Assignment of the Purine Resonances

The assignments of the purine resonances [460, 749, 751, 758, 759, 761] of proton broadband-decoupled 13C NMR spectra of nucleosides and nucleotides are made using the following aids [749, 761] Correlation with the signals of the parent bases and analogous nucleosides or nucleotides comparison of the 13C NMR spectrum with that of a specifically analogue correlations of the chemical shift values with the 7r-electron densities [749, 761] and, in addition, proton off-resonance decoupling, which affords the identification of quaternary carbon atoms. 

Some of the common purine nucleosides are symbolized by the following formula  

Converting purine to the nucleoside nebularine causes considerable shifts of resonances in the spectrum of the parent base (cf Table 5.22). 

Comparison of the spectrum of inosine with that of guanosine leads to the assignments of the signals of C-2 and C-4 both resonances are shifted upfield in inosine by 5.4 ppm because of the lack of an electron-withdrawing amino group at C-2. 

The signal of the quaternary carbon atom C-4 of inosine can be easily distinguished from that of C-2 by proton off-resonance decoupling. Comparison of the spectrum of inosine with the spectral data of 8-deuterioinosine and 6-thioinosine leads to the unequivocal signal assignment of the base residue of this nucleoside [749] (see Table 5.22). 

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