13C NMR Study of the Natural Glycosides Vicine and Convicine

¹³C NMR parameters have been obtained for vicine and convicine in DMSO, D₂O/DMSO and D₂O. Complete assignment of the spectra has been achieved. Interpretation of spin-lattice relaxation rates and heteronu-clear NOES has yielded evidence of intramolecular structuring in the case of vicine and not in that of ronvirine and also of a complex network of solute-solvent interactions.     

Fast 13C-NMR Spectral Editing for Determining CHn Multiplicities

With an improved version of the ¹³C DEPTQ NMR experiment all carbon multiplicities (C_q, CH, CH₂ und CH₃) can be identified unequivocally and most conveniently in two experiments and at best with only one scan each. This simplifies the analysis of ¹³C-NMR data on a routine level in general. With higher sample amounts and/or exploiting the high sensitivity of cryogenically cooled ¹³C probeheads the efficiency of such investigations may be improved. This makes this method attractive for fast ¹³C analysis of small-to-medium sized molecules in high-throughput laboratories.     

A 13C-NMR Study of Some 9-Ahinoacridines and Their Cations in Relation with Tautomaric Problems

¹³C-NMR chemical shifts have been measured for some 9-aminoacridine derivatives, substituted on positions 9 and 10, in DMSO and for some related salts in D₂O. The results are discussed with regard to amino/imino tautomerism, structure of the cations, and slow rotation about a formal C-N double bond.     

Structural Assignment of Isomeric S‐ and S,N‐1‐Alkoxycarbonylalkylated 6‐Amino‐2‐thiouracil Derivatives by Means of 1H and 13C NMR Spectroscopy

Abstract

The structures of new isomeric 2‐alkoxycarbonylalkylthio‐ and 2‐alkoxy‐ carbonylalkylthio‐1‐alkoxycarbonylalkyl‐6‐aminouracils (1–21) have been established on the basis of the ¹H NMR and ¹³C NMR spectroscopic data. The ¹H NMR and ¹³C NMR spectra of 1–21 have been fully assigned by a combination of two‐dimensional experiments [heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond correlation (HMBC)]. The ¹³C NMR spectra have been shown to be able to differentiate between isomers.     

13C-NMR Spectroscopy of para-Substituted Benzylideneacetones.: I. Long Distance Electronic Effects

para-benzylideneacetones present a characteristic long distance charge transfer pattern, where the olefinic bridge (CH=CH) and the aromatic ring (Ph) carbon centers are perturbed according to the nature of the para-substituent groups.

By means of ¹³C-NMR spectroscopy and AMl molecular orbital calculations we have found that in this molecular series the chemical shifts (Δ) and the charge densities (qAMI) corresponding to the C₃, C₁ and C_β centers follow a functional dependence of the type: Δ = a qAMl + Δ°, while C₂, C_α and C_CO are practically constants.

On the other hand, after a complete spectral assignment of the ¹³C-NMR signals, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C₄ center, has permitted us to find a good correlation between the C₄ spectral shift and the electronegativity of this vicinal center.     

13C NMR Chemical Shifts of Heterocycles: 3∗. Empirical Substituent Effects in 2-Halomethyl-2-hydroxy-tetrahydrofurans and -5,6-tetrahydro-4H-pyrans

Abstract

Evaluation by empirically derived equations for the Substituent effect (α, β, γ, δ) on the ¹³C NMR chemical shifts for C-2, C-3, C-4, C-5, C-6, the halomethyl-substituted carbon (C-7) and the cyano or oxymic carbon (C-8) in 2-halomethyl-2-hydroxy-tetrahydrofurans 1a-c, 2, 3a, b, 4a and -5,6-tetrahydro-4H-pyrans 5a-c, 6a [with C-2-substituents (R²): CF₃, CCl₃ or CHCl₂, C-3-substituents (R³): CN, C(Me)=NOH, CH=NOMe, C(Me)=NOMe or CH=NOH], taking as reference the 2-trifluoromethyl-2-hydroxy-tetrahydrofuran (la), is reported. From the additivity properties of the α-, β-, γ-, δ-and ?-effects for each Substituent it is possible to predict the chemical shift of each carbon of the compounds 1–6.

