An ESR study of metal ion-guanosine monophosphate interaction

Abstract

The addition of guanosine 5-monophosphate (5'-GMP) to an aqueous solution of Mn²⁺ ions results in a decrease in ESR signal intensity and an increase in line-width of Mn²⁺ ions. This can be interpreted in terms of stepwise formation of outersphere and inner-sphere complexes as Mn²⁺(H2O)6 +5'-GMP ⇆ ^K₁[ Mn²⁺(H₂O)₆, 5'-GMP] ⇆^K₁[Mn²⁺(H₂O)₅, 5'-GMP] +H₂O. From the measurement of ESR signal intensity and linewidth as a function of concentration of 5'-GMP at room temperature (23°C ), the stability constants K₁ and K₂ are calculated as 180 M^-1 and 1.14, respectively. When Mg²⁺ is added to a mixture of Mn²⁺ and 5'-GMP, ESR signal intensity increases, presumably due to the replacement of Mn²⁺ by Mg²⁺ in the complex. From the variation of ESR signal intensity as a function of concentration of Mg²⁺, the product K₁K₂ for the magnesium complex is calculated as 125 M^-1. This difference in stability constants may indicate that both phosphate group and guanine base are involved in the formation of Mn²⁺-5'-GMP complex.