Molecular Weight Distributions of Polymer Solutions: Combination of Field Flow Fractionation (FFF) and Analytical Ultracentrifugation (AUC)

Molecular weight, distribution, as well as other molecular characteristics are important drivers in determining the potential behaviors and hence applications of polymeric materials. Out of different methods available for the determination of molecular weight and its distribution, field flow fractionation (FFF) provides absolute molecular weight values and accurate molecular weight distributions. Analytical ultracentrifugation (AUC), on the other hand, relies on the exact density of the polymer materials in solution to determine the accurate molecular weight and its distribution and in the absence of knowledge of exact density, AUC is less accurate than the FFF method. However, combination of the two methods can be achieved to gain insights into the other molecular characteristics of swollen polymer chains. One such example is the determination of the exact density of the swollen polymer chains by the incorporation of the molecular weight information from FFF into AUC analysis. Based on the comparison of the optimized polymer chain density with the bulk density, it was observed that the polyacrylic acid and polyacrylamide chains were swollen in the range of 27 to 29%. Moreover, the FFF and AUC can also complement each other in enhancing the range of characterization possible with the two methods when used separately.     

Data processing method for the determination of accurate molecular weight distribution of polymers by SEC/MALDI-MS.

A novel data processing method for a hyphenated technique, size exclusion chromatography/matrix-assisted laser desorption/ionization-mass spectrometry (SEC/MALDI-MS), has been proposed to determine accurate molecular weight distributions on the basis of the individual oligomer species of a polymer. This method is based on the concept that the individual peak intensities of MALDI mass spectrum observed for every SEC fraction with narrow molecular weight distribution could be adjusted to the quantified values to reveal the accurate molecular weight distribution using the signal intensity of the corresponding fraction on the SEC chromatogram observed with a refractive index detector. At first, the theory of the proposed date processing is described in detail. Then, experimental verification of the method is described. This was performed through the characterization of mixtures of three kinds of monodispersed polystyrene reference materials (weight average molecular weight = ca. 6000, 10000, and 18000) as model samples. An accurate trimodal molecular weight distribution for the individual oligomer species of the sample was obtained without any influence of the chromatographic band broadening observed in the original SEC chromatogram. Moreover, the method for depicting the elution profiles of individual oligomer species during SEC separation was also obtained as a "mass chromatogram" using the data processing procedure.     

Assessing the accuracy of improved force‐matched water models derived from Ab initio molecular dynamics simulations

The accuracy of water models derived from ab initio molecular dynamics simulations by means on an improved force‐matching scheme is assessed for various thermodynamic, transport, and structural properties. It is found that although the resulting force‐matched water models are typically less accurate than fully empirical force fields in predicting thermodynamic properties, they are nevertheless much more accurate than generally appreciated in reproducing the structure of liquid water and in fact superseding most of the commonly used empirical water models. This development demonstrates the feasibility to routinely parametrize computationally efficient yet predictive potential energy functions based on accurate ab initio molecular dynamics simulations for a large variety of different systems. © 2016 Wiley Periodicals, Inc.     

Designing electrostatic interactions in biological systems via charge optimization or combinatorial approaches: insights and challenges with a continuum electrostatic framework

Electrostatic interactions between biological molecules are crucially influenced by their aqueous environment, with efficient and accurate models of solvent effects required for robust molecular design strategies. Continuum electrostatic models provide a reasonable balance between computational efficiency and accurate system representation. In this article, I review two specific molecular design strategies, charge optimization and combinatorial design, paying particular attention to how the continuum framework (also briefly described herein) successfully enables both theoretical insights and molecular designs and presents a challenge in design applications due to what I call “the isostericity constraint.” Efforts to work around the isostericity constraint and other challenges are discussed. Additionally, particular emphasis is placed on using such models in the rational design of particularly tight, specific, or promiscuous interactions, in keeping with the increased sophistication of current molecular design applications.     

