Use of chemometrics to characterize tropical wines from different vintages and grape cultivars according to the 1H NMR spectroscopy data.

2011

Metabolic fingerprint of açaí (Euterpe oleracea) berry pulp using 1H NMR spectroscopy and chemometric analysis.

Portanto, o objetivo deste trabalho foi avaliar a influência de diferentes cultivares e sazonalidade do açaí utilizando a técnica de 1H NMR para criação de perfis de impressão digital associada a métodos quimiométricos, obtidos a partir de frutos geneticamente modificados (cultivada no controle agronômico) e o comércio na cidade de Belém

Interactions of Polyvinylpyrrolidone with Chlorin e6-Based Photosensitizers Studied by NMR and Electronic Absorption Spectroscopy

Polyvinylpyrrolidone (PVP) can act as potential drug delivery vehicle for porphyrin-based photosensitizers in photodynamic therapy (PDT) to enhance their stability and prevent porphyrin self-association. In the present study the interactions of PVP (MW 10 kDa) were probed with five different derivatives of chlorin e6 (CE6) bearing either one of the amino acids serine, lysine, tyrosine or arginine, or monoamino-hexanoic acid as substituent. All derivatives of CE6 (xCE) formed aggregates of a similar structure in aqueous buffer in the millimolar range. In the presence of PVP monomerization of all xCE aggregates could be proved by 1H NMR spectroscopy. xCE-PVP complex formation was confirmed by 1H NMR T2 relaxation and diffusion ordered spectroscopy (DOSY). 1H1H-NOESY data suggested that the xCE uptake into the PVP polymer matrix is governed by hydrophobic interactions. UV–vis absorption and fluorescence emission bands of xCE in the micromolar range revealed characteristic PVP-induced bathochromic shifts. The presented data point out the potential of PVP as carrier system for amphiphilic derivatives of chlorin e6. The capacity of PVP to monomerize xCE aggregates may enhance their efficiency as possible photosensitizers in PDT.

Sensitivity of the NMR density matrix to pulse sequence parameters : a simplified analytic approach

We present a formalism for the analysis of sensitivity of nuclear magnetic resonance pulse sequences to variations of pulse sequence parameters, such as radiofrequency pulses, gradient pulses or evolution delays. The formalism enables the calculation of compact, analytic expressions for the derivatives of the density matrix and the observed signal with respect to the parameters varied. The analysis is based on two constructs computed in the course of modified density-matrix simulations: the error interrogation operators and error commutators. The approach presented is consequently named the Error Commutator Formalism (ECF). It is used to evaluate the sensitivity of the density matrix to parameter variation based on the simulations carried out for the ideal parameters, obviating the need for finite-difference calculations of signal errors. The ECF analysis therefore carries a computational cost comparable to a single density-matrix or product-operator simulation. Its application is illustrated using a number of examples from basic NMR spectroscopy. We show that the strength of the ECF is its ability to provide analytic insights into the propagation of errors through pulse sequences and the behaviour of signal errors under phase cycling. Furthermore, the approach is algorithmic and easily amenable to implementation in the form of a programming code. It is envisaged that it could be incorporated into standard NMR product-operator simulation packages.

Diffusion-sensitive magnetic resonance spectroscopy and imaging in biomedical sciences

We present a mini-review of the development and contemporary applications of diffusion-sensitive nuclear magnetic resonance (NMR) techniques in biomedical sciences. Molecular diffusion is a fundamental physical phenomenon present in all biological systems. Due to the connection between experimentally measured diffusion metrics and the microscopic environment sensed by the diffusing molecules, diffusion measurements can be used for characterisation of molecular size, molecular binding and association, and the morphology of biological tissues. The emergence of magnetic resonance was instrumental to the development of biomedical applications of diffusion. We discuss the fundamental physical principles of diffusion NMR spectroscopy and diffusion MR imaging. The emphasis is placed on conceptual understanding, historical evolution and practical applications rather than complex technical details. Mathematical description of diffusion is presented to the extent that it is required for the basic understanding of the concepts. We present a wide range of spectroscopic and imaging applications of diffusion magnetic resonance, including colloidal drug delivery vehicles; protein association; characterisation of cell morphology; neural fibre tractography; cardiac imaging; and the imaging of load-bearing connective tissues. This paper is intended as an accessible introduction into the exciting and growing field of diffusion magnetic resonance.

