Acid-base properties of the 5 '-triphosphates of guanosine and inosine (GTP(4-) and ITP4-) and of several related nucleobase derivatives

The acidity constants of H-2(NTP)(2-) [NTP4- = guanosine 5'-triphosphate (G TP(4-)) or inosine 5'-triphosphate (ITP4-)], which may lose three protons g iving (NTP - H)(5-), were determined by potentiometric pH titrations (25 de greesC; I = 0.1 M, NaNO3) and values for the deprotonation of H-3(NTP)(-) w ere estimated via micro-acidity-constant evaluations. Complete micro-acidit y-constant schemes for both H,(GTP)- and H,(ITP)- are given. From the evalu ations it follows that the ratios, R, of the twofold protonated and isochar ged species (H . NTP .H)(2-) and (NTP .H-2)(2-) for ITP and GTP, which carr y one proton at N7 and one at the terminal gamma -phosphate group or both p rotons 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 basic ity 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 con sider the acid-base properties of guanosine, inosine, adenosine, and of the 5'-triphosphates of adenosine, cytidine, uridine and thymidine; the corres ponding acidity constants were in part taken from our earlier work and in p art are also measured now.