der Waals interactions, enforcing a specific conformation of the carbon-radical FePP complex within the binding pocket. Meanwhile, the hydrogen-bond between water and the nitrogen atom in the active intermediate dictates the stereochemical outcome. Quantum mechanics/molecular mechanics (QM/MM (ABEEM PFF)) and free-energy perturbation calculations elucidate that the 3RTS is characterized by sandwich-like structure among adjacent amino acid residues, which exhibits greater stability than crowed arrangement in 3STS and enables the R enantiomer to form more favorably. Thus, this study provides mechanistic insight into the catalytic reaction of hemoproteins.

Occurrence of higher chlorinated benzenes in livers of grass carp and common carp collected from markets of Xinxiang, China.

Five higher chlorinated benzenes (including hexachlorobenzene (HCB), pentachlorobenzene (PeCB) and three isomers of tetrachlorobenzens) were measured in the livers of grass carp and common carp collected from five markets in Xinxiang city, China. HCB and PeCB were detected in all samples. The major component of the higher CBs residue was HCB and significant correlations existed between HCB and PeCB in both grass carp and common carp livers. The ratio range of HCB/PeCB in grass carp and common carp were 3.4-6.2 and 4.9-7.7, respectively, which implied the sources of higher CBs originate mainly from the revolatilization of industrial HCB with a minor impact from PeCB.

A simple, rapid method for simultaneous determination of multiple elements in serum by using an ICP-MS equipped with collision cell.

We developed an inductively coupled plasma mass spectrometry method for testing 23 elements, namely, Mg, Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Mo, Cd, Sn, Sb, Ba, W, Tl, Pb, and U, in human serum. The serum samples were analyzed after diluting 1/25 with 0.5% nitric acid, 0.02% Triton-X-100, and 2% methanol. Sc, In, Y, Tb, and Bi were assigned internal standards to correct the baseline drift and matrix interference. The kinetic energy discrimination mode of the instrument with helium gas as the collision gas eliminated polyatomic interference. All 23 elements exhibited excellent linearity in their testing range, with a coefficient of determination ≥ 0.9996. The limits of detection of the 23 elements were within the range of 0.0004-0.2232 µg/L. The intra- and inter-day precision (relative standard deviation) were < 12.19%. The recoveries of the spiked standard for all elements were 88.98-109.86%. Among the 23 elements of the serum reference materials, the measured results of Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Se were within the specified range of the certificate, and the results of the other elements were also satisfactory. The developed method was simple, rapid, and effective, and only 60 µL sample was consumed. A total of 1000 serum samples from healthy individuals were randomly selected from the Henan Rural Cohort, which reflects the status of serum elements in rural adults from the Northern Henan province of central China.

Direct evaluation of individual hydrogen bond energy in situ in intra- and intermolecular multiple hydrogen bonds system.

The results of evaluating the individual hydrogen bond (H-bond) strength are expected to be helpful for the rational design of new strategies for molecular recognition or supramolecular assemblies. Unfortunately, there is few obvious and unambiguous means of evaluating the energy of a single H-bond within a multiple H-bonds system. We present a local analytic model, ABEEMσπ H-bond energy (HBE) model based on ab initio calculations (MP2) as benchmark, to directly and rapidly evaluate the individual HBE in situ in inter- and intramolecular multiple H-bonds system. This model describes the HBE as the sum of electrostatic and van der Waals (vdW) interactions which all depend upon the geometry and environment, and the ambient environment of H-bond in the model is accounted fairly. Thus, it can fairly consider the cooperative effect and secondary effect. The application range of ABEEMσπ HBE model is rather wide. This work has discussed the individual H-bond in DNA base pair and protein peptide dimers. The results indicate that the interactions among donor H atom, acceptor atom as well as those atoms connected to them with 1,2 or 1,3 relationships are all important for evaluating the HBE, although the interaction between the donor H atom and the acceptor atom is large. Furthermore, our model quantitatively indicates the polarization ability of N, O, and S in a new style, and gives the percentage of the polarization effect in HBE, which can not be given by fixed partial charge force field.