NMR (1H, 13C) and MO (ab initio, CNDO/2, EHT) studies on basic and N-protonated hydrazinecarbothioamide

The conformational preferences of hydrazinecarbothioamide (HCTA, H2NNHCSNH2) in its basic and N-protonated (PHCTA, H3NNNHCSNH2) forms have been studied by 1H and 13C NMR spectroscopy and by theoretical LCAO-MO methods (ab initio, CNDO/2 and EHT). The hindered rotation around the C---N bond has been investigated by a total line shape analysis for the thioamide protons and by the three MO methods. Changes in the molecular conformation and electronic structure on protonation are briefly discussed.

A H-1-NMR STUDY OF BILIRUBIN IX-ALPHA SOLUBILIZATION BY CHOLATE MICELLES - APPLICATION OF NUCLEAR OVERHAUSER EFFECTS

The solubilization of bilirubin IX-Alpha in aqueous solution by sodium cholate micelles has been examined by 270 MHz 1H-NMR spectroscopy. Incorporation of bilirubin into the micelles is accompanied by specific shifts of bilirubin vinyl and bridgehead protons and the C18 and C19 methyl groups of the steroid. The observed chemical shifts show a monotonic concentration dependence suggesting that changes in aggregation size are continuous. Nuclear Overhauser effects (NOE) have been shown to be a useful probe or micellization. A 4:1 cholate/bilirubin mixture has been investigated by difference NOE spectroscopy. The observation of intermolecular nuclear Overhauser effects between peripheral protons of bilirubin and cholate are diagnostic of spatially proximate groups. Inter-cholate nuclear Overhauser effects increase in magnitude upon bilirubin incorporation suggesting closer packing of steroid molecules on solubilization of the pigment. Intramolecular nuclear Overhauser effects observed for solubilized bilirubin are consistent with a compact intramolecularly hydrogen-bonded conformation resembling that determined for bilirubin in the solid state.

A 13C-NMR study of non-planar distortions in amides

The technique of 13C-NMR spectroscopy of oriented systems to problems of biological importance has been suggested and used to investigate non-planar distortions in substituted amides—models for peptides. The studies in conjunction with the proton magnetic resonance data on 5N-[13C]methyl[13C]formamide oriented in a nematic solvent provide all the direct dipolar couplings between the interacting nuclei in the system. When the 13C- and the 1H-NMR experiments are performed under non-identical conditions, 22 different direct dipolar couplings are obtained. It is demostrated that they can be used to determine unambiguously non-planar distortions around the nitrogen atom together with other geometrical data and the molecular order.

