Combination Of 13C NMR Spectroscopy Research Articles

13C NMR study on carbamate hydrolysis reactions in aqueous amine/CO2 solutions

Carbamates of primary or secondary amines play a central role in amine CO2 absorbents. The equilibrium constant of a carbamate hydrolysis reaction, Kch, is important for evaluating the performance of absorbents. However, few Kch values have been reported to date. We determined the Kch values of normal amines, such as monoethanolamine (MEA), diethanolamine (DEA), and hindered amines, such as 2-amino-2-methyl-1-propanol (AMP) and 2-(isopropylamino)ethanol (IPAE), using a method involving a combination of 13C NMR spectroscopy and a chemical equilibrium analysis. The Gibbs energy changes derived from Kch for AMP and IPAE at 25 °C were negative, whereas those for normal amines, MEA and DEA, were positive. These properties are characteristic of hindered amines. We determined the standard enthalpy and entropy changes of the carbamate hydrolysis reactions from the temperature dependence of the equilibrium constant. The equilibrium constants and standard enthalpies obtained using the present method will be valuable for evaluating the performance of amine CO2 absorbents.

Trimorphism of Betamethasone Valerate: Preparation, Crystal Structures, and Thermodynamic Relations

The glucocorticoide bethamethasone valerate was investigated for polymorphism. The commercially available modification (Form I) and one methanol solvate were characterized by single crystal structure analysis. DTA-TG and DSC investigations reveal that on solvent removal the methanol solvate decomposes into a new polymorphic modification (Form III), which, upon further heating, transforms into another modification (Form II) via melting and recrystallization. The crystal structures of forms II and III were determined by a combination of 13C NMR spectroscopy and X-ray powder diffraction. In all modifications as well as in the methanol solvate differences in the conformation and the packing of the molecules are observed. Solvent mediated conversion experiments reveal that form I represents the thermodynamically stable form at room temperature. Thermomicroscopic, XRPD, and heating rate dependent DSC measurements on forms I–III do not show any interconversion of these forms via the solid. Further DSC measurements indicate both a higher melting point and heat of fusion for form I compared to form II and form III. Therefore, form I should also be the thermodynamically stable form over the whole temperature range and all forms seems to be monotropically related.

Generation of an S-Peptide via an N–S Acyl Shift Reaction in a TFA Solution

Abstract An N–S acyl shift reaction of thiol-containing peptides in a trifluoroacetic acid (TFA) solution was confirmed by a combination of 13C NMR spectroscopy, reversed-phase (RP) HPLC, and MS analyses. A model peptide containing a cysteine residue was transformed into an S-peptide in a TFA solution. The S-peptide was quickly transformed to the original peptide during RP-HPLC analysis even when an eluent that contained 0.1% TFA was used. A peptide that had an N-2-mercapto-4,5-dimethoxybenzyl (Dmmb) group was also transformed into an S-peptide, forming a thioester bond with the thiol of the Dmmb group by TFA treatment. The generated S-peptide was isolated by directly injecting the reaction mixture into a RP-HPLC and was readily converted to the corresponding 2-sulfoethyl thioester via an intermolecular thiol exchange reaction with sodium 2-mercaptoethanesulfonate. The 2-sulfoethyl peptide thioester is widely used as a building block in polypeptide synthesis. The N–S acyl shift reaction observed in peptides with or without an auxiliary group provides a new route for the preparation of peptide thioesters.

