Xanthosine 5'-monophosphate (XMP) : acid-base and metal ion-binding properties of a chameleon-like nucleotide

The four acidity constants of threefold protonated xanthosine 5'-monophosphate, H-3(XMP)(+), reveal that in the physiological pH range around 7.5 (X - H.MP)(3-) strongly dominates and not XMP2- as commonly given in textbooks and often applied in research papers. Therefore, this nucleotide, which participates in many metabolic processes, should be addressed as xanthosinate 5'-monophosphate as is stated in this critical review. Micro acidity constant schemes allow quantification of intrinsic site basicities. In 9-methylxanthine nucleobase deprotonation occurs to more than 99% at (N3) H, whereas for xanthosine it is estimated that about 30% are (N1) H deprotonated and for (X - H.MP)(3-) it is suggested that (N1)H deprotonation is further favored, especially in macrochelates where the phosphate-coordinated M2+ interacts with N7. The formation degree of these macrochelates in the (X - H.MP.M)(-) species of Co2+, Ni2+, Cu2+, Zn2+ or Cd2+ amounts to 90% or more. In the monoprotonated (M.X - H.MP.H)(+/-) complexes, M2+ is located at the N7/[(C6)O] unit as the primary binding site and it forms macrochelates with the P(O)(2)(OH)(-) group to about 65% for nearly all metal ions considered (i. e., including Ba2+, Sr2+, Ca2+, Mg2+); this indicates outer-sphere binding to P(O)(2)(OH). Finally, a new method quantifying the chelate effect is applied to the M(X - H.MP)(-) species, stabilities and structures of mixed-ligand complexes are considered, and the stability constants for several M(X - H.DP)(2-) and M(X - H.TP)(3-) complexes are estimated (112 references).