Macrochelate formation in monomeric metal ion complexes of nucleoside 5'-triphosphates and the promotion of stacking by metal ions. Comparison of the self-association of purine and pyrimidine 5'-triphosphates using proton nuclear magnetic resonance

The concentration dependence of the chemical shifts of the protons H-2, H-8, and H-I’ or H-5, H-6, and H-1’ of A T P , I T P , and G T P or C T P and U T F ( = N T P ) , respectively, and of the corresponding nucleosides has been measured. The results are consistent with the isodesmic model of indefinite noncooperative stacking; the association constants for NTP4are between 1.3 (ATP4-) and about 0.4 M-’ (UTP4-) and for the nucleosides between 15 (adenosine) and 1.2 M-’ (uridine). The self-stacking tendency decreases within the series adenosine > guanosine > inosine > cytidine uridine. Due to the repulsion of the negatively charged phosphate moieties, this trend is much less pronounced for the corresponding N T F series. The charge effect also governs the series adenosine >> AMP2> ADP3ATP4-. Likewise the self-association tendency of ATP4-, ITP, and GTP‘is promoted by a factor of about 3-5 by the coordination of Mg2+ to the phosphate moiety, which neutralizes part of the negative charge a t this residue. However, the self-association tendency of Zn(ATP)2and Cd(ATP)2is much larger than of Mg(ATP)’-; this is explained by an increased tendency to form an intermolecular metal ion bridge in the dimeric stacks in which Zn2+ or Cd2+ is coordinated to the phosphate moiety of one ATP4and to N-7 of the adenine residue of the other A T P . The shifts of H-8 for complete stacking (6,) agree with this interpretation. There is no significant increase in stability in Zn(ITP)2and Zn(GTP)2-: Le. , the stability of these stacks is governed only by the charge neutralization-the effect of Zn2+ is the same as that of Mg2+. Comparison of the shifts of H-8 at infinite dilution (6,) for several systems reveals that an M2+/N-7 interaction exists in the monomeric Zn2+ and CdZt complexes of the purine 5’-triphosphates; Le. , a macrochelate is formed through an inrramolecular coordination of the metal ion to the phosphate moiety and to N-7. The position of this concentration-independent equilibrium between the open isomer (with phosphate coordination only) and the macrochelated isomer is estimated by comparing 6o of M(NTP)2with the shifts of H-8 for complete complex formation of the corresponding metal ion-nucleoside complexes, which were also determined. The N M R study gives no hint for such an N-7 interaction either for the corresponding Mg(NTP)2complexes or for a base interaction in any of the pyrimidine 5’-triphosphate complexes. These N M R results prompted an evaluation of stability data (obtained earlier under conditions where no self-association occurs), which give further evidence that macrochelate formation also occurs in the M(NTP)’complexes of purine nucleotides with Mn2+ Co2+, Ni2+, Cu2+, and Zn2+; no evidence for such an interaction is observed in the pyrimidine nucleotide complexes. Ionizkon of the base moiety in I T P , G T F , UTF, and TTP favors, however, the base-metal ion interaction and therefore also the formation of macrochelates in the M(NTP-H)3complexes. The percentage of the macrochelated isomer is estimated for all these systems: the whole range from nearly 100% ring back-bonding to only insignificant traces is observed. The ambivalent coordinating properties of nucleotides and their structural versatility are discussed. I t is now well-known tha t metal ions a r e essential in a large variety of biological processes, including those with nucleic acids and their