Conformational studies of cyclic pentapeptides by proton magnetic resonance spectroscopy

The proton magnetic resonance spectra of c-(-Gly-L -Ala-Gly-Gly-L -Pro-) (I) and four analogous cyclopentapeptides are presented. At ambient temperature the spectra contain two sets of resonances which correspond to two different molecular conformations of the peptides. The relative concentrations of the two forms depend on the peptide, the solvent, and the temperature. For the two molecular species of peptide I in DMSO solution, the NMR. data imply that the peptide linkage involving the nitrogen of proline is respectively in the cis- and the trans-form, and both conformations contain intramolecular hydrogen bridges. Replacement of L -alanine in I by L -cysteine leaves the molecular conformations essentially unalteed. On the other hand substitution of L -proline by L -proline, or replacement of the two glycines in positions 3 and 4 by two sarcosyl residues gives rise to markedly different types of peptide backbone conformation.     

Indirect detection of magnetic resonance by heteronuclear two-dimensional spectroscopy

A new technique is proposed to indirectly record with high sensitivity the nuclear magnetic resonance of nuclei with a low gyromagnetic ratio. The method relies on a coherent transfer of transverse magnetization to nuclei of high gyromagnetic ratio.     

Identification of isomeric amides of itaconic acid by proton magnetic resonance spectroscopy

Two half amides of itaconic acid, ( 1 ) and ( 2 ), have been synthesized and identified by proton magnetic resonance spectroscopy as a preliminary to an interpretation of the mechanism of action of succinate thiokinase on guanosine triphosphate substrates.     

Determination of acid dissociation constants of histidine-containing peptides by proton magnetic resonance spectroscopy

The acid dissociation constant of the imidazolium ring of the decapeptide luliberin has been determined by ¹H NMR-followed titration in D₂O. The normal procedure for the analysis of the titration curve, i.e. direct use of the Henderson-Haselbalch equation, is still applicable in this case, but for more complex peptides a modified calculation procedure is proposed. Results obtained when both methods were applied to luliberin are compared. The influence of D₂O when used as the solvent in this type of determination has been studied using N^α-acetyl-L -histidine methyl ester as a model compound. The difference between the acid dissociation constant of this molecule determined in H₂O and in D₂O implies that a correction of ?.25 unit is needed for those pK_a values calculated by plotting the chemical shifts in D₂O vs the apparent pH meter readings. The pK_a found for N^α-acetyl-L-histidine methyl ester, 6.30 ± 0.04, can be taken as a standard value for histidine-containing peptides.     

The detection of hidden proton magnetic resonance signals by means of INDOR

The INDOR technique appears to be a very useful and rapid tool for the detection of hidden proton lines as an aid in the elucidation of the structure of complex organic molecules. The experimental conditions for obtaining optimum results in a limited time are discussed. Analysis of the INDOR spectra of a number of selected spin systems are given; these show the capabilities of this method. Examples are given in which this technique is applied.     

Use of in vivo magnetic resonance spectroscopy for studying metabolic diseases

Owing to the worldwide obesity epidemic and the sedentary lifestyle in industrialized countries, the number of people with metabolic diseases is explosively increasing. Magnetic resonance spectroscopy (MRS), which is fundamentally similar to magnetic resonance imaging, can detect metabolic changes in vivo noninvasively. With its noninvasive nature, ¹H, ¹³C and ³¹P MRS are being actively utilized in clinical and biomedical metabolic studies to detect lipids and important metabolites without ionizing radiation. ¹H MRS can quantify lipid content in liver and muscle and can detect other metabolites, such as 2-hydroxyglutarate, in vivo. Of interest, many studies have indicated that hepatic and intramyocellular lipid content is inversely correlated with insulin sensitivity in humans. Thus, lipid content can be utilized as an in vivo biomarker for detecting early insulin resistance. Employing ¹³C MRS, hepatic glycogen synthesis and breakdown can be directly detected, whereas ³¹P MRS provides in vivo adenosine triphosphate (ATP) synthesis rates by saturation transfer methods in addition to ATP content. These in vivo data can be very difficult to assess by other methods and offer a critical piece of metabolic information. To aid the reader in understanding these new methods, fundamentals of MRS are described in this review in addition to promising future applications of MRS and its limitations.     

High-resolution proton nuclear magnetic resonance studies of urine from asphyxiated newborn infants

High-resolution proton nuclear magnetic resonance spectroscopy was used to study human urine obtained from 10 normal babies and twenty babies with various degrees of neonatal asphyxia, respiratory distress syndrome (RDS), and meconium aspiration syndrome (MAS). All sick babies showed different degrees of oxygen deficiency, indicated by an obvious increase of the lactate signal level in the urine spectra. Changes in the concentration of other urinary metabolites produced from the citric acid cycle were also observed. In extremely serious cases, the signals of some of the major components, including citrate, α-ketoglutarate, and succinate, simply disappeared. The spectra of urine, serum, and CSF of an infant suffering from SIDS showed common characteristics of the metabolites.     

Reexamination of reduced pteridine cofactors behaviour by proton nuclear magnetic resonance spectroscopy and mass spectrometry

In aqueous solutions, the autoxidation by air of 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetra-hydropteridine, a hydroxylase cofactor, leads to the corresponding 7,8-dihydro derivate. Oxidation by hydrogen peroxide and Horseradish peroxidase does not give a stable quinonoid form as previously claimed but affords two metabolic products. Kinetics of the two pathways and structures of the different final compounds were determined by ¹H nmr and mass spectrometry.     

