A ¹³Cmr study of 2′-deoxynucleotides in the syn and anti conformation

The geminal and vicinal 13C—31P coupling constants for the 3′- and 5′-monophosphates and 3′,5′-diphosphates of thymidine and its isomer 6-methyl-2′-deoxyuridine have been obtained for aqueous solutions at varions pH values. Since the thymine base is anti and the 6-methyluracil base is syn, the data provide information about the influence of the orientation about the N-glycosyl linkage on the conformation about the [Formula: see text] and [Formula: see text] bonds. The 3J(C4′—P5′) couplings reveal a preference for the [Formula: see text] conformer of the 5′-nucleotidyl units, in agreement with previous measurements of 1H—31P coupling constants. The 3J(C4′—P3′) and 3J(C2′—P3′) couplings are consistent with a preference for the [Formula: see text] conformer for both the syn and anti3′-nucleotidyl units. The response of these couplings to secondary ionization of the phosphate is different for the thymine and 6-methyluracil derivatives and suggests a phosphate–phosphate interaction for the syn, but not the anti, 3′,5′-diphosphates. A correlation between the sugar-pucker and the [Formula: see text] orientation is revealed. The 2J(C5′—P5′) and 2J(C3′—P3′) are relatively insensitive to the ionization state of the phosphate as well as to other structural features of the deoxyribose phosphate backbone.