Effects of data analysis on accuracy and precision of GPC results

The effects of data imprecision and baseline uncertainties have been investigated by computer simulation of GPC data from a polymer sample with a truncated log-normal molecular weight distribution. If the data are very accurate, as few as five data points can be sampled without serious error in computed molecular weight averages. The number of data points required is much larger, however, if these are taken at equal increments of molecular weight rather than elution volume. The effects of noise can be counteracted by increasing the sampling frequency. Baseline uncertainties present a more serious problem, especially for broad-distribution samples. If the detected signal is too noisy to permit accurate location of the baseline, errors can be minimized by using a second, more sensitive detector to determine the peak start and end. It is very difficult to estimate M?_z and higher molecular weight averages accurately if the noise level is greater than 0.5%.     

Observation of sodium gel-induced protein modifications in dodecylsulfate polyacrylamide gel electrophoresis and its implications for accurate molecular weight determination of gel-separated proteins by matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry (TOFMS) can potentially provide accurate molecular weight information of proteins separated by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE). Several issues related to resolution and accuracy of molecular weight measurement are investigated by using a time-lag focusing MALDI-TOF mass spectrometer. The effects of the gel components SDS, glycerol, and tris buffer on the mass spectral signals are studied systematically. Glycerol and tris buffer are shown to have little or no effect on resolution and mass accuracy, whereas SDS degrades sensitivity, resolution, and mass accuracy even at low concentrations. A simple and fast gel extraction technique is presented which is capable of detecting proteins loaded at the low-picomole level on the gel. The sample preparation procedure used in this work appears to remove most of SDS from the gel, thereby reducing the peak broadening effect caused by SDS and resulting in high resolution and accurate measurement of proteins. However, for proteins containing cysteines, the molecular ions are composed of a distribution of acrylamide-protein adducts likely formed by reaction with unpolymerized acrylamide in the gel during the gel separation process. The implications of gel-induced protein modifications on the accurate molecular weight measurement of gel-separated proteins are discussed.     

Concentration effect in hyaluronan analysis by size exclusion chromatography

Summary The concentration effect in size-exclusion chromatographic analysis of hyaluronans of various relative molecular masses (RMM) has been studied. A critical concentration has been found that is negatively dependent on the hyaluronan molecular mass; the higher the biopolymer molecular mass, the smaller the injected sample where the concentration effect should be taken into account for accurate evaluation of molecular mass distribution. At higher temperatures, however, the concentration effect diminishes significantly.     

The chiral molecule CHClFI: first determination of its molecular parameters by Fourier transform microwave and millimeter-wave spectroscopies supplemented by ab initio calculations

The rotational spectrum of chlorofluoroiodomethane (CHClFI) has been investigated. Because its rotational spectrum is extremely crowded, extensive ab initio calculations were first performed in order to predict the molecular parameters. The low J transitions were measured using a pulsed-molecular-beam Fourier transform spectrometer, and the millimeter-wave spectrum was measured to determine accurate centrifugal distortion constants. Because of the high resolution of the experimental techniques, the analysis yielded accurate rotational constants, centrifugal distortion corrections, and the complete quadrupole coupling tensors for the iodine and chlorine nuclei, as well as the contribution of iodine to the spin-rotation interaction. These molecular parameters were determined for the two isotopologs CH35ClFI and CH37ClFI. They reproduce the observed transitions within the experimental accuracy. Moreover, the ab initio calculations have provided a precise equilibrium molecular structure. Furthermore, the ab initio molecular parameters are found in good agreement with the corresponding experimental values.     

Evaluation of field desorption mass spectrometry for the analysis of polyethylene

Field desorption mass spectrometry (FD-MS) has been evaluated for the analysis of low molecular weight polyethylene by using samples in the molecular weight range 600–2000 u as determined by gel permeation chromatography. The repeat units and end groups were characterized by FD-MS, but it was demonstrated that accurate molecular weight distribution data cannot be obtained for polyethylene by FD-MS because there is mass discrimination against the higher molecular weight polymers.     

