Precise Molecular Weight Determination and Structure Characterization of End-functionalized Polymers: An NMR Approach via Combination of One-dimensional and Two-dimensional Techniques

We present here the application of one-dimensional and two-dimensional NMR techniques to characterize the structure of methoxyl end-functionalized polystyrenes (PS).The peaks in 1H-NMR spectra corresponding to main-chain,side-chain and chain-end groups are assigned by 1H-1H gCOSY,1H-13C gHSQC and gHMBC spectra.For the first time,the spin-lattice relaxation time (T1) of protons of the chain-ends is revealed to be affected more by polymer molecular weight (MW) than by the protons of the main-chains and the side-chains (almost independent from MW).As a result,a much higher delay time (d1) for chain-ends (d1 ＞ 20T1) is needed for quantitative NMR measurement when using end-group estimation method to obtain the MW of PS,which is in accordance with the value estimated by GPC.An improved method for the polymer MW determination is established,by combination of different NMR techniques to distinguish the peaks,and a large dl setting to achieve quantitative NMR analysis.     

Ligand-observed in-tube NMR in natural products research: A review on enzymatic biotransformations,protein–ligand interactions,and in-cell NMR spectroscopy

Natural product-observed NMR methods have considerably expanded the potentialities for in-tube NMR monitoring of complex enzymatic biotransformations and investigation of protein-natural product interactions even in living cells. We review, herein, the significant advantages of ligand-observed in-situ NMR monitoring of enzymatic biotransformations without restoring to laborious and time-consuming chromatographic methods. Emphasis will be given to the potentialities of the use of the NMR bioreactor: (i) to investigate through saturation transfer difference (STD), the capacity of natural products to serve as enzyme substrates, (ii) to monitor multiple biotransformation products of natural products with the use of immobilized enzymes and (iii) to investigate interactions of biotransformed products with protein targets. The use of STD and its variants, transfer effect Noes for PHArmacophore Mapping (INPHARMA) NMR, in conjunction with computational methods, can provide excellent tools in investigating competitive binding modes even in proteins with multiple binding sites. The method has been successfully applied in the study of unsaturated free fatty acids (UFFAs)-serum albumin complexes in which the location and conformational states of UFFAs could not be determined accurately, despite numerous X-ray structural studies, due to conformational averaging. This combined method, thus, may find promising applications in the field of protein-natural product recognition research. The emerging concept of in-cell NMR and recent applications will be discussed since they can provide atomic level insights into natural product-protein interactions in living cells without the need of isotope labelled techniques.     

Competition STD NMR for the detection of high-affinity ligands and NMR-based screening

The reported competition STD NMR method combines saturation transfer difference (STD) NMR with competition binding experiments to allow the detection of high-affinity ligands that undergo slow chemical exchange on the NMR time-scale. With this technique, the presence of a competing high-affinity ligand in the compound mixture can be detected by the disappearance or reduction of the STD signals of a low-affinity indicator ligand. This is demonstrated on a BACE1 (beta-site amyloid precursor protein cleaving enzyme 1) protein-inhibitor system. This method can also be used to derive an approximate value, or a lower limit, for the dissociation constant of the potential ligand based on the reduction of the signal intensity of the STD indicator, which is illustrated on an HSA (human serum albumin) model system. This leads to important applications of the competition STD NMR method for lead discovery: it can be used (i) for compound library screening against a broad range of drug targets to identify both high- and low-affinity ligands and (ii) to rank order analogs rapidly and derive structure-activity relationships, which are used to optimize these NMR hits into viable drug leads.     

