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By comparative NMR study of 2′,3′-dideoxynucleosides (see ref 1) with or , we have been able to quantify for the first time the competing medium-dependent influences of the 3′-OH promoted gauche and the aglycone-configuration dependent anomeric effects that result in the overall drive of the sugar conformation in 2′-deoxynucleosides. It has been shown that although the pKas of the nucleobases in α- and are identical, the transmission of the free-energy of protonation-deprotonation equilibria to steer the sugar conformation is not the same, indeed it is finely tuned by the balance between the 3′-gauche and anomeric effect. It has emerged that the counteracting 3′-OH gauche effect reduces the influence of the pH-dependent anomeric effect, thereby limiting the conformational flexibility of with respect to the corresponding . |
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