Complexes of cobalt (II) and manganese (II) with adenosine 5′-diphosphate and adenosine 5′-triphosphate. A circular dichroism study

For studies of interactions between Co²⁺ and adenosine 5′-diphosphate or adenosine 5′-triphosphate (ADPH₄⁺ and ATPH₅⁺ in strongly acidic medium) visible circular dichroism (d-d transitions of Co²⁺) and ultraviolet circular dichroism (adenine transitions) have proven to be very sensitive to structural changes. Drastic variation of spectra as a function of pH and concentration enabled us to show the existence of various species, to state their stoichiometry and eventually, their self-association. With ATPH₂^2-, C.D. results are in agreement with recent N.M.R. results. With ligands bearing three negative charges, complexes (1 metal:2 nucleotides)_n are formed in which bases of the two nucleotides of the molecule are self-associated. With ADP^3-, the visible C.D. spectrum of this complex is intense and hides the spectra of the complexes formed with other protonated species of ADP; this self-associated complex is detected up to a lower limit of 5 × 10^-4 M concentration. With ATPH^3-, a complex exhibiting the same characteristics as the one with ADP^3- is formed but in about twenty times less amount which explains why it was not detected by potentiometry. With 0.1 M ATP^4-, dimeric (or polymeric) complexes, of 1:2 and 1:1 stoichiometry are observed. With 0.01 M ATP^4-, 1:1 monomeric and 2:1 dimeric (or polymeric) complexes are detected. The interactions between Mn²⁺ ions and ADP or ATP have been studied by C.D. on the UV range. The same species as with CO²⁺ ions have been found but the 1:2 complex formation with ADP^3- was shown to occur to a lesser exten and was not observed below a 10^-2 M ADP concentration.