C enrichment

Figure 9.1. 395 MHz M NMR spectra of 2-iiorbornyl 50 MHz proton decoupled C NMR spectra cation in SbF5/S02CIF/S02F2 solution of 2-norbornyl cation ( C enriched) in SbF5/S02ClF/S02F2 solution.

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

An elegant NMR experiment by the group of Lynn Jelinski at Cornell University has established that at least part of the microcrystals is built up from the polyalanine repeats in the protein chains. These experiments, which were made on C-enriched proteins produced by feeding the spiders C-labeled alanine, showed that there were two populations of alanine side chains, one ordered and oriented perpendicular to the fiber axis and a second less ordered. Jelinski s interpretation is that parts of the polyalanine sequences are incorporated as p strands in the microcrystals with an orientation parallel to the fiber axis. Whether or not the Gly-Gly-X repeats also form P strands in the microcrystals remains an open question. 

Boutton, T.W. 1991 Tracer studies with C-enriched substrates humans and large animals. In Coleman, D.W. and Fry, B., eds.. Carbon Isotope Techniques. Academic Press, San Diego 219-242. 

Carnivores rely on a protein-rich diet and produce new biomass primarily from dietary amino acids, although the enzymes required for de novo amino acid synthesis are present (Garmes et al., 1998). Bone collagen, muscle (meat) and apatite were analyzed for a set of modern southern African herbivores and carnivores (Lee-Thorp et al., 1989). The isotopic analyses showed i C enrichment in bone collagen, apatite and muscle, and depletion in lipids. Difference in values between herbivores and carnivores indicates a trophic effect, which for carbon in bone collagen is 2.5-3%o (Fig. 2). 

Not all of the spectral data are given, The C-enriched-methyl resonance of the di[ C]methylSer residue overlaps with this resonance. 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Tetrahydrophenanthrene is the angular benzologue of Tetralin. Both natural and 10% 1- C-enriched H Ph were... 

Fig. 2 also compares the relative N/C enrichments for an initial 40 Mq star. For high mass stars (> 30 Mq), higher Z models have the larger enrichments, i.e. a situation opposite to that described above. The reason is that mass loss effects dominate over mixing. Higher Z stars have a higher mass loss, which... 

Scheme 8.1 Synthesis of a sialyl-Lewis tetrasaccharide employing C-enriched protecting groups for the quantitative reaction monitoring using gated decoupling NMR spectroscopy.

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

Fig. 3.11. Comparison of the molecular ion signals, M , of [60]fullerene with natural isotopic abundance and of C-enriched Cgo- By courtesy of W. Kratschmer, Max Planck Institute for Nuclear Physics, Heidelberg.

FTIR and NMR. FTIR spectra of the O-alkylated extracts were obtained (KBr pellets) using a Nicolet lODX spectrometer. The O-methylated extract was prepared using C-enriched methyl iodide (99% C) to enhance the NMR signal of the methyl carbon in the extract. The spectrum was obtained on a pyridine solution of the extract using a Varian XL 300 NMR Spectrometer. Proton decoupling techniques were used and the data was collected in a quantitative mode. 

The O-methylated extract was derivatized with 99 percent C-enriched methyl iodide. The NMR spectrum of the extract showed a strong absorption at 56 ppm (relative to TMS), which can be attributed to unhindered aryl methyl ethers (67%), a smaller absorption at 60 ppm, attributable to hindered aryl methyl ethers (23%), and a resonance at 51 ppm, assignable to methyl esters derived from carboxylic acids (10%).(16) TTiese results are consistant with those of Liotta l al, who studied the alkylation of whole Illinois No. 6 coal.(12)... 

The distribution of 5 C-values with water depth is mainly controlled by biological processes Conversion of CO2 into organic matter removes C resulting in a C enrichment of the residual DIG. In turn, the oxidation of organic matter releases C-enriched carbon back into the inorganic reservoir, which results into a depth-dependent isotope profile. A typical example is shown in Fig. 3.21. 

Jeffrey et al. (1983) interpreted this trend as the loss of labile, C-enriched amino acids and sugars through biological reworking which leaves behind the more refractory, isotopically light lipid components. 

Methane extracted from air bubbles in polar ice up to 350 years in age has a 8 C-value which is 2%c lower than at present (Craig et al. 1988). This may indicate that anthropogenic burning of the Earth s biomass may be the principal cause of the recent C enrichment in methane. 

Stratospheric methane collected over Japan gave a 8 C-value of —47.5%c at the tropopause and increased to —38.9%c at around 35 km (Sugawara et al. 1998). These authors suggested that reaction with Cl in the stratosphere might be responsible for the C-enrichment. 

Figure 11.6 2D /-resolved solid-state NMR spectrum of [(=SiO)Ta(=CH Bu)(CH2 Bu)2], 10% C enriched at the R positions ( ). Traces extracted along the 8i dimension of the 2DJ-resolved spectrum at different carbon chemical shift frequencies 31,47, 95 and 247 ppm. (Reprinted from Reference [18].)... 

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