Conformational Transmission in Four- and Five-Coordinated Phosphorus Compounds. Solvent Effects on the C4'-C5' Conformation in 5'-Phosphorylated Model Nucleosides

Article abstract of DOI:10.1021/ja00299a045

The C_4'-C_5' conformation of the 5'-P(IV) model compounds 1.3 and the 5'-P(V) TBP model compounds 4-6 has been determined in various solvents with 300- and 500-MHz 1H NMR.It is found that lowering the solvent polarity results in a substantial increase of the g^- populations for the model compounds 1, 2, 4, and 5 (e.g., for 1 in D2O, x(g^-)=0.00; CCl4, x(g^-)=0.23).This effect can be attributed to an enhanced electrostatic repulsion between the charge densities on O5 and the endocyclic oxygen(s) at lower solvent polarities.This conclusion is supported by the experimental finding that the 5'-P(IV) system 3 and the 5'-P(V) TBP system 6, in which the endocyclic oxygen(s) are replaced by C(H2), do not show a C_4'-C_5' conformational change when the medium polarity is changed.Furthermore, it is found that the 5'-P(IV) nucleotides 7 and 8, which are made soluble in apolar solvents by triesterification of the phosphorus, show also increased g^- populations upon lowering the solvent polarity.The present results confirm our earlier proposal that enhanced charge repulsion between O_5' and O_1' in mononucleotides drives a rotation around the C_4'-C_5' bond toward g^-.