2-氨基-5-取代基4-噻唑基膦酸酯的质谱研究

本文对一系列2-氨基-5-取代基-4-噻唑基膦酸酯进行了质谱研究,发现2-氨基-5-取代基-4-噻唑基膦酸酯的膦酰基中的烷基在碎裂过程中产生了一种新的γ-烷基重排反应,并且讨论了这类化合物的质谱碎裂行为,用电子轰击碰撞活化质量分析离子动能谱(EI-CA-MIKES)和高分辨精确质量测量技术(AMMT)作了进一步的研究。     

MALDI／MS快速测定蛋清中溶菌酶的相对分子质量

本文介绍了一种用基本辅助激光解吸电离质谱法快速测定蛋清中溶菌酶相对分子质量的方法。与传统的蛋白质相对分子质量测定相比，ＭＡＬＤＩ／ＭＳ法具有灵敏度高、精度高、易操作和分析时间短的优点，包括样品处理在内，一次分析不超过３０ｍｉｎ。     

Structure elucidation of degradation products of the antibiotic amoxicillin with ion trap MS<Superscript>n</Superscript> and accurate mass determination by ESI TOF

Today, it is necessary to identify relevant compounds appearing in discovery and development of new drug substances in the pharmaceutical industry. For that purpose, the measurement of accurate molecular mass and empirical formula calculation is very important for structure elucidation in addition to other available analytical methods. In this work, the identification and confirmation of degradation products in a finished dosage form of the antibiotic drug amoxicillin obtained under stress conditions will be demonstrated. Structure elucidation is performed utilizing liquid chromatography (LC) ion trap MS/MS and MS3 together with accurate mass measurement of the molecular ions and of the collision induced dissociation (CID) fragments by liquid chromatography electro spray ionization time-of-flight mass spectrometry (LC/ESI-TOF).     

A new method for chemical identification based on orthogonal parallel liquid chromatography separation and accurate molecular weight confirmation

Recent advances in the theory and application of orthogonal LC separation have allowed for the establishment of a more effective method for the chemical identification of target compounds in complex samples, especially structurally similar compounds. In this study, a new chemical identification method based on orthogonal parallel separation and accurate molecular weight confirmation was developed. An orthogonal separation system consisting of an XTerra MS C(18) column, a home-made Click OEG column, and a Click CD column was established for separation and identification. In addition, 82 flavonoids were selected as references, to be used for the construction of a library. Retention times of each reference flavonoid on each column and accurate molecular weights were recorded and imported into a searchable library as "tags" for the unknown screening. For the method validation, two complex mixtures, fractions of Dalbergia odorifera T. Chen and Scutellaria baicalensis Georgi, specifically, were separated and identified. In total, nine compounds were unequivocally identified by retention time and confirmation of accurate molecular weight, demonstrating that this method is suitable and efficient for the chemical identification of complex samples.     

Isotope abundance analysis methods and software for improved sample identification with supersonic gas chromatography/mass spectrometry

We present newly developed isotope abundance analysis (IAA) methods and software which are used to derive elemental formula information from experimental mass spectral data of molecular ion isotopomeric abundances. The software, using a novel method, can also be used to automatically confirm or reject NIST library search results, thereby significantly improving the confidence level in sample identifications. In the case of IAA confirmation of the NIST library results, sample identification is unambiguous, since the confirmation is achieved by two independent sets of data and analytical methods. In the case of a rejection, such as when the molecule is not included in the library's databases, the IAA software independently provides a list of elemental formulae with declining order of matching to the isotopomeric experimental data, in a similar way to accurate mass measurements with costly instruments. IAA is ideally applicable to gas chromatography/mass spectrometry (GC/MS) (and liquid chromatography/electron ionization mass spectrometry (LC/EI-MS)) with a supersonic molecular beam (SMB) since it requires a trustworthy and highly abundant true molecular ion that is unique to the SMB-MS systems, plus the absence of self chemical ionization and vacuum background noise, again unique features of GC/SMB-MS. The various features of the IAA methods and software are described, their performance is demonstrated with the analysis of experimental GC-SMB-MS data and the IAA concept is compared with accurate mass alternatives. The combination of IAA and GC/SMB-MS is believed to be superior to accurate mass GC/MS in view of the general availability of trustworthy molecular ions for an extended range of compounds.     