     

DEPTQ: Distorsionless Enhancement by Polarization Transfer Including the Detection of Quaternary Nuclei

A pulse sequence DEPTQ yielding the signals and multiplicity information for all carbon types including the signals of quaternary carbons and encompassing all the known advantages of the basic DEPT experiment is proposed. Its behavior has been studied theoretically and experimentally and has been compared critically with alternative methods dedicated for the same purpose. Its marked insensitivity to experimental parameters and its potential for complete and efficient spectral editing makes DEPTQ the ideal experimental platform for a semi- or fully automated analysis of 1D¹³C spectra.     

13C NMR Chemical Shifts and CH Coupling Constants of Substituted 4-Ylidenebutenolides

The ¹³C chemical shifts and CH coupling constants for 6 substituted 4-ylidenebutenolides are reported. The CH coupling constants are useful in helping to determine the stereochemistry of the substituted exocyclic double bond.     

13C-Nmr Spectroscopy of β-Nitrostyrenes. II. Mono-, Bi- and Tri-Methoxy Phenyl-Substitutions and Long Distance Electronic Effects

By means of ¹³C-NMR spectroscopy and AM1 molecular orbital calculations of mono-, bi- and tri-methoxy-β-nitrostyrenes at the meta and para positions, we have characterized a long distance electronic charge transfer pattern on the ethylenic bridge (CH=CH) and on the aromatic ring (Ph) carbon centers, determined by the electron-donor nature of the methoxy-substituent groups.

After a complete spectral assignment of the ¹³C-NMR signals, we have found a functional dependence of the chemical shifts on the C₁ and C_β centers respect to the C₄ and C₃ methoxy subtitution sites on the aromatic ring, while in the same molecular series C_alfa-chemical shifts are practically constants. on the other hand, the ¹³C-NMR chemical shifts of the C₃ and C₄ centers plus the analysis of the AM1 electronic charge density have permitted us determine the long distance charge transfer effect induced by the C₄ methoxy substitutions as well as the attenuation of this effect due to the C₃ methoxy substitutions.     

Analysis of 13C-mixed Triacylglycerol in Stool by Bulk (Ea-IRMS) and Compound Specific (GC/MS) Methods

Abstract

This paper was presented in poster form at the 17th International Congress of Nutrition, August 27-31, Vienna. Austria (Annals of Nutrition & Metabolism 2001; 45(Suppl.1):349). Some of the data were also presented in poster form at the British Society of Gastroenterology Meeting, March 18-21, Glasgow, UK (Gut 2001; 48(Suppl.1):A91).

The ¹³C-mixed triacylglycerol (MTG) breath test is used to measure intraluminal fat digestion. In normal digestion. 20–40% of the ingested ¹³C label is recovered in breath CO₂. We aimed to identify the proportions of ingested label excreted in stool, as well as breath following ingestion of ¹³C-MTG by children with impaired exocrine pancreatic function and healthy controls. ¹³C enrichment of breath samples was measured by continuous flow isotope ratio mass spectrometry (IRMS) and cumulative percent dose recovered (cPDR) in 10 h was calculated. Total ¹³C of a faecal fat extract from each stool was measured by elemental analyser-IRMS, and ¹³C enrichment and concentration of the TBDMS derivative of octanoic acid was measured by GC/MS after hydrolysis of the fat extract. Stool 5-day cPDR was calculated. Mean breath cPDR was 35%. Mean cPDR in stool by combustion-IRMS and GC/ MS, respectively, was 0.8% and 1.0%. Therefore, the remaining 64% of the ¹³C label must remain in the body and variability in breath cPDR is due to postabsorptive rather than predigestive factors.     

