Simulation Of NMR Spectra Research Articles

Neural Networks for Conversion of Simulated NMR Spectra from Low-Field to High-Field for Quantitative Metabolomics

Background: The introduction of benchtop NMR instruments has made NMR spectroscopy a more accessible, affordable option for research and industry, but the lower spectral resolution and SNR of a signal acquired on low magnetic field spectrometers may complicate the quantitative analysis of spectra. Methods: In this work, we compare the performance of multiple neural network architectures in the task of converting simulated 100 MHz NMR spectra to 400 MHz with the goal of improving the quality of the low-field spectra for analyte quantification. Multi-layered perceptron networks are also used to directly quantify metabolites in simulated 100 and 400 MHz spectra for comparison. Results: The transformer network was the only architecture in this study capable of reliably converting the low-field NMR spectra to high-field spectra in mixtures of 21 and 87 metabolites. Multi-layered perceptron-based metabolite quantification was slightly more accurate when directly processing the low-field spectra compared to high-field converted spectra, which, at least for the current study, precludes the need for low-to-high-field spectral conversion; however, this comparison of low and high-field quantification necessitates further research, comparison, and experimental validation. Conclusions: The transformer method of NMR data processing was effective in converting low-field simulated spectra to high-field for metabolomic applications and could be further explored to automate processing in other areas of NMR spectroscopy.

Correlated Anion Disorder in Heteroanionic Cubic TiOF2.

Resolving anion configurations in heteroanionic materials is crucial for understanding and controlling their properties. For anion-disordered oxyfluorides, conventional Bragg diffraction cannot fully resolve the anionic structure, necessitating alternative structure determination methods. We have investigated the anionic structure of anion-disordered cubic (ReO3-type) TiOF2 using X-ray pair distribution function (PDF), 19F MAS NMR analysis, density functional theory (DFT), cluster expansion modeling, and genetic-algorithm structure prediction. Our computational data predict short-range anion ordering in TiOF2, characterized by predominant cis-[O2F4] titanium coordination, resulting in correlated anion disorder at longer ranges. To validate our predictions, we generated partially disordered supercells using genetic-algorithm structure prediction and computed simulated X-ray PDF data and 19F MAS NMR spectra, which we compared directly to experimental data. To construct our simulated 19F NMR spectra, we derived new transformation functions for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF4. We find good agreement between our simulated and experimental data, which supports our computationally predicted structural model and demonstrates the effectiveness of complementary experimental and computational techniques in resolving anionic structure in anion-disordered oxyfluorides. From additional DFT calculations, we predict that increasing anion disorder makes lithium intercalation more favorable by, on average, up to 2 eV, highlighting the significant effect of variations in short-range order on the intercalation properties of anion-disordered materials.

Structural Characterization and Molecular Model Construction of Lignite: A Case of Xianfeng Coal

The object of the study is lignite. Analytical testing techniques, such as elemental analysis, 13C nuclear magnetic resonance (13C NMR) spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), were used to acquire information on the structural parameters of lignite. The aromaticity of Xianfeng lignite is 43.57%, and the aromatic carbon structure is mainly naphthalene and anthracene/phenanthrene. The aliphatic carbon structure is dominated by cycloalkanes, alkyl side chains, and hydrogenated aromatics. Oxygen is mainly present in ether oxygen, carboxyl, and carbonyl groups. Nitrogen is mainly in the form of pyrrole nitrogen and quaternary nitrogen. Sulfur is mainly thiophene sulfur. According to the analysis results, the molecular structure model of XF lignite was constructed. The molecular formula is C184H172O39N6S2. The 2D structure was converted to a 3D structure using computer simulation software and optimized. The optimized model has a remarkable stereoconfiguration, and the aromatic lamellae are irregularly arranged in space. The aromatic rings were mainly connected by methylene, hypomethylene, methoxy, and aliphatic rings. In addition, the simulated 13C NMR spectra are in good agreement with the experimental spectra. This shows the rationality of the 3D chemical structure model.