气液色谱法测定工业直链烷基磺酸盐数均分子量

本文介绍用气相色谱法测定工业直链烷基磺酸盐在浓磷酸介质中脱磺酸基后,用石油醚萃取释出的不同碳数烷基的含量,然后,根据烷基的数均分子量,分别加—SO_3Na基的值,从而计算出工业直链烷基磺酸盐的数均分子量。该法快速、简易和准确。     

A distance-dependent parameterization of the extended Hubbard model for conjugated and aromatic hydrocarbons derived from stretched ethene

The Hubbard model, which is widely used in physics but is mostly unfamiliar to chemists, provides an attractive yet simple model for chemistry beyond the self consistent field molecular orbital approximation. The Hubbard model adds an effective electron-electron repulsion when two electrons occupy the same atomic orbital to the familiar Hückel Hamiltonian. Thus it breaks the degeneracy between excited singlet and triplet states and allows an explicit treatment of electron correlation. We show how to evaluate the parameters of the model from high-level ab initio calculations on two-atom fragments and then to transfer the parameters to large molecules and polymers where accurate ab initio calculations are difficult or impossible. The recently developed MS-RASPT2 method is used to generate accurate potential energy curves for ethene as a function of carbon-carbon bond length, which are used to parameterize the model for conjugated hydrocarbons. Test applications to several conjugated/aromatic molecules show that even though the model is very simple, it is capable of reasonably accurate predictions for bond lengths, and predicts molecular excitation energies in reasonable agreement with those from the MS-RASPT2 method.     

Studies on the P-branch spectral lines of rovibrational transitions of diatomic system

An analytical formula is used to predict the accurate P-branch spectral lines of rovibrational transitions for diatomic systems. The formula is derived from elementary expression of molecular total energy by taking multiple spectral differences. It is not only reproduces the known experimental transition lines by using a group of fifteen known experimental transition data, but also predicts the accurate spectral lines that may not be available experimentally. The P-branch emission spectra of the (0,1), (0,2) and (0,3) bands of the B(2)∑(+)→X(2)∑(+) system in the (12)C(17)O(+) molecular ion are studied, and correct values of the unknown spectral lines up to J=80.5 for each band are predicted using the formula.     

Multipole moments for embedding potentials: Exploring different atomic allocation algorithms

Polarizable quantum mechanical (QM)/molecular mechanics (MM)‐embedding methods are currently among the most promising methods for computationally feasible, yet reliable, production calculations of localized excitations and molecular response properties of large molecular complexes, such as proteins and RNA/DNA, and of molecules in solution. Our aim is to develop a computational methodology for distributed multipole moments and their associated multipole polarizabilities which is accurate, computationally efficient, and with smooth convergence with respect to multipole order. As the first step toward this goal, we herein investigate different ways of obtaining distributed atom‐centered multipole moments that are used in the construction of the electrostatic part of the embedding potential. Our objective is methods that not only are accurate and computationally efficient, but which can be consistently extended with site polarizabilities including internal charge transfer terms. We present a new way of dealing with well‐known problems in relation to the use of basis sets with diffuse functions in conventional atomic allocation algorithms, avoiding numerical integration schemes. Using this approach, we show that the classical embedding potential can be systematically improved, also when using basis sets with diffuse functions, and that very accurate embedding potentials suitable for QM/MM embedding calculations can be acquired. © 2016 Wiley Periodicals, Inc.     

Accurate Calculation of Equilibrium Reduced Density Matrix for the System-Bath Model: a Multilayer Multiconfiguration Time-Dependent Hartree Approach and its Comparison to a Multi-Electronic-State Path Integral Molecular Dynamics Approach?

An efficient and accurate method for computing the equilibrium reduced density matrix is presented for treating open quantum systems characterized by the system-bath model. The method employs the multilayer multiconfiguration time-dependent Hartree theory for imaginary time propagation and an importance sampling procedure for calculating the quantum mechanical trace. The method is applied to the spin-boson Hamiltonian, which leads to accurate results in agreement with those produced by the multi-electronic-state path integral molecular dynamics method.