An ESR study of metal ion-guanosine monophosphate interaction

Abstract

The addition of guanosine 5-monophosphate (5′-GMP) to an aqueous solution of Mn 2+ ions results in a decrease in ESR signal intensity and an increase in line-width of Mn 2+ ions. This can be interpreted in terms of stepwise formation of outersphere and inner-sphere complexes as Mn 2+( H 2 O) 6+5′− GMP ⇌ K 1 ( Mn 2+( H 2 O) 6, 5′− GMP) ⇌ K 1 ( Mn 2+( H 2 O) 6, 5′− GMP)+ H 2 O . 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 K 1 and K 2 are calculated as 180 M −1 and 1.14, respectively. When Mg 2+ is added to a mixture of Mn 2+ and 5′-GMP, ESR signal intensity increases, presumably due to the replacement of Mn 2+ by Mg 2+ in the complex. From the variation of ESR signal intensity as a function of concentration of Mg 2+, the product K 1 K 2 for the magnesium complex i s 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 Mn 2+-5 ′-GMP complex.