The application of quantitative NMR for the facile,rapid and reliable determination of clindamycin phosphate in a conventional tablet formulation

Spectroscopic tools such as NMR can be applied to the quantitative analysis of active pharmaceutical ingredients with relative ease and accuracy. Here, we demonstrate the quantification of clindamycin phosphate (CLP) in a conventional tablet formulation, performed using potassium hydrogen phthalate (KHP) as the internal standard and deuterium oxide (D₂O) as the NMR solvent. The methyl protons signal of CLP at 0.72 ppm (triplet) relative to the signal of KHP at 7.37–7.40 ppm (multiplet) was used for quantification purposes using ¹H NMR. This method was shown to be specific and linear (r = 0.9997) within the CLP concentration range from 7.2 to 23.1 mg per 0.5 ml of D₂O. The maximum relative standard deviation (RSD) of accuracy and precision was calculated at 0.39% and 0.64%, respectively. The limits of detection (LOD) and quantification were 0.04 and 0.11 mg/ml, respectively. The method was highly stable with a calculated RSD of 0.03%. The robustness of the method was demonstrated by changing four different parameters, and the difference among each parameter was ≤ 0.78%. The findings of this work were in good agreement with previously reported conventional HPLC‐based approaches, highlighting its applicability in the determination of other active pharmaceutical ingredients in conventional formulations for quality control purposes. Copyright © 2014 John Wiley & Sons, Ltd.     

Application of a new method for simultaneous phase and baseline correction of NMR signals (SINC)

Recently, we presented a new approach for simultaneous phase and baseline correction of nuclear magnetic resonance (NMR) signals (SINC) that is based on multiobjective optimization. The algorithm can automatically correct large sets of NMR spectra, which are commonly acquired when reactions and processes are monitored with NMR spectroscopy. The aim of the algorithm is to provide spectra that can be evaluated quantitatively, for example, to calculate the composition of a mixture or the extent of reaction. In this work, the SINC algorithm is tested in three different studies. In an in-house comparison study, spectra of different mixtures were corrected both with the SINC method and manually by different experienced users. The study shows that the results of the different users vary significantly and that their average uncertainty of the composition measurement is larger than the uncertainty obtained when the spectra are corrected with the SINC method. By means of a dilution study, we demonstrate that the SINC method is also applicable for the correction of spectra with low signal-to-noise ratio. Furthermore, a large set of NMR spectra that was acquired to follow a reaction was corrected with the SINC method. Even in this system, where the areas of the peaks and their chemical shifts changed during the course of reaction, the SINC method corrected the spectra robustly. The results show that this method is especially suited to correct large sets of NMR spectra and it is thus an important contribution for the automation of the evaluation of NMR spectra.     

Solution-State 2D NMR Spectroscopy of Mixtures HyperpolarizedUsing Optically Polarized Crystals

The HYPNOESYS method (Hyperpolarized NOE System), which relies on the dissolution of optically polarized crystals, has recently emerged as a promising approach to enhance the sensitivity of NMR spectroscopy in the solution state. However, HYPNOESYS is a single-shot method that is not generally compatible with multidimensional NMR. Here we show that 2D NMR spectra can be obtained from HYPNOESYS-polarized samples, using single-scan acquisition methods. The approach is illustrated with a mixture of terpene molecules and a benchtop NMR spectrometer, paving the way to a sensitive, information-rich and affordable analytical method.     

Quantitative determination and validation of avermectin B1a in commercial products using quantitative nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance is defined as a quantitative spectroscopic tool that enables a precise determination of the number of substances in liquids as well as in solids. There is few report demonstrating the application of NMR in the quantification of avermectin B_1a (AVB_1a); here, a proton nuclear magnetic resonance spectroscopy (¹H NMR) using benzene [1‐methoxy‐4‐(2‐nitroethyl) (PMN)] as an internal standard and deuterochloroform as an NMR solvent was tested for the quantitative determination of AVB_1a. The integrated signal of AVB_1a at 5.56 ppm and the signal of PMN at 8.14 ppm in the ¹H NMR spectrum were used for quantification purposes. Parameters of specificity, linearity, accuracy, precision, intermediate precision, range, limit of detection (LOD), limit of quantification (LOQ), stability and robustness were validated. The established method was accurate and precise with good recovery (98.86%) and relative standard deviation (RSD) of assay (0.34%) within the linearity of the calibration curve ranging from 5.08 to 13.58 mg/ml (R² = 0.9999). The LOD and LOQ were 0.009 and 0.029 mg/ml, which indicated the excellent sensitivity of the method. The stability of the method was testified by a calculated RSD of 0.11%. The robustness was testified by modification of four different parameters, and the differences among each parameter were all less than 0.1%. Comparing with the assay described by the manufacturer of avermectin tablets, there was no significant difference between the assay obtained by HPLC and quantitative NMR (qNMR), which indicated qNMR was a simple and efficient method for the determination of AVB_1a in commercial formulation products. Copyright © 2014 John Wiley & Sons, Ltd.     