Generalized kekulenes and clarenes as novel families of cycloarenes: structures, stability, and spectroscopic properties.

Cycloarenes constitute a captivating class of polycyclic aromatic hydrocarbons with unique structures and properties, but their synthesis represents a challenging task in organic chemistry. Kekulenes and edge-extended kekulenes as classic types of cycloarenes play an important role in the comprehension of π electron distribution, but their sparse molecular diversity considerably limits their further development and application. In this work, we propose two novel classes of cycloarenes, the generalized kekulenes and the clarenes. Using density functional theory, we carry out a comprehensive study of all possible isomers of the generalized kekulenes and clarenes with different sizes. By applying a simple Hückel model, we show that π delocalization plays a crucial role in determining the relative stability of isomers. We also discover that π-π stacking is commonly present in certain larger clarenes and provides a considerable additional stabilization effect, making the corresponding isomers the lowest-energy ones. Among all considered typical looped polyarenes, generalized kekulenes and/or clarenes are revealed to be the energetically most stable forms, suggesting that these novel cycloarenes proposed here would be viable targets for future synthetic work. The simulated 1H NMR spectra and UV-vis absorption spectra provide valuable information about the electronic and optoelectronic properties for the most stable generalized kekulene and clarene species and may support their identification in future synthesis and experimental characterization.

Structure, chemical reactivity, NBO, MEP analysis and thermodynamic parameters of pentamethyl benzene using DFT study

This study explores the structural, molecular and electronic properties of the pentamethyl benzene (PMB) using quantum chemical calculations employing DFT/ B3LYP/6–311++G(d,p) level of theory. Structural parameters, HOMO-LUMO energies, chemical reactivity descriptors and molecular electrostatic potential (MEP) were evaluated from the optimized molecular geometry. The computed small band gap energy, correlated with the UV–visible spectrum, demonstrates that the test molecule has good biological activity. NLO activity of the molecule was also studied by evaluating dipole moment and first order hyperpolarizability values. Intra-molecular charge transfer, donor-acceptor transitions and stabilizations energies were determined from the analyses of natural bond orbital (NBO) and natural population. 1H and 13C chemical shifts were computed from the simulated NMR spectrum following GIAO approach in DMSO‑d6 solvent at the same level of DFT. Furthermore, thermodynamic parameters and rotational constants were also calculated for this molecule.

Simulation of NMR spectra at zero and ultralow fields from A to Z - a tribute to Prof. Konstantin L'vovich Ivanov.

Simulating NMR experiments may appear mysterious and even daunting for those who are new to the field. Yet, broken down into pieces, the process may turn out to be easier than expected. Quite the opposite, it is in fact a powerful and playful means to get insights into the spin dynamics of NMR experiments. In this tutorial paper, we show step by step how some NMR experiments can be simulated, assuming as little prior knowledge from the reader as possible. We focus on the case of NMR at zero and ultralow fields, an emerging modality of NMR in which the spin dynamics are dominated by spin-spin interactions rather than spin-field interactions, as is usually the case with conventional high-field NMR. We first show how to simulate spectra numerically. In a second step, we detail an approach to construct an eigenbasis for systems of spin- nuclei at zerofield. We then use it to interpret the numerical simulations.

Expanding the Chemical Space of Arsenicin A-C Related Polyarsenicals and Evaluation of Some Analogs as Inhibitors of Glioblastoma Stem Cell Growth