Ontogeny and Cellular Localization of the Pyruvate Recycling System in Rat Brain

The ontogeny of the cerebral pyruvate recycling pathway and the cellular localization of associated enzymes, malic enzyme (ME) and phosphoenolpyruvate carboxykinase (PEPCK), have been investigated using a combination of 13C NMR spectroscopy, enzymatic analysis, and molecular biology approaches. Activity of the pathway, using [1,2-(13)C2]acetate as a substrate, was detected by 13C NMR in brain extracts 3 weeks after birth, increasing progressively up to the third month of age. In whole-brain homogenates, ME activity increased to adult levels with the same time course as the recycling pathway. PEPCK activity was low during the first 2 weeks of life and decreased further toward adulthood. ME and PEPCK activity were found in primary cultures of astrocytes and in synaptosomal fractions of adult brain. Primary cultures of cortical neurons showed PEPCK activity but no detectable ME activity. The cytosolic ME gene was expressed in primary cultures of neurons and in astrocytes as well as in the neonatal and adult brain. The PEPCK gene was expressed both in primary cultures of cortical neurons and in astrocytes, but the level of its expression in the neonatal and adult brain was undetectable.

Glucose metabolism distal to a critical coronary stenosis in a canine model of low-flow myocardial ischemia.

Myocardial regions perfused through a coronary stenosis may cease contracting, but remain viable. Clinical observations suggest that increased glucose utilization may be an adaptive mechanism in such "hibernating" regions. In this study, we used a combination of 13C-NMR spectroscopy, GC-MS analysis, and tissue biochemical measurements to track glucose through intracellular metabolism in intact dogs infused with [1-13C]glucose during a 3-4-h period of acute ischemic hibernation. During low-flow ischemia [3-13C]alanine enrichment was higher, relative to plasma [1-13C]glucose enrichment, in ischemic than in nonischemic regions of the heart, suggesting a greater contribution of exogenous glucose to glycolytic flux in the ischemic region (approximately 72 vs. approximately 28%, P < 0.01). Both the fraction of glycogen synthase present in the physiologically active glucose-6-phosphate-independent form (46 +/- 10 vs. 9 +/- 6%, P < 0.01) and the rate of incorporation of circulating glucose into glycogen (94 +/- 25 vs. 20 +/- 15 nmol/gram/min, P < 0.01) were also greater in ischemic regions. Measurement of steady state [4-13C)glutamate/[3-13C]alanine enrichment ratios demonstrated that glucose-derived pyruvate supported 26-36% of total tricarboxylic acid cycle flux in all regions, however, indicating no preference for glucose over fat as an oxidative substrate in the ischemic myocardium. Thus during sustained regional low-flow ischemia in vivo, the ischemic myocardium increases its utilization of exogenous glucose as a substrate. Upregulation is restricted to cytosolic utilization pathways, however (glycolysis and glycogen synthesis), and fat continues to be the major source of mitochondrial oxidative substrate.

Steroidal glycosides from Agave cantala

Two new steroidal glycosides, agaveside A and B, isolated from the fruits of Agave cantala were characterized as 3β- O-{β- d-xylopyranosyl-(1→2),β- d-xylopyranosyl-(1→3),β- d-glucopyranosyl-(1→3)-[β- d-xylopyranosyl-(1→3)-β- d-galactopyranosyl-(1→2)]-β- d-glucopyranosyl}-(25 R)-5α-spirostane and 3β- O-{β- d-xylopyranosyl-(1→2),β- d-xylopyranosyl-(1→3)-β- d-glucopyranosyl-(1→3)-[β- d-galactopyranosyl-(1→2)]-β- d-glucopyranosyl}-(25 R)-5α-spirostane. The structures were elucidated by a combination of 13C NMR spectroscopy, chemical degradation and fast atom bombardment mass spectrometry.

Synthesis and 13C-NMR analysis of 5α- and 5β-cholestane-3,6-dione

Starting from cholesterol a simple and efficient synthesis of 5 α-cholestane-3,6-dione and 5 β-cholestane-3,6-dione is described. The 13C shielding data of C-7, C-9, and C-19 in both isomers can be used in the determination of the stereochemistry at C-5 of these compounds. The combination of 13C NMR spectroscopy and the simple synthesis of both isomers offers good opportunities for the determination of the stereochemistry at C-5 of 3,6-dioxosteroids.