The magnesium interaction with the mononucleotide guanosine-5'-monophosphate disodium salt (5'-GMP Na2 or G5'p) has been studied by Fourier transform infrared spectroscopy. The results have been compared with previously obtained proton nuclear magnetic data on the same system as well as other similar systems. The spectra here show that the natural conformation of the mononucleotide changes significantly in the presence of normal concentrations of magnesium chloride (approximately 10(-3) M) from the predominantly C2'-endo,anti into C3'-endo,anti conformation of the sugar ring, and the phosphate group becomes virtually gauche-gauche. The marker bands for C2'-endo,anti and C3'-endo,anti are shown near 820 cm-1 and 803 cm-1, respectively. The infrared spectroscopic results found here indicate that the unit Mg(H2O)2+5 is coordinated at the N7 site of the base guanine and hydrogen bonded via the water molecules to the carbonyl C6 = O and to the phosphate negative oxygens. This type of bonding is very similar to the binding of the antitumor drug cis-platinum, which is chemically (covalently) bound to the N7 site of guanine in the same molecule or in nucleic acids.