13C-Leucin-Atemtest zur Beurteilung der anabolen Verwertung von diätetischen Aminosäurengemischen bei Phenylketonurie

It is essential to establish whether and how environmental factors affect the reliability of [¹³C]methacetin breath test (¹³C-MBT). In 12 healthy volunteers (smokers), a standard ¹³C-MBT with 75 mg [¹³C]methacetin was performed twice in random order: on a control day without smoking and on another day with smoking two cigarettes antecedently. A considerable flattening of the curve of the momentary ¹³C recovery within the expiratory air was observed when the ¹³C-MBT was performed after smoking. The maximum of the momentary ¹³C recovery, D _max, decreased from 37.20±2.58 to 25.39±2.29% dose/h (p=0.00052). Moreover, the time to reach D _max was prolonged after cigarette smoking (26.5±3.1 vs. 16.5±1.9 min, p=0.0199). The curve of the cumulative ¹³C recovery on the cigarette smoking day appeared to be shifted downwards, and statistically significant differences relative to the control situation were found between the 24th and 75th minute following [¹³C]methacetin administration. Smoking cigarettes immediately prior to the ¹³C-MBT diminishes the ability of the liver to handle methacetin, and hence a possibility of such an interaction should be excluded in order to interpret the results of the test correctly.     

Scrutiny of 13C-phenylalanine breath test reproducibility

We evaluated the reproducibility of the ¹³C-phenylalanine breath test (¹³C-PheBT). On three separate days, 21 healthy volunteers (11 F and 10 M) underwent ¹³C-PheBT with 100 mg l-[1-¹³C]phenylalanine taken orally. Short-term reproducibility was evaluated with paired examinations taken 3 days apart; paired examinations separated by 23 days (median) served for the medium-term reproducibility assessment. Expiratory air was sampled at 19 points throughout 3?h. Determined limited reproducibility of the ¹³C-PheBT must be taken into consideration while interpreting the results of this diagnostic tool. The results of this study imply the following conclusions: (i) From among the three parameters examined, the cumulative ¹³C recovery area under the curve (AUC) offers much better reproducibility than the maximum momentary ¹³C recovery in the expiratory air (D_max) or the time to reach the maximum momentary ¹³C recovery (T_max) (ii) Collection of the breath air samples for 2?h results in a much better reproducibility of AUC, than for 1?h only; (iii) Reproducibility of ¹³C-PheBT is affected neither by the duration of the time gap between repeated tests nor by gender; (iv) Comparison with data obtained formerly reveals that reproducibility of the ¹³C-PheBT is worse than either that of of the ¹³C-methacetin (¹³C-MBT) or the ¹³C-alpha-ketoisocaproic acic (¹³C-KICA-BT) breath tests. This finding will have to be taken into consideration while interpreting the results of this diagnostic tool.     

13CH3OH and13CD3OH optically pumped fir laser: New large offset emission and optoacoustic spectroscopy

27 new, large offset, FIR laser lines from¹³CH₃OH and one from¹³CD₃OH have been discovered by pumping with a high tunability waveguide CW CO₂ laser. Optoacoustic measurements of isotopic methyl alcohol have also been performed and the pump offsets of the new and of previously known lines have been measured and checked. Frequency tunability by Stark effect has been observed for 6 strong lines. Some assignments are discussed.     

Diode laser absorption spectra of HCCD and HCCD at 6500 cm

The absorption spectra of H¹²C¹³CD and H¹³C¹²CD have been observed at high resolution between 6480 and 6610 cm⁻¹ using an external cavity diode laser. The strong 2ν₁ band has been observed for each species using a sample enriched in deuterium at natural abundance of ¹³C. Rotational analyses reveal bands of both species to be essentially unperturbed. Centers of unblended lines are determined with an accuracy of approximately 10 MHz.     

Biomedical Application of an Isotope Selective Nondispersive Infrared Spectrometer for 13CO2 and 12CO2 Concentration Measurements in Breath Samples

A newly developed isotope selective nondispersive infrared (NDIR) spectrometer for the measurement of ¹³CO₂ and ¹²CO₂ concentrations in breath samples was applied as a low cost and very simple to operate alternative to isotope ratio mass spectrometry (IRMS). We used this device for several biomedical applications ([¹³C]urea breath test, [¹³C]leucine metabolism, [¹³C]methacetin catabolism of rats) and found that the results agree very well with IRMS.     