The marine polyarsenical metabolite arsenicin A is the landmark of a series of natural and synthetic molecules characterized by an adamantane-like tetraarsenic cage. Arsenicin A and related polyarsenicals have been evaluated for their antitumor effects in vitro and have been proven more potent than the FDA-approved arsenic trioxide. In this context, we have expanded the chemical space of polyarsenicals related to arsenicin A by synthesizing dialkyl and dimethyl thio-analogs, the latter characterized with the support of simulated NMR spectra. In addition, the new natural arsenicin D, the scarcity of which in the Echinochalina bargibanti extract had previously limited its full structural characterization, has been identified by synthesis. The dialkyl analogs, which present the adamantane-like arsenicin A cage substituted with either two methyl, ethyl, or propyl chains, were efficiently and selectively produced and evaluated for their activity on glioblastoma stem cells (GSCs), a promising therapeutic target in glioblastoma treatment. These compounds inhibited the growth of nine GSC lines more potently than arsenic trioxide, with GI50 values in the submicromolar range, both under normoxic and hypoxic conditions, and presented high selectivity toward non-tumor cell lines. The diethyl and dipropyl analogs, which present favorable physical-chemical and ADME parameters, had the most promising results.

An account of noncovalent interactions in homoleptic palladium(II) and platinum(II) complexes within the DFT framework: A correlation between geometries, energy components of symmetry-adapted perturbation theory and NCI descriptors

The present work addresses the underlying nature of weak noncovalent interactions (NCIs) in the self-assembled dimers of two square planar palladium(II) and platinum(II) complexes trans-[Pd(Hida)2] (1) and trans-[Pt(Hida)2] (2) (Hida = monoprotonated iminodiacetate) within the framework of density functional theory (DFT) in gas phase. Initial geometries of the dimers in different spatial orientations were extracted from the X-ray crystal structures, reported earlier, and optimized with three dispersion-corrected functionals that are frequently used to explore NCIs. The BP86-D3, M062X-D3 and ωB97X-D3 functionals have been used to test their performances over the present systems. The SARC-ZORA-TZVP and ZORA-def2-TZVP basis sets were applied for the metals and the remaining elements, respectively. The optimizations resulted in equilibrium geometries where the monomers are self-assembled through NCIs to form dimers in a cyclic fashion. This type of structural pattern is absent in the crystal structures of both 1 and 2. Physical components of interaction energies were investigated by symmetry-adapted perturbation theory (SAPT). The UV-Vis absorption spectra of the dimers are described by time-dependent density functional theory (TD-DFT). Global reactivity parameters for the dimers have been computed within the framework of conceptual density functional theory (CDFT). Detailed investigations on NCIs were performed for all dimer geometries. Simulated IR and 1H NMR spectra, charge transfer, QTAIM, NCI-RGD, IGM, ETS-NOCV and ELF studies confirmed the presence of intermolecular hydrogen bonds (HBs) and weak van der Waals interactions. Energies of the hydrogen bonds and associated orbital interaction energies were computed by QTAIM and ETS-NOCV methods, respectively.

Conformational Analysis of [60]PCBM from DFT Simulations of Electronic Energies, Bond Strain and the 13C NMR Spectrum: Input Geometry Determination and Ester Bond Rotation Dynamics

With the aim of determining the best input geometry for DFT calculations of [60]PCBM, the geometry of 24 chemically possible [60]PCBM conformers were optimised and their electronic energies and average bond strains were determined. A DFT analysis of the relevant dihedral angles provided insights into the dynamical behaviour of the ester group through sterically restricted bond rotations. In addition, the 13C NMR spectra of the six better performing conformers were simulated and compared with an experiment. There is a close correlation between average bond strain, total electronic energy and mean absolute error of the simulated 13C NMR spectra of the ester carbons. The best overall candidate conformer for the input geometry had the C61-C4, C4-C3 and C3-C2 single bonds of the alkyl chain in syn, anti and anti arrangements, respectively, and had the C2-C1 and C1-O single bonds of the ester in syn and anti arrangements, respectively. This contrasts strikingly with most representations of PCBM in the literature, which depict all relevant bonds in anti arrangements.