Molecular Interactions Chlorin Assemblies and NMR Separations

Photosynthesis is a chemical process in which the energy of the light quanta is transformed into chemical energy. Chlorophyll (Chl) molecules play a key role in photosynthesis; they function in the antennae systems and in the photosynthetic reaction center where the primary charge separation (CS) takes place. Bio-inspired mimicry of the CS is an essential unit in dye-sensitized solar cells. Aim of this study was to design and develop electron donor-acceptor (EDA) pairs from Chls and fullerenes (C60) or carbon nanotubes (CNT). The supramolecular approach was chosen, as long synthetic sequences required by the covalent approach lead to long reaction schemes and low yields. Here, a π-interaction between soluble CNTs and Chl was used in EDA construction. Also, a beta-face selective two-point bound Chl-C60 EDA was introduced. In addition, the photophysical properties of the supramolecular EDA dyads were analyzed. In organic chemistry, nuclear magnetic resonance (NMR) spectroscopy is the most vital analytical technique in use. Multi-dimensional NMR experiments have enabled a structural analysis of complex natural products and proteins. However, in mixture analysis NMR is still facing difficulties. In many cases overlapping signals can t be resolved even with the help of multi-dimensional experiments. In this work, an NMR tool based on simple host-guest chemistry between analytes and macromolecules was developed. Diffusion ordered NMR spectroscopy (DOSY) measures the mobilities of compounds in an NMR sample. In a liquid state NMR sample, each of the analytes has a characteristic diffusion coefficient, which is proportional to the size of the analyte. With normal DOSY experiment, provided that the diffusion coefficients of the analytes differ enough, individual spectra of analytes can be extracted. When similar sized analytes differ chemically, an additive can be introduced into the sample. Since macromolecules in a liquid state NMR sample can be considered practically stationary, even faint supramolecular interaction can change the diffusion coefficient of the analyte sufficiently for a successful resolution in DOSY. In this thesis, polyvinylpyrrolidone and polyethyleneglycol enhanced DOSY NMR techniques, which enable mixture analysis of similar in size but chemically differing natural products, are introduced.

Aspects and Applications of Pulse Sequence Design for Solution-State Nuclear Magnetic Resonance Spectroscopy

NMR spectroscopy enables the study of biomolecules from peptides and carbohydrates to proteins at atomic resolution. The technique uniquely allows for structure determination of molecules in solution-state. It also gives insights into dynamics and intermolecular interactions important for determining biological function. Detailed molecular information is entangled in the nuclear spin states. The information can be extracted by pulse sequences designed to measure the desired molecular parameters. Advancement of pulse sequence methodology therefore plays a key role in the development of biomolecular NMR spectroscopy. A range of novel pulse sequences for solution-state NMR spectroscopy are presented in this thesis. The pulse sequences are described in relation to the molecular information they provide. The pulse sequence experiments represent several advances in NMR spectroscopy with particular emphasis on applications for proteins. Some of the novel methods are focusing on methyl-containing amino acids which are pivotal for structure determination. Methyl-specific assignment schemes are introduced for increasing the size range of 13C,15N labeled proteins amenable to structure determination without resolving to more elaborate labeling schemes. Furthermore, cost-effective means are presented for monitoring amide and methyl correlations simultaneously. Residual dipolar couplings can be applied for structure refinement as well as for studying dynamics. Accurate methods for measuring residual dipolar couplings in small proteins are devised along with special techniques applicable when proteins require high pH or high temperature solvent conditions. Finally, a new technique is demonstrated to diminish strong-coupling induced artifacts in HMBC, a routine experiment for establishing long-range correlations in unlabeled molecules. The presented experiments facilitate structural studies of biomolecules by NMR spectroscopy.

The 310 Helical Conformation of the Amino Terminal Decapeptide of Suzukacillin. 270 MHz 'H NMR Evidence for Eight Intramolecular Hydrogen Bonds

The amino terminal suzukacillin decapeptide fragment, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aitbh-eO Me, two pentapeptides Boc-AibPrc-Val-AibVal-OMe and Boc-Ala-AibAla-AibAibOMe, and the tripeptide Boc-Ala-AibAibOMe have been studied by 270-MHz 'H NMR spectroscopy. By use of solvent dependence of chemical shifts in a CDC13-(CD3),S0 system and temperature dependence of amide NH chemical shifts in (CD3),S0, the intramolecularly hydrogen bonded NH groups in these peptides have been identified. The tripeptide possesses one hydrogen bond, both pentapeptides show evidence for three intramolecular hydrogen bonds, and the decapeptide has eight NH groups participating in hydrogen bonding. An Ala( 1)-Aib(2) @ turn is proposed for the tripeptide. Both pentapeptides favor 310 helical conformations composed of three consecutive B turns. The decapeptide adopts a 310 helical conformation with some flexibility at the Va1(5)-Ala(6) segment. The proposed conformations are consistent with the known stereochemical preferences of Aib residues.

