Acid–base properties of the 5′-triphosphates of guanosine and inosine (GTP4− and ITP4−) and of several related nucleobase derivatives

Abstract

The acidity constants of H₂(NTP)²⁻ [NTP⁴⁻ = guanosine 5′-triphosphate (GTP⁴⁻) or inosine 5′-triphosphate (ITP⁴⁻)], which may lose three protons giving (NTP − H)⁵⁻, were determined by potentiometric pH titrations (25 °C; I = 0.1 M, NaNO₃) and values for the deprotonation of H₃(NTP)⁻ were estimated via micro-acidity-constant evaluations. Complete micro-acidity-constant schemes for both H₃(GTP)⁻ and H₃(ITP)⁻ are given. From the evaluations it follows that the ratios, R, of the twofold protonated and isocharged species (H·NTP·H)²⁻ and (NTP·H₂)²⁻ for ITP and GTP, which carry one proton at N7 and one at the terminal γ-phosphate group or both protons at the triphosphate chain, respectively, are about 1 ∶ 1 for the ITP system and about 10 ∶ 1 for the GTP system. This confirms the higher basicity of N7 in the guanosine residue compared with that in the inosine moiety. For the evaluation of the indicated analysis it was also necessary to consider the acid–base properties of guanosine, inosine, adenosine, and of the 5′-triphosphates of adenosine, cytidine, uridine and thymidine; the corresponding acidity constants were in part taken from our earlier work and in part are also measured now.