Possible Magnetic Structure with a Tilted Helical Plane in SmBe13 Probed by 9Be-NMR Study

9Be-NMR measurements were performed using single crystalline SmBe13 in order to investigate a magnetic structure of a low-temperature ordering state microscopically. We observed a spectral broadening in the ordered state, and the broadened spectral shape depends on the magnetic field directions. By comparing the experimentally obtained and simulated NMR spectra for magnetic fields along the cubic [001] and [011] directions, we argue a helical structure with a basal plane tilted from the (001) plane in low-field ordering region under the assumption of a helical with a propagation vector of (0, 0, 1/3) found in other RBe13 compounds (R = rare earths). Such a tilted helical structure is explained by a combination of an ellipse helical in the (001) plane and a longitudinal magnetic density wave along the [001] direction. Considering a magnetic easy axis parallel to [001] revealed by the magnetization measurements, the peculiar helical structure in SmBe13 may be built on delicate balance among exchange interactions, dipole-dipole interaction, and single-ion magnetic anisotropy due to the crystalline electric field.

Structural characteristics of Mile lignite and its molecular model construction

The understanding of the structural characteristics of lignite is of very importance to the lignite utilization. The structure parameters of Mile lignite in Yunnan were ascertained via Fourier transform infrared spectroscopy, 13C solid-state nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and ultimate analysis. The results indicate that the aromaticity and number of aromatic ring substituents of Mile lignite are 38.79% and 3, respectively. The aromatic carbon structure typically contains benzene and naphthalene, while the aliphatic carbon structure mainly includes methyl and methylene. Oxygen dominantly exists in ether oxygen, carboxyl and carbonyl; nitrogen occurs in the form of pyrrole nitrogen and pyridine nitrogen; and sulfur is mainly present in thiophenol and mercaptan. According to the analysis results, a molecular structure model of Mile lignite is constructed, with a molecular formula of C147H148O36N2S. The semi-empirical PM3 basis set and the density functional theory M06-2X were adopted to optimize the molecular configuration. The optimized model has a significant three-dimensional configuration, in which the aromatic layers are arranged irregularly in space and the aromatic rings are connected by methyl, methylene, methoxy and alicyclic rings. The simulated FT-IR spectrum and simulated 13C NMR spectrum are in great agreement with the experimental spectrum, which proves the accuracy and rationality of the molecular model of Mile lignite.

2D NMR spectroscopy of refolding RNase Sa using polarization transfer from hyperpolarized water

Polarization transfer from hyperpolarized water through proton exchange is used to enhance the NMR signals of amide protons of the Ribonuclease Sa protein. Spectra of the refolding protein are measured within 6 s after dilution of the denaturant urea, at urea-dependent folding rates adjusted in the range of 0.3–0.8 s−1. Peak patterns including a mixture of folded and unfolded protein at different ratios are observed. The changes in the observed signals indicate that each spectrum accesses a different point in the partial completion of the folding. A comparison to simulated 2D NMR spectra suggests a lower polarization transfer efficiency from water when the protein folds slowly, which may result from the molecular motions in the unfolded protein and the absence of long-range contacts. The ability to acquire 2D NMR spectra under different refolding conditions may open a new avenue for residue specific characterization of the folding process.

Can NMR spectroscopy discriminate between NPS amphetamines and cathinones? An evaluation by in silico studies and chemometrics

The use of new psychoactive substances (NPS) has become a problem that affects many countries. Because new chemical structures appear frequently and reliable reference data do not always exist, identifying these compounds is an operational problem. Theoretical chemistry associated with statistical methods has scarcely been used to tackle such problems and can provide an unvaluable aid. In this study, we have employed quantum chemistry tools to simulate NMR spectra and to evaluate, as a paradigm, the main differences between amphetamines and cathinones. We have generated and examined the 1H and 13C chemical shifts of 21 homologous amphetamine and cathinone structures. In our computational approach, we used a test set and a proof set to generate the shifted factors on the basis of experimental observations. We employed three different DFTs (B3LYP, M062X, and PBE0) with a TZVP basis set and different solvents (water, ethanol, chloroform, and gas-phase). To evaluate the data, we applied statistical approaches, which allowed us to assess the methods and to distinguish between the studied NPS. This study provided reliable information that helps to explain how NMR evaluation differs in assessing these molecules and promoting their discrimination in forensic analysis.