Carbon Isotope Analysis in Urea at High 13C-Abundances Using the 13/12CO2-Breath Test Device Fanci2

Abstract

The increasing application of ¹³C-labelled urea in medicine requires simple and reasonable methods for measuring highly enriched ¹³C in urea. The combination: ultimate organic analysis—mass spectrometry so far prescribed is complicated and expensive. For medical diagnosis, however, isotope selective nondispersive infrared spectrometers (NDIRS) have been available for many years. One of these tools is FANci2 which is very reasonable and easily to be operated. By means of such devices also urea highly enriched in ¹³C can be analysed, provided that the samples are first diluted with a defined amount of urea of natural isotopic composition and then transformed into carbon dioxide by means of urease. The relative abundance of ¹³C in this carbon dioxide, measured by nondispersive infrared spectrometry, is then a measure of the ¹³C abundance in the initial urea sample. Comparison of results of such measurements with those attained by mass spectrometry proves that this procedure is feasible and yields precise results.     

Line positions and energy levels of the O substitutions from the HDO/D2O spectra between 5600 and 8800 cm

Absorption spectra of HDO/D₂O mixtures recorded in the 5600-8800 cm⁻¹ region with a total pressure of water from 13 up to 18 hPa and an absorption path length of 600 m have been analyzed in order to obtain new spectroscopic data for HD¹⁸O and D₂¹⁸O. In spite of the low natural ¹⁸O concentration (about 2×10⁻³ with respect to the ¹⁶O one), about 1100 transitions belonging to HD¹⁸O and more than 280 transitions belonging to D₂¹⁸O have been assigned. Most of the D₂¹⁸O transitions belong to the ν₁+ν₂+ν₃ and 2ν₁+ν₃ bands. Sets of energy levels for seven vibrational states of D₂¹⁸O and four states of HD¹⁸O are reported for the first time. The comparison of the experimental data with the calculated values based on Partridge-Schwenke global variational calculations is discussed.     

Heteronuclear Multiple-Quantum Coherence NMR Spectroscopy of Im-cyt C

Some ¹³C chemical shifts of the CH_n groups in the aliphatic side chains of Im-cyt c have been determined for the first time based on the H chemical shifts of their attached protons with the aid of heteronuclear multiple-quantum coherence (HMQC) spectroscopy. Comparison of chemical shifts of these specifically assigned ¹³C and H resonances from Im-cyt c with those from cyt c indicates that ¹³C-NMR spectra may provide an opportunity to probe the electronic structure and conformational changes induced by axial ligand substitution.     

The C1 IIg-A1 IIu Emission Spectrum of 12C13C

A few bands of the C¹II_g -A¹II_u system of ¹²C¹³C molecule have been photographed and five of them have been rotationally analyzed. Molecular constants for v = 0, 1 and 2 in the C¹ II_g and in A¹II_u states have been obtained using a nonlinear least-squares procedure in which all analyzed bands were fitted simultaneosly.     

Effect of alcohol consumption on the liver detoxication capacity as measured by [13C]methacetin- and [methyl-13C]methionine-breath tests

The aim of this study was to investigate the hepatic microsomal and mitochondrial functions by using the ¹³CO₂-breath test in healthy subjects either before or after the consumption of red wine. Fourteen adults received [¹³C]methacetin and [methyl-¹³C]methionine together with a standardised dinner. Expired air samples were taken over 6 h. After a wash-out period, the subjects consumed 0.4 ml ethanol/kg/day together with dinner over a 10-day period. Thereafter, ¹³C-tracer administration was repeated under identical conditions. The ¹³CO₂-enrichments were measured by isotope ratio mass spectrometry. The mean cumulative percentage ¹³C-dose recovery (CPDR) after administration of [¹³C]methacetin and [methyl-¹³C]methionine either without or with red wine consumption amounted to 38.2±6.3 vs. 36.3±6.7% (p=0.363) and 9.5±3.3 vs. 8.8±2.5% (p=0.47), respectively. Moderate alcohol consumption does not induce significant short-term changes of the microsomal and the mitochondrial functions of the human liver in healthy subjects.