Molecular vibrations and conformation of primary dithiocarbamate ester. Infrared and NMR studies on S-methyl dithiocarbamate

The i.r. spectra of a primary dithiocarbamate ester namely, S-methyl dithiocarbamate (SMDTC) and its N-dideuterated compound have been measured between 4000 and 30 cm−1. Spectra in solution and at liquid nitrogen temperature have also been obtained. Assignment of all the fundamentals has been proposed and supported from a full normal coordinate analysis. The band assignments for SMDTC have been compared with those of related molecules and the characteristic bands of primary thioamides are derived. Conformational flexibility of SMDTC has been examined by i.r. and proton NMR spectroscopy. The hindered rotation around the C---N bond has been studied by a complete line shape analysis. The magnitude of ---NH2 and ---CH3 torsional barriers is also estimated from vibrational frequencies and force constants.

The conformation of (μ-butatriene)hexacarbonyldiiron complex from proton NMR studies in three nematic phases

Proton NMR spectroscopy in three different liquid crystals has been used to determine two conformational angles of (μ-butatriene)hexacarbonyldiiron complex, namely the angle between the two CH2 planes and the dihedral angle between the two planes containing four carbon atoms of the butatriene moiety. The values are 44 and 46°, respectively. The direct and the indirect geminal HH couplings are shown to be of the same sign in the liquid crystals with positive diamagnetic anisotropy.

Solution cation-binding properties of segmented polyethylene oxide-A C-13 NMR spectroscopic study

Two segmented polyethylene oxides, SPEO-3 and SPEO-4, were prepared using a novel transetherification methodology. Their structures were confirmed by H-1 and C-13 NMR spectroscopy. The complexation of these SPEO's with alkali-metal ions in solution was investigated by C-13 NMR spectroscopy. The mole-fraction method was used to determine the complexation ratio of SPEO with LIClO4 at 25 degrees C, which showed that these formed 1:1 (polymer repeat unit/salt) complexes. The association constant, K, for the complex formation was calculated from the variation of the chemical shift values with salt concentration, using a standard nonlinear least-square fitting procedure. The maximum change in chemical shift (Delta delta) and the K values suggest that both SPEO-3 and SPEO-4 formed stronger complexes with lithium salts than with sodium salts. Unexpectedly, the K values were found to be different, when the variation of delta of different carbons was used in the fitting procedure. This suggests that several possible complexed species may be in equilibrium with the uncomplexed one. Structurally similar model compounds were also prepared and their complexation studies indicated that all of them also formed 1:1 complexes with Li salts. Interestingly, it was observed that the polymers gave higher K values suggesting the formation of more stable complexes in polymers when compared to the model analogues. (C) 2000 John Wiley & Sons, Inc.

Conformations of dehydrophenylalanine containing peptides. NMR studies on three tripeptides with a central dehydrophenylalanyl residue.

Three tripeptides containing a central Z-dehydrophenylalanine residue (Δz-Phe), Boc-L-Phe-Δz-Phe-X-OMe (X = L-Val 1, L-Leu 2 and X = L-Ala 3) have been synthesized and their solution conformations investigated by 270 MHz 1H NMR spectroscopy. In all three peptides, conformations involving the X residue NH in an intramolecular hydrogen bond were favoured in CDCl3 solutions. Studies of the nuclear Overhauser effect (NOE) provided support for a Type II β turn conformation in these peptides with Phe and Δz-Phe occupying the i + 1 and i + 2 positions, respectively. Significantly different conformations lacking any intramolecular hydrogen bonds were observed for peptide 1 in (CD3)2SO. NOE results were consistent with a significant population of molecules having semi-extended conformations (ø > 100°) at the Δz-Phe residue.