N-NMR

Broad lines, low sensitivity, and low resonance frequency have for a long time prevented NMR spectroscopy from being applied to the study of amino acids and peptides. However, with the advent of high field superconducting solenoids and accumulation techniques, these difficulties are less formidable. 

In a recent study 321) the shifts and linewidths of aqueous solutions of amino acids, peptides, and derivatives have been correlated. It was seen that the groups directly bonded to the a-nitrogen atom determine the region in which the shift occurs, but other groups in the molecule, or intermolecular interactions, may cause small perturbations within that region. The extreme of the observed shifts in the oc-amino acids are 332 and 346 ppm (with trimethyl-phenyl-ammonium chloride chosen as external secondary standard). In the case of tryptophan at pH 7.15 the shift was observed at 343 ppm. 

The secondary amino group of the indole ring in tryptophan has a low field shift (293 ppm), probably because of rapid tautomerism with the C = N indolenine form, in which the nitrogen has a 7i-bonded structure. However, the equilibrium favours the indole form, the deshielding contribution from the ru-electrons in the indolenine form is, therefore, small, moving the chemical shift of the indole nitrogen atom in tryptophan to much higher field than in imidazole, but to lower field than in the saturated counterpart, proline. 

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C-nmr data have been recorded and assigned for a great number of hydantoin derivatives (24). As in the case of H-nmr, useful correlations between chemical shifts and electronic parameters have been found. For example, Hammett constants of substituents in the aromatic portion of the molecule correlate weU to chemical shifts of C-5 and C-a in 5-arylmethylenehydantoins (23). Comparison between C-nmr spectra of hydantoins and those of their conjugate bases has been used for the calculation of their piC values (12,25). N-nmr spectra of hydantoins and their thio analogues have been studied (26). The N -nmr chemical shifts show a linear correlation with the frequencies of the N—H stretching vibrations in the infrared spectra. 

Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

Nuclear magnetic resonance (nmr) spectroscopy is useful for determining quaternary stmcture. The N-nmr can distinguish between quaternary ammonium compounds and amines, whether primary, secondary, or tertiary, as well as provide information about the molecular stmcture around the nitrogen atom. The C-nmr can distinguish among oleic, tallow, and hydrogenated tallow sources (194). 

The structure of cytosine is certainly the aminooxo form (60) which has been confirmed by X-ray analyses <73AX(B)1234), UV and NMR data <63JCS3046), Raman spectra <67SA(A)255l) and even by N NMR spectra (72JPC5087). 

The possibility offered by new instruments to obtain N NMR spectra using natural abundance samples has made " N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incomparably better with the N nucleus (/ = ) than with " N (/ = 1). H- N coupling constants could be obtained from natural abundance samples by N NMR and more accurately from N-labelled compounds by H NMR. Labelled compounds are necessary to measure the and N- N coupling constants. 

With improvements in Instrument sensitivity and the use of techniques such as enhancement by polarization transfer (INEPT), it can be expected that natural abundance N NMR spectra will become increasingly Important in heterocyclic chemistry. The chemical shifts given in Table 10 illustrate the large dispersion available in N NMR, without the line broadening associated with N NMR spectra. 

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