Carbon-13 NMR spectra of nucleoside 3′,5′-cyclic phosphates. Proposition for revised signal assignments

On the basis of the data obtained from ¹³C NMR spectra of 8,2′-S-cycloadenosine 3′,5′-cyclic phosphate and other nucleoside 3′,5′-cyclic phosphate analogues, it is suggested that the published assignments of the C-3′ and C-4′ signals in nucleoside 3′,5′-cyclic phosphates should be reversed. According to the revised assignments, C-4′, which is fixed very closely to the diesterified phosphate group is markedly shielded (?12.5 to ?15 ppm), and the C-3′ signal shows a downfield shift (+6 to +8 ppm) which is comparable to that for the C-5′ signal, for all compounds so far measured when compared with the chemical shifts for the corresponding nucleosides. The 3′,5′-cyclic phosphates of thymidine and 8,2′-S-cycloadenosine, which have no α-OH group on C-2′, show similar chemical shift changes for the corresponding sugar carbons which are different from those observed for ribonucleoside derivatives.