Xanthosine 5′-monophosphate (XMP). Acid–base and metal ion-binding properties of a chameleon-like nucleotide?

Chemical Society Reviews Pub Date: 2009-06-22 DOI: 10.1039/B902181G

Abstract

The four acidity constants of threefold protonated xanthosine 5′-monophosphate, H3(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).

Graphical abstract: Xanthosine 5′-monophosphate (XMP). Acid–base and metal ion-binding properties of a chameleon-like nucleotide
Recommended Literature