Premium
Acid–Base and Metal‐Ion‐Binding Properties of Xanthosine 5′‐Monophosphate (XMP) in Aqueous Solution: Complex Stabilities, Isomeric Equilibria, and Extent of Macrochelation
Author(s) -
Sigel Helmut,
Massoud Salah S.,
Song Bin,
Griesser Rolf,
Knobloch Bernd,
Operschall Bert P.
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600160
Subject(s) - chemistry , protonation , tautomer , aqueous solution , guanosine , crystallography , stereochemistry , inosine monophosphate , medicinal chemistry , inorganic chemistry , ion , nucleotide , organic chemistry , biochemistry , gene
The four acidity constants of threefold protonated xanthosine 5′‐monophosphate, H 3 (XMP) + , reveal that at the physiological pH of 7.5 (XMP−H) 3− strongly dominates (and not XMP 2− as given in textbooks); this is in contrast to the related inosine (IMP 2− ) and guanosine 5′‐monophosphate (GMP 2− ) and it means that XMP should better be named as xanthosinate 5′‐monophosphate. In addition, evidence is provided for a tautomeric (XMP−H⋅N1) 3− /(XMP−H⋅N3) 3− equilibrium. The stability constants of the M(H;XMP) + species were estimated and those of the M(XMP) and M(XMP−H) − complexes (M 2+ =Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ ) measured potentiometrically in aqueous solution. The primary M 2+ binding site in M(XMP) is (mostly) N7 of the monodeprotonated xanthine residue, the proton being at the phosphate group. The corresponding macrochelates involving P(O) 2 (OH) − (most likely outer‐sphere) are formed to ≈65 % for nearly all M 2+ . In M(XMP−H) − the primary M 2+ binding site is (mostly) the phosphate group; here the formation degree of the N7 macrochelates varies widely from close to zero for the alkaline earth ions, to ≈50 % for Mn 2+ , and ≈90 % or more for Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , and Cd 2+ . Because for (XMP−H) 3− the micro stability constants quantifying the M 2+ affinity of the xanthosinate and ${{\rm PO}{{2- \hfill \atop 3\hfill}}}$ residues are known, one may apply a recently developed quantification method for the chelate effect to the corresponding macrochelates; this chelate effect is close to zero for the alkaline earth ions and it amounts to about one log unit for Co 2+ , Ni 2+ , Cu 2+ . This method also allows calculation of the formation degrees of the monodentatally coordinated isomers; this information is of relevance for biological systems because it demonstrates how metal ions can switch from one site to another through macrochelate formation. These insights are meaningful for metal‐ion‐dependent reactions of XMP in metabolic pathways; previous mechanistic proposals based on XMP 2− need revision.