One‐step synthesis of 6‐acetamido‐3‐(N‐(2‐(dimethylamino) ethyl) sulfamoyl) naphthalene‐1‐yl 7‐acetamido‐4‐hydroxynaphthalene‐2‐sulfonate and its characterization with 1D and 2D NMR techniques

A one‐step method was reported for the synthesis of 6‐acetamido‐3‐(N‐(2‐(dimethylamino) ethyl) sulfamoyl) naphthalene‐1‐yl 7‐acetamido‐4‐hydroxynaphthalene‐2‐sulfonate by treating 7‐acetamido‐4‐hydroxy‐2‐naphthalenesulfonyl chloride with equal moles of N, N‐dimethylethylenediamine in acetonitrile in the presence of K₂CO₃. The chemical structure of the obtained compounds was characterized by MS, FTIR, ¹H NMR, ¹³C NMR, gCOSY, TOCSY, gHSQC, and gHMBC. The chemical shift differences of ¹H and ¹³C being δ 0.04 and 0.2, respectively, were unambiguously differentiated. Copyright © 2013 John Wiley & Sons, Ltd.     

(29)Si NMR in LC-NMR analysis of silicones

It is demonstrated using a practical example that indirect detection of (29)Si NMR signals is sufficiently sensitive in LC-NMR stop-flow arrangement to analyze mixtures of siloxane polymers. New cryogenic probes with better signal-to-noise ratio will turn this version of LC-NMR into a routine method for analysis of siloxane polymers.     

Optimal flip angle for robust practical quantitative NMR measurement using a fixed pulse length

NMR quantification has been traditionally performed by using internal standards. Although methods using external reference in NMR quantification have been developed, the major obstacles in using external referencing method are the measurement deviations associated with changing sample conditions and the requirement of pulse width calibration for every sample in order to compensate these errors. The calibration process is time consuming and in some cases impossible. We developed a quantitative NMR method fixed pulse length (FIXPUL) for all measurements without sample-by-sample calibration. The method is based on the use of an optimal flip angle calibrated for an external standard so that the quantitative errors associated with the pulse width variations are minimized. FIXPUL can be implemented on most basic NMR spectrometers and is robust and easily automated. The method is applicable to a wide range of solution NMR samples in chemistry, biology, and drug research and discovery.     

31P NMR assessment of the phosvitin-iron complex in mayonnaise

Lipid oxidation is the main reason for the limited shelf life of mayonnaise. One of the main catalysts of this process is iron, which is introduced in its ferric (Fe(III)) form via phosvitin, an egg yolk phosphoprotein rich in phosphoserines. The binding of Fe(III) to phosvitin and its ability to establish a redox couple with Fe(II) is believed to determine the oxidation rate of unsaturated lipids. In this work, a ³¹P NMR based method was developed to quantify loading of phosvitin with Fe(III) and its reductive release. Both features could be quantified in model phosvitin solutions by exploiting the paramagnetic broadening of ³¹P NMR signal of phosphoserine residues by Fe(III). This method was then successfully applied to quantify the phosvitin-Fe(III) loading in mayonnaise water phase by liquid NMR, whereas ³¹P NMR MAS could only provide a qualitative measure. The ³¹P NMR method showed a direct relation between loading of the Fe(III)-phosvitin complex and lipid oxidation.     