Synthesis, spectral characterization, and theoretical investigations of a new azo-stilbene dye for acrylic resins

This paper presents the synthesis of a new symmetrical disazo direct dye with good coloring properties for acrylic resins and several theoretical approaches to choose the best simulation method of the dye structure and spectra. The synthesized dye contains 4,4′-diaminostilbene-2,2′-disulfonic acid, as middle component and 2,7-dihydroxynaphthalene, as coupling component. It was analyzed by thin-layer chromatography (TLC), FT/IR, UV/VIS, 1H NMR, 13C NMR spectroscopy, and HRMS spectrometry. The molecular dye structure was investigated using molecular mechanics and the Density Functional Theory (DFT) approach at the B3LYP/6-31G(d,p), B3LYP/6-31+G(d,p) and B3PW91/6-311G(d,p) levels. Theoretical vibrational frequencies and structural parameters were calculated and the dye UV/VIS spectrum was simulated using the CIS, TD and ZINDO methods and compared with the experimental data. The ZINDO approach gave better maximum absorption predicted values compared to the CIS and TD methods. A generally good agreement between the experimental and theoretical computed absorption maxima and excellent correspondences between the experimental and calculated vibrational frequencies and chemical shifts were found. The experimental and simulated vibrational, UV/VIS and 13C NMR spectra confirmed the trans-configuration form of the π-bonds of the investigated dye. The CIEL∗a∗b∗ color space was used in all color measurements for the dye in powder and its mixtures with a water-based acrylic resin and confirmed the color properties of the newly synthesized dye.

Synthesis and structure of 9-hydroxy-1,2-dicarba-closo-dodecaborane(11)

The oxidation of 1,2-C2B10H12 (1) with 100% nitric acid was studied in two solvents (CH2C12 and CCl4). Under the action of superacid (CF3SO3H), the compound 9-HO-1,2-C2B10H11 (2) gives the onium cation 9-H2O+-1,2-C2B10H11 involved in the salt [9-H2O+-1,2-C2B10Hn]-CF3SO3???, as demonstrated by uB NMR spectroscopy. The experimental and simulated uB NMR spectra of the cation 9-H2O+-1,2-C2B10H11 are in satisfactory agreement with each other. In the presence of a base, compound 2 is transferred from an ethereal solution to an aqueous alkaline solution giving the anion 9-O???- 1,2-C2B10H11. The structure of compound 2 was confirmed by 1H, 11B, 11B1H, 11B-11B COSY NMR spectroscopy, IR spectroscopy, and gas chromatography mass spectrometry and was additionally established by X-ray diffraction.

Spectroscopic Identifications, Molecular Docking, Neuronal Growth and Enzyme Inhibitory Activities of Steroidal Nitro Olefin: Quantum Chemical Study

AbstractFT‐IR, NMR and ultraviolet spectroscopy by means of DFT / B3LYP‐6‐311G(d, p), molecular docking and biological activity of 6‐nitrocolest‐5‐en‐3‐beta‐yl acetate (steroid nitro‐olefin) was reported in this article. The optimized molecular geometric structure and its associated parameters including vibrational frequencies, carbon and 1H NMR data of the title compound in ground electronic state was obtained at DFT approach applying B3LYP/6‐311G(d,p). The ultraviolet absorbance data is obtained at TDB3LYP/6‐311G(d,p) basis set. The present computational data are found in reasonable agreement with the experimental results. Simulated electronic and NMR (1H and 13C) spectra have been reported in solution phase. Parameters like thermodynamic quantities, dipole moment, FMO energies, MEP and global reactivity descriptors of steroid nitro olefin were determined at same level of theory. The neuromodulatory effect of the compound on central nervous system development of rat embryonic hippocampal neurons has been studied. Based on the outcome; compound demonstrated suppression activity despite the promotion of neuronal cytoarchitecture leading to a negative effect on hippocampal neurons. The nature of the compound found to be toxic to the cells. Besides, steroid compound is also used to perform inhibitory activity against acetylcholinesterase (AChE) compared to the reference drug i. e. Galanthamine. The molecular docking of the synthesized compound is carried out with the structure of the acetylcholinesterase (PDB: 1DX6) protein to check the mode of interactions.

