Determination of the Tautomeric Equlibrium of ψ-Uridine in the Basic Solution

The pD-dependent ¹³C shift and ³JCH of ψ-uridine have shown that the pK_a of its N1 and N3 are 9.6 and 9.3, respectively. For the monoanionic ψ-uridine, the N1-anion population (64%) is favored over the N3-anion (36%), suggesting a larger preference of the population for N1-Hg²⁺ complex for ψ-uridine over its N3-Hg²⁺ complex in the neutral pH, which has also been proven by the similarity of the pD-dependent titration curve of ψ-uridine and its N3-methyl-ψ-uridine. Finally, ab initio molecular orbital calculations with a 6-31G** basis set (Gaussian 94) have been used to explain why the Nl-anion tautomer of ψ-uridine is thermodynamically more stable than the N3-anion tautomer of ψ-uridine, which cannot be corroborated by the results of the calculations at 6-31+G** and 6-31++G** levels. The fact that N1 of ψ-uridine (pK_a 9.3) is more acidic than N3 (pK_a 9.6) implies that the N1-H will compete favorably for any potential hydrogen bonding with a complementary nucleotide over the N3-H.