AbstractThe interaction of guanosine‐5′‐monophosphoric acid (H2‐GMP) with the alkaline earth metal ions has been studied in aqueous solution at neutral pH. The crystalline salts of the type Mg‐GMP·5H2O, Ca‐GMP·6H2O, Sr‐GMP·7H2O, and Ba‐GMP·7H2O were isolated and characterized by Fourier transform ir,1H‐nmr and x‐ray powder diffraction measurements. Two types of macrochelate complexes have been identified: (a) The direct metal–base and indirect metal–phosphate bindings (inner and outer sphere interaction) for the Mg(II), Ca(II), and Sr(II), ions; and (b) the indirect metal–base and direct metal–phosphate bindings (outer and inner sphere interaction) for the Ba(II) ion. In aqueous solution, an equilibrium exists between the base–metal–H2O…PO3and base … H2O–M–PO3interactions. The ribose moiety shows C3′‐endo/anticonformation in the free acid; C2′‐endo/antiin the Na2‐GMP salt; C3′‐endo/antiin the Mg(II)‐, Ca(II)‐, an