Ensemble-Based Modeling of the NMR Spectra of Solid Solutions: Cation Disorder in Y2(Sn,Ti)2O7.

The sensitivity of NMR to the local environment, without the need for any long-range order, makes it an ideal tool for the characterization of disordered materials. Computational prediction of NMR parameters can be of considerable help in the interpretation and assignment of NMR spectra of solids, but the statistical representation of all possible chemical environments for a solid solution is challenging. Here, we illustrate the use of a symmetry-adapted configurational ensemble in the simulation of NMR spectra, in combination with solid-state NMR experiments. We show that for interpretation of the complex and overlapped lineshapes that are typically observed, it is important to go beyond a single-configuration representation or a simple enumeration of local environments. The ensemble method leads to excellent agreement between simulated and experimental spectra for Y2(Sn,Ti)2O7 pyrochlore ceramics, where the overlap of signals from different local environments prevents a simple decomposition of the experimental spectral lineshapes. The inclusion of a Boltzmann weighting confirms that the best agreement with experiment is obtained at higher temperatures, in the limit of full disorder. We also show that to improve agreement with experiment, in particular at low dopant concentrations, larger supercells are needed, which might require alternative simulation approaches as the complexity of the system increases. It is clear that ensemble-based modeling approaches in conjunction with NMR spectroscopy offer great potential for understanding configurational disorder, ultimately aiding the future design of functional materials.

Assessment of interatomic parameters for the reproduction of borosilicate glass structures via DFT‐GIPAW calculations

AbstractBorates and borosilicates are potential candidates for the design and development of glass formulations with important industrial and technological applications. A major challenge that retards the pace of development of borate/borosilicate based glasses using predictive modeling is the lack of reliable computational models to predict the structure‐property relationships in these glasses over a wide compositional space. A major hindrance in this pursuit has been the complexity of boron‐oxygen bonding due to which it has been difficult to develop adequate B–O interatomic potentials. In this article, we have evaluated the performance of three B–O interatomic potential models recently developed by Bauchy et al [J. Non‐Cryst. Solids, 2018, 498, 294–304], Du et al [J. Am. Ceram. Soc. https://doi.org/10.1111/jace.16082] and Edèn et al [Phys. Chem. Chem. Phys., 2018, 20, 8192–8209] aiming to reproduce the short‐to‐medium range structures of sodium borosilicate glasses in the system 25 Na2O x B2O3 (75 − x) SiO2 (x = 0‐75 mol%). To evaluate the different force fields, we have computed at the density functional theory level the NMR parameters of 11B, 23Na, and 29Si of the models generated with the three potentials and the simulated MAS NMR spectra compared with the experimental counterparts. It was observed that the rigid ionic models proposed by Bauchy and Du can both reliably reproduce the partitioning between BO3 and BO4 species of the investigated glasses, along with the local environment around sodium in the glass structure. However, they do not accurately reproduce the second coordination sphere of silicon ions and the Si–O–T (T = Si, B) and B‐O‐T distribution angles in the investigated compositional space which strongly affect the NMR parameters and final spectral shape. On the other hand, the core‐shell parameterization model proposed by Edén underestimates the fraction of BO4 species of the glass with composition 25Na2O 18.4B2O3 56.6SiO2 but can accurately reproduce the shape of the 11B and 29Si MAS‐NMR spectra of the glasses investigations due to the narrower B–O–T and Si‐O‐T bond angle distributions. Finally, the effect of the number of boron atoms (also distinguishing the BO3 and BO4 units) in the second coordination sphere of the network former cations on the NMR parameters have been evaluated.

