Proton noise

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

Carbon-13 nuclei, due to their low natural abundance, do not interact with each other in a molecule, though they are affected by adjacent protons. In practice, such couplings are removed by irradiation of the whole spectrum as it is recorded, in a technique known as proton noise decoupling. This means that practical NMR spectra exhibit one unsplit signal for each type of carbon atom present in the sample. 

Proton-noise decoupled and single-frequency off-resonance decoupled carbon-13 NMR spectra were determined for the CTC Working Standard (Figure 13). 

Figure 5. Proton noise-decoupled 22.6-MHz C-13 NMR spectrum of the hydrogenated 1,4-polyisoprene sample. Perdeuteriohenzene solution at 25°C with TMS as internal reference. Approximately 5000 pulses with an acquisition time of 0.7 sec and a flip angle of 30°.

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Figure 1 shows the proton noise-decoupled C-NMR spectrum of a polytetrahydrofurein (polytetramethylene ether glycol, PTMEG) dissolved in THF. In this spectrum the carbons numbered 1, 2 and 3 which cure a to the oxygen appear at lower field them the 6-carbons labeled as 4, 5 and 6. The carbon atoms in the polymer are clearly resolved from the corresponding carbons of the THF monomer. The fact that carbons 3 and 4 near the hydroxyl end-groups can be easily identified shows the excellent resolution of this technique. 

The cyclopentenebromonium ion 181 was also obtained via protonation of 4-bromocyclopentene in HS03F-SbF5-S02ClF solution at 120°C, through the reaction sequence shown in Eq. (4.125).413 The proton noise-decoupled 13C NMR spectrum of the cyclopentenebromonium ion 181 shows three carbon resonances at 813C 114.6 (doublet, 7C-h = 190.6Hz), 31.8 (triplet,. /cll 137.6Hz), and 18.7 (triplet, 7c-h= 134.0 Hz). 

The 1H NMR spectra of parbendazole was recorded with a JEOL-PS 100 NMR spectrometer operating at a frequency of 100 MHz and a magnetic field strength of 2.349 T. Spectra were determined over the region 10.8-0.0 parts per million (ppm), with a sweep time of 250 s. Chemical shifts were recorded as S (delta) ppm downfield from tetra-methylsilane (TMS). Proton noise and off-resonance decoupled 13C NMR spectra were measured on a JEOL FX 90Q Fourier Transform NMR spectrometer operating at 90 MHR and spectral width of 5000 Hz (220 ppm). All measurements were obtained with the compound being dissolved in deuterated dimethyl sulfoxide (DMSO-d6) for dT NMR and in deuterated trifluoroacetic acid (TFA-dx) for 13C NMR. 

The natural abundance carbon-13 NMR spectrum was recorded under the same experimental conditions except that the frequency was 75.46 MHz. Because of the low solubility of silver sulfadiazine in DMSO-dg an incomplete spectrum was obtained. The chemical shifts observed in the proton-noise decoupled spectrum are given in Table II (11). Again the chemical shifts of silver sulfadiazine compared to sulfadiazine are shifted slightly to high field, e.g. 4.5ppm and less (11). Carbon-13 NMR data of sulfadiazine are reported by Chang et al. (13). 

The 145.7 MHz proton noise decoupled 31p nmR spectra of the poly(dA-dT) duplex in 10 mM cacodylate buffer between 28° and 54°C are presented in Figure 9. A broad symmetrical unresolved resonance is observed at 28°C. By contrast, two resolved narrow resonances separated by 90.2 ppm have been observed for 150 base pair long (dA-dT)n (41). Thus, though the resolution of dT dA and dApdT phosphodiesters cannot be achieved at the synthetic DNA level in solution (18), it has been observed for the same sequence at a shorter well defined length (41). More recently, two resolved 31p resonances have also been reported in poly(dA-dT) fibers oriented parallel to the direction to the magnetic field by solid state 3lp nmr spectroscopy (42). 

Figure 9. The proton noise decoupled 145.7-MHz 3,P NMR spectra of polyfdA-dT) in 0.1 M cacodylate, lOmM EDTA, 2H.O, pH 7.08 between 28° and 54°C ft,., of complex is 60°C). The scale is upheld from standard trimethylphosphate.

Figure 24. The proton noise decoupled 145.7-MHz 31P NMR spectra of (A) poly-(dA-dT) in 1M NaCl, lOmM cacodylate, lOmM EDTA, H20, pH 6.2 at 65°C and (B) the proflavine poly(dA-dT) complex, Nuc/D = 10, in 1M NaCl, lOmNi cacodylate, lOmM EDTA, 2H20 at 65°C. The scale is upfleld from standard trimethylphosphate.

Phosphodiester Linkages The proton noise decoupled lp nmr spectra of the daunomycin poly(dA-dT) complex in 1 M NaCl solution at 67°C have been recorded at 1 antibiotic per 6 base pairs (Nuc/D = 11.8) and 1 antibiotic per A,3 base pairs (Nuc/D = 5.9). Resolved resonances are observed for the complex at both Nuc/D ratios (Figure 32). One of the resonances in the complex exhibits a chemical shift similar to that observed for poly(dA-dT) in 1 M NaCl alone ( 4.1 ppm) at this temperature while the other resonance is shifted downfield by 0.3 ppm in the Nuc/D = 11.8 complex and by 0.45 ppm in the NucD = 5.8 complex (Table XI). The results suggest that daunomycin intercalates at either the dTgdA or dApdT sites, resulting in a downfield shift of the 31p resonance of the corresponding phosphodiester grouping at the intercalation site. 

Figure 32. The proton noise decoupled 145.7-MHz 3,P NMR spectra of the daunomycin poly(dA-dT) complex in IM NaCl, 10/nM cacodylate, WmM EDTA, 80% H/O-20% D O. Spectrum A corresponds to the Nuc/D = 11.8 complex, pH 6.0 at 67°C and Spectrum B corresponds to the Nuc/D — 5.9 complex, pH 6.05 at 67°C. The chemical shifts are upfield from standard trimethyl-...

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