A comparative study of ATP and GTP complexation with trivalent Al, Ga and Fe cations. Determination of cation binding site and nucleotide conformation by FTIR difference spectroscopy.

The interactions of adenosine-5'-triphosphate (Na2H2ATP) and guanosine-5'-triphosphate (Na2H2GTP) with trivalent Al, Ga and Fe cations are investigated in aqueous solution at pH = 6-7 with metal/nucleotide ratios (r) 1/10, 1/2, 1 and 4. Fourier Transform infrared (FTIR) difference spectroscopy is used to characterize the nature of metal cation binding and nucleotide conformational variations, in aqueous solution. Spectroscopic evidences show that at low cation concentration (r = 1/10), metal binding is mainly through the -PO2- and -PO3(2-) groups of the nucleotide moiety (outer-sphere), while at r > 1/10, in addition to direct metal-phosphate binding (chelation), minor metal-base complexes are also formed via indirect metal-NH2 or metal-carbonyl interaction (through H2O). At r = 1, metal macrochelate formation occurs, involving direct cation coordination to phosphate groups and imidazol N-7 atom with an indirect metal-NH2 or metal-carbonyl interaction. At r = 4, polymerization of these metal complexes is observed through nucleotide phosphate chain. The amount of cation/phosphate binding is considerably larger than that of metal/base binding. Evidence for metal-phosphate coordination comes from major spectral changes (shifting and intensity variations) of the alpha-, beta- and gamma-PO2- vibrations in the region 1250-900 cm-1, while metal-base binding is characterized by spectral alterations of the bands at 1700 (guanine carbonyl stretch), 1695 (adenine N-1-H+ mode), 1650-1660 (NH2 bending), 1610-1613 (pyrimidine), 1575 (pyrimidine), 1535-1540 (imidazol), 1470-1480 (imidazol) and 1377 cm-1 (imidazol). The ribose moiety shows C2'-endo/anti conformation for the free ATP and its metal-ATP complexes with an infrared marked band at 821 cm-1. A mixture of both C2'-endo/anti and C3'-endo/anti conformations are occurring for the free GTP and its trivalent metal-complexes with marker bands at 820 and 800 cm-1.