Combined experimental and theoretical studies on the diorganotin(IV) complexes of sparfloxacin: Synthesis, spectroscopic and DFT studies, and biological activity

New diorganotin(IV) derivatives of sparfloxacin having general formula R2SnCl(L), (where L = monoanion of sparfloxacin (HL) and R = n-Bu (1)/Ph (2)) have been synthesized and structurally characterized by elemental analysis, IR, NMR (1H, 13C, 119Sn), HMQC, ESI-MS, UV–Vis and emission spectroscopy. These investigations suggest that, in these 1:1 monomeric derivatives the sparfloxacin ligand acts as monoanionic bidentate coordinating through the Ocarboxylate and Opyridone, and the polyhedron around tin is intermediate between pseudotetrahedral and cis-trigonal bipyramidal. The proposed structures have been validated by density functional theory (DFT) based electronic structure calculations at B3LYP/6-31G(d,p)/Def2-SVP(Sn) level of theory through the calculation of the atomic charges at the selected atoms, molecular electrostatic potential (MEP) map to assign sites on the surface of the molecules, the selected conceptual-DFT based global reactivity descriptors to obtain an insight into the structure and reactivity behaviour, and the frontier molecular orbital analysis to analyze the nature of frontier orbitals. A comparative analysis of the experimental vibrational frequencies and simulated harmonic frequencies indicates good correlation between them. The simulated UV–Vis spectrum was obtained with time dependent-DFT method in gas phase and in the solvent field with IEFPCM model. The simulated 1H and 13C NMR spectra were obtained by gauge-independent atomic orbital (GIAO) method and the results were in good agreement to the experimental results. The complexes were screened for their in vitro antibacterial activity against two Gram-positive and five Gram-negative bacterial strains. Both the complexes exhibited promising antibacterial activity against all the chosen strains (MIC: 0.062–0.125 μg ml−1).

Improved variable reduction in partial least squares modelling by Global-Minimum Error Uninformative-Variable Elimination

The calibration performance of Partial Least Squares regression (PLS) can be improved by eliminating uninformative variables. For PLS, many variable elimination methods have been developed. One is the Uninformative-Variable Elimination for PLS (UVE-PLS). However, the number of variables retained by UVE-PLS is usually still large.In UVE-PLS, variable elimination is repeated as long as the root mean squared error of cross validation (RMSECV) is decreasing. The set of variables in this first local minimum is retained. In this paper, a modification of UVE-PLS is proposed and investigated, in which UVE is repeated until no further reduction in variables is possible, followed by a search for the global RMSECV minimum. The method is called Global-Minimum Error Uninformative-Variable Elimination for PLS, denoted as GME-UVE-PLS or simply GME-UVE. After each iteration, the predictive ability of the PLS model, built with the remaining variable set, is assessed by RMSECV. The variable set with the global RMSECV minimum is then finally selected. The goal is to obtain smaller sets of variables with similar or improved predictability than those from the classical UVE-PLS method.The performance of the GME-UVE-PLS method is investigated using four data sets, i.e. a simulated set, NIR and NMR spectra, and a theoretical molecular descriptors set, resulting in twelve profile-response (X-y) calibrations. The selective and predictive performances of the models resulting from GME-UVE-PLS are statistically compared to those from UVE-PLS and 1-step UVE, one-sided paired t-tests.The results demonstrate that variable reduction with the proposed GME-UVE-PLS method, usually eliminates significantly more variables than the classical UVE-PLS, while the predictive abilities of the resulting models are better. With GME-UVE-PLS, a lower number of uninformative variables, without a chemical meaning for the response, may be retained than with UVE-PLS. The selectivity of the classical UVE method thus can be improved by the application of the proposed GME-UVE method resulting in more parsimonious models.