The conformational study of the new ergot alkaloids agroclavine-I and epoxyagroclavine-I by proton magnetic resonance spectroscopy

The 100 MHz p.m.r. data on the new ergot alkaloids agroclavine-I and epoxyagroclavine-I, metabolites of fungus Penicillium kapuscinskii strain UCM F-2156D, are reported. Their C and D rings are shown to exist in an “envelope” form. A cis orientation of the C/D ring junction is confirmed for both alkaloids.     

Bridging the gap between metabolic profile determination and visualization in neurometabolic disorders: a multivariate analysis of proton magnetic resonance in vivo spectra

The purpose of this work was to check the degree of overlap between rare inborn errors of metabolism and other neurological disorders using principal component analysis of proton magnetic resonance spectroscopy (¹H MRS) in vivo data. We examined 60 patients (median age of 22 months). Fourteen of them were diagnosed with neurometabolic disorders (three cases of metachromatic leukodystrophy, two cases of Canavan disease, two cases of megalencephalic leukoencephalopathy with subcortical cysts, three cases of mitochondrial cytopathy, one case of nonketotic hyperglycinemia, one case of globoid leukodystrophy, one case of congenital disorders of glycosylation, and one case of ethylmalonic encephalopathy). The remaining 46 patients were diagnosed with epilepsy, cerebral palsy, and developmental delay. Results obtained from principal component analysis of complete unresolved ¹H MRS in vivo spectra were interpreted parallelly with LCModel‐derived metabolite levels. The main attention was paid to the following metabolites: N‐acetylaspartate, glutamate + glutamine, creatine, choline, myo‐inositol signal with an uncertain contribution of glycine, and glucose. ¹H MRS in vivo coupled with multivariate analysis is an efficient tool in visualization of metabolic abnormalities in several inborn errors of metabolism (metachromatic leukodystrophy, globoid leukodystrophy, megalencephalic leukoencephalopathy with subcortical cysts, and Canavan disease). Copyright © 2013 John Wiley & Sons, Ltd.     

Quality evaluation and species differentiation of Rhizoma coptidis by using proton nuclear magnetic resonance spectroscopy

Rhizoma coptidis, a broadly used traditional Chinese medicine, derives from the dried rhizomes of Coptis chinensis Franch, Coptis deltoidea C.Y. Cheng et Hsiao and Coptis teeta Wall. Quantitative determination of protoberberine alkaloids in R. coptidis is critical for controlling its quality. In this study, a rapid, simple and accurate quantitative ¹H NMR (qNMR) method was developed for simultaneous determination of berberine, jatrorrhizine, epiberberine, coptisine, palmatine and columbamine in R. coptidis from the three species. Method validation was performed in terms of selectivity, precision, repeatability, stability, accuracy, robustness and linearity. The average recoveries obtained were in the range of 96.9–102.4% for all the six alkaloids. In addition, the qNMR data were analyzed with analysis of variance (ANOVA), hierarchical clustering analysis (HCA) and principal component analysis (PCA), and the results showed that the contents of the active alkaloids have significant difference among the three species. Compared with the conventional HPLC approach, the proposed qNMR method was demonstrated to be a powerful tool for quantifying the six alkaloids due to its unique advantages of high robustness, rapid analysis time and no need of standard compounds for calibration curves preparation. These findings indicate that this method has potential as a reliable method for quality evaluation of herb medicines, especially for protoberberine alkaloid-containing ones.     

Absolute quantitative analysis for sorbic acid in processed foods using proton nuclear magnetic resonance spectroscopy

An analytical method using solvent extraction and quantitative proton nuclear magnetic resonance (qHNMR) spectroscopy was applied and validated for the absolute quantification of sorbic acid (SA) in processed foods. The proposed method showed good linearity. The recoveries for samples spiked at the maximum usage level specified for food in Japan and at 0.13 g kg⁻¹ (beverage: 0.013 g kg⁻¹) were larger than 80%, whereas those for samples spiked at 0.063 g kg⁻¹ (beverage: 0.0063 g kg⁻¹) were between 56.9 and 83.5%. The limit of quantification was 0.063 g kg⁻¹ for foods (and 0.0063 g kg⁻¹ for beverages containing Lactobacillus species). Analysis of the SA content of commercial processed foods revealed quantities equal to or greater than those measured using conventional steam-distillation extraction and high-performance liquid chromatography quantification. The proposed method was rapid, simple, accurate, and precise, and provided International System of Units traceability without the need for authentic analyte standards. It could therefore be used as an alternative to the quantification of SA in processed foods using conventional method.     

Simultaneous determination of theophylline and caffeine by proton magnetic resonance spectroscopy using partial least squares regression techniques.

A 1H-NMR procedure based on an analysis of its data by a multivariate calibration method was conducted for the simultaneous determination of theophylline and caffeine in synthetic and real samples. Partial least squares regression (PLS) was chosen as the calibration method. The methyl signals of theophilline at 3.36 and 3.54 ppm that overlapped with those of caffeine were significant characteristics which were employed in this study for their analyses. The proposed method was successfully applied to recovery studies of theophylline and caffeine from real tablet samples.     

A study of imidazole by the proton magnetic resonance method

The concentration dependence of the chemical shifts of the protons of imidazole in six polar organic solvents and water has been studied. In aqueous solutions an exchange of the NH protons between the imidazole and the water molecules takes place. In organic solvents, the position and half-width of the NH line of imidazole depend on the concentration. With an increase in the concentration, this line shifts in the downfield direction. The shift (1 ppm for a solution of imidazole in DMSO) may be due to the formation of an ordinary hydrogen bond (> NH N ) between the imidazole molecules.Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 6, No. 6, pp. 810–813, June, 1970.