Chemical shift proton resonance assignment

The H NMR spectram (CDClj, 500 MHz) of 12 showed two singlets (8 0.83 and 8 0.95), each integrating for three protons due to the C-18 and C-19 methyl protons. Three 3H doublets at 8 0.78 (J= 6.5 Hz), 8 0.79 (J= 6.5 Hz) and 8 0.85 (J = 7.0 Hz) were due to the secondary C-26, C-27 and C-21 methyl protons, respechvely. The C-3 methine proton resonated as a one-proton double doublet at 8 3.63 (JJ= 10.5 Hz and J2= 3.5 Hz) and its downfield chemical shift value was indicative of the presence of a geminal hydroxyl funchonality. A one-proton mulhplet at 8 5.21 was ascribed to the C-6 olefinic proton. The C-28 exocyclic methylene protons appeared as two broad singlets at 8 5.40 and 5.58. The C-NMR spectram (CDCl, 125 MHz) showed the resonance of all 28 carbon atoms. The combination of H and C-NMR data suggested that compound 12 has a sterol like structure as most of the H and C-NMR chemical shift values of 12 were similar to those of sterols reported in the literature [19, 20]. The H and C-NMR chemical shift values were assigned with the aid of COSY-45 , HSQC and HMBC spectral data. Compound 12 was found to have modest inhibitory activity against C. xerosis and S. aureus with minimal inhibitory concentration values of 82.35 and 146 pg/ml, respectively. 

The potential of the method of using mutant and chemically modified Hbs to assign proton resonances of Hb is greatly enhanced when correlated with both intensity measurements and calculations from the crystal structure of Hb as determined by X-ray diffraction. A representative example of this approach is El 1 Val in HbCO A. Based on intensity measurements on a mixture of HbCO A and ferrocy-tochrome c, Lindstrom etal. (1972b) have shown for the ring-current-shifted proton resonances at —5.86, -6.48, and -6.58 ppm from HDO that each arises from one CH3 per a(3 dimer. In the spectrum of a mixture of HbCO A and HbCO Sydney [(367(E1 l)Val — Ala],... 

A fully characterised nmr spectrum is always a useful piece of data and can also be used as evidence of purity. A spectrum is particularly useful when a compound has most of its proton resonances in the same region of the spectrum. The level of assignment will depend on how well the spectrum can be interpreted. It may simply be a list of chemical shifts, it may be a list of chemical shifts with signals assigned as CH3, CH2, CH or C, or you may be able to give actual assignments to individual carbon atoms. 

The proton nuclear magnetic resonance spectrum of lomefloxacin mesylate obtained in D2O at 25° C is given in Figure 5 (9). The spectrum was obtained on a Bruker AM-500 NMR Spectrometer operating at 500.13 MHz and was referenced to external TSP [3-(trimethylsilyl)propionic-2,2,3,3-d4 acid]. The chemical shifts and spectral assignments are provided in Table 2 (9,10). The effect of increasing concentrations of Al3+ on the... 

Llsinoprll, Proton Magnetic Resonance Assignments Chemical Shift. 8H (ppm) Assignment ... 

As shown in Figure 7.2, an NMR spectrometer consists of a strong magnet, a radio station for sending pulses and a radio frequency receiver to detect the FID, and a computer to control acquisition, record and transform the data. Frequencies for a given nucleus are referenced to a standard, for example the proton peak of tetramethylsilane is assigned 0 frequency and all proton peaks are related to that position in terms of chemical shifts in parts per million (ppm). Chemical shifts place resonances of different types in characteristic regions for example aromatic protons appear at 6-7 ppm, methyl protons at 0.5-1.5 ppm, amide protons at 7.5-9 ppm, etc. The representation of the acquisition of a proton. 

On the other hand, very recently Asakura et al. reported application of high-resolution H SS NMR performed under F-MAS and GIPAW chemical shift calculations for assignment of resonances and structure of alanine tripeptides [168]. The information on the exact H positions is important for fine refinement of peptides because their higher order structure is determined mainly by the intramolecular and intermolecular hydrogen bonds. The homonuclear DQMAS experiment allowed to precisely assign proton signals in the H spectra of (Ala)3 (Fig. 2.34). 

The broad-band decoupled C-NMR spectrum of ethyl acrylate shows five carbon resonances the DEPT (6 = 135°) spectrum displays only four signals i.e., only the protonated carbons appear, since the quaternary carbonyl carbon signal does not appear in the DEPT spectrum. The CH and CH3 carbons appear with positive amplitudes, and the CHj carbons appear with negative amplitudes. The DEPT (6 = 90°) spectrum displays only the methine carbons. It is therefore possible to distinguish between CH3 carbons from CH carbons. Since the broadband decoupled C spectrum contains all carbons (including quaternary carbons), whereas the DEPT spectra do not show the quaternary carbons, it is possible to differentiate between quaternary carbons from CH, CHj, and CH3 carbons by examining the additional peaks in the broad-band spectrum versus DEPT spectra. The chemical shifts assigned to the various carbons are presented around the structure. 

The one-bond HETCOR spectrum and C-NMR data of podophyllo-toxin are shown. The one-bond heteronuclear shift correlations can readily be made from the HETCOR spectrum by locating the posidons of the cross-peaks and the corresponding 5h and 8c chemical shift values. The H-NMR chemical shifts are labeled on the structure. Assign the C-NMR resonances to the various protonated carbons based on the heteronuclear correlations in the HETCOR spectrum. 

Figure 6.12 A 3D heteronuclear HMQC-NOESY spectrum of a tripeptide. The (o,-axis represents N chemical shifts, whereas <1)2- and (Uj-axes exhibit proton chemical shifts. The assignment pathways are indicated in the top spectrum for reference purposes, not as part of the 3D experiment. (Reprinted from J. Mag. Reson. 78, S. W. Fesik and E. R. P. Zuiderweg,. 588, copyright (1988), with permission from Academic Press, Inc.)...

Hill et al. (127) reported the first proton magnetic resonance work on a series of cobalamins. This work was carried out at 60 MHz and the spectra are, therefore, of quite low resolution. Subsequently, this work was extended to a wider variety of molecules and also spectra were recorded at 100 MHz (128). Five low field resonances and those of the metal alkyl groups were assigned. Some representative chemical shifts for the low field resonances are shown in Table 1. 

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