The coupling constant polarizability and hyperpolarizabilty of 1J(NH) in N-methylacetamide, and its application for the multipole spin-spin coupling constant polarizability/reaction field approach to solvation

We present a benchmark study of a combined multipole spin-spin coupling constant (SSCC) polarizability/reaction field (MJP/RF) approach to the calculation of both specific and bulk solvation effects on SSCCs of solvated molecules. The MJP/RF scheme is defined by an expansion of the SSCCs of the solvated molecule in terms of coupling constant dipole and quadrupole polarizabilities and hyperpolarizabilities derived from single molecule ab initio calculations. The solvent electric field and electric field gradient are calculated based on data derived from molecular dynamics (MD) simulations thereby accounting for solute-solvent dynamical effects. The MJP/RF method is benchmarked against polarizable QM/MM calculations for the one-bond N-H coupling constant in N-methylacetamide. The best agreement between the MJP/RF and QM/MM approaches is found by truncating the electric field expansion in the MJP/RF approach at the linear electric field level. In addition, we investigate the sensitivity of the results due to the choice of one-electron basis set in the ab initio calculations of the coupling constant (hyper-)polarizabilities and find that they are affected by the basis set in a way similar to the coupling constants themselves.     

Structural elucidation by 1D and 2D NMR of three isomers of a carotenoid lysophosphocholine and its synthetic precursors

A carotenoic acid was used to obtain a long-chain unsaturated lysophosphocholine. The carotenoid lysophosphocholine was synthesized by two methods. The first method resulted in mixtures of regioisomers for each step in the synthetic route. Homo- and heteronuclear 1D and 2D NMR methods were employed to elucidate the structures of the individual isomers and their intermediates. The pure regioisomer [1-(beta-apo-8'-carotenoyl)-2-lyso-glycero-3-phosphocholine] was obtained by a second method, but in low yield. The 1D 1H NMR subtraction spectrum of the mixture and the pure regioisomer was used to interpret the 1H shifts of the unsaturated acyl moieties. The 1H and 13C signals of the acyl chain show characteristic shifts depending on the positions of the choline and the acyl group attached to the glycerol backbone. Therefore, the unsaturated acyl chain signals have diagnostic values for the identification of isomers of unsaturated (lyso)phosphocholines. Chemical shifts and indirect coupling constants are reported for each of the major components of the mixtures. The methods used were 1D (1H, 13C and 31P) and 2D (H,H-COSY, HMBC, HSQC and HETCOR) NMR.     

Assignment of 13C resonances of a nematic liquid crystal using off-magic angle spinning

A novel method for assigning the resonances in the 13C NMR spectrum of a static liquid crystalline sample in its nematic phase is proposed. The method is based on the fact that the carbon chemical shifts in the isotropic phase and in the oriented phase under static and off-magic angle spinning (OMAS) conditions are uniquely related by the tensorial property of the CSA tensor, requiring just one OMAS spectrum and the assignment in the isotropic phase. A computational procedure is proposed to take into account deviations arising out of non-ideal experimental conditions and the assignments are made by identifying the minimum in the differences in the frequencies between calculated and experimental line positions. Practical implementation of the method has also been demonstrated in the case of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline.     

Phenylselenenylmenthane derivatives and their enantiomeric discrimination by 1H and 13C NMR spectroscopy in the presence of a chiral dirhodium complex

The phenylselenenylmenthane and ‐menthene derivatives 1–4 were studied in terms of ¹H and ¹³C signal assignments, conformational analysis and also complexation shifts (Δδ) and dispersion effects (Δν) observed when non‐racemic (ca 2 : 1) mixtures of the chiral selenides were exposed to an equimolar amount of the chiral dirhodium complex Rh^*. The complexation site is the selenium atom exclusively. Whereas Δδ values are moderate or small, dispersed signals (split into two owing to the existence of diastereomeric adducts) can be observed, many of which are large enough for a facile determination of enantiomeric ratios of the selenides irregardless of the conformational behaviour of the selenides and the adduct composition. Thus, the ‘dirhodium method’ is simple and reliable for chiral recognition in the class of organoselenium compounds which is gaining increasing importance in developing new techniques of asymmetric synthesis. Copyright © 2002 John Wiley & Sons, Ltd.     

一种金属洗涤剂的NMR分析

由于精细化工行业的迅速发展,作为“工业味精”的表面活性剂,已广泛进入日化、轻工、纺织、建筑、石化、金属加工等生产领域,对其的分析也显得日益重要。表面活性剂的分析历来是比较困难的课题之一,而通过核磁共振(NMR)进行定性和定量的文献则更少[1]。本文利用NMR为多种表面活     

Improved method for unambiguous amino acid side-chain 1H and 13C resonance assignment

Side-chain proton and carbon-13 resonance assignments of [13C;15N]-enriched proteins usually rely on combinations of several multi-dimensional experiments. Here, we describe a four-dimensional pulse sequence, H(C)C-COSY-TOCSY-(CACO)NH, which provides the information required to assign completely aliphatic side-chain resonance frequencies. As in widely used HCC(CO)NH-TOCSY experiments, problems due to spectral crowding are alleviated by exploiting the dispersion of backbone amide 1H and 15N signals. The modification introduced here allows signals from different side-chains to be distinguished even in the case of overlap in the 1H(N)-15N plane of the spectra. For illustration, the new method is applied to two proteins with molecular masses of 11 and 23 kDa.     

DFT and multinuclear magnetic resonance studies of diazenedicarboxylates and related compounds

A number of diazenedicarboxylates have been studied by multinuclear magnetic resonance ((17)O, (15)N, (13)C) and compared with analogous fumaric, maleic, and phthalic diesters; the investigation of selected compounds of these classes was complemented by density functional theory (DFT) calculations using a polarizable continuum model (PCM) for the solvent, employing the PBE0 functional together with the 6-311G(d,p) basis set for geometry optimization, and the 6-311 + G(2d,p) basis set for calculating the NMR shielding using the gauge-including atomic orbital (GIAO) method. This combined approach provided important information about the preferred conformations in chloroform and their influence on the NMR parameters.     

Quantitative Analysis of Clove Oil by NMR Spectrometry

A procedure utilizing NMR spectrometry in the quantitative analysis of clove oil is described. The ¹H-NMR spectrum is determined with acetophenone as internal standard, the ¹³C-NMR spectrum is determined with chloroform as internal standard and 0.1M Cr(acac)₃ as relaxation reagent. The contents of eugenol are compared to the results obtained from G.C. (90.8%, s.d. 0.7%) and from chemical method (89.6% s.d. 1.2%). The result obtained from ¹H-NMR method is 90.4% (s.d. 0.3%) and from ¹³C-NMR method is 90.5% (s.d. 0.7%). The time required for the ¹H-NMR method is about 20mins and for the ¹³C-NMR method is about 2-3hours. Apparently the ¹H-NMR method proves to be more simple, rapid and accurate.     

Is high‐resolution magic angle spinning NMR a practical speciation tool for cheese samples? Parmigiano Reggiano as a case study

High-resolution magic angle spinning (HRMAS) NMR is probably the most apt NMR method to analyze complex materials involving a solid phase, e.g. foodstuffs. We present here an HRMAS analysis of grated cheese (Parmigiano Reggiano). A full NMR characterization of this cheese allows the identification of the presence of fatty acids (saturated and unsaturated), amino acids and other small organic molecules. Since the presence and relative concentration of these molecules have previously been shown to correlate with organoleptic, origin and age characterization, HRMAS NMR of cheese is likely to provide a good complimentary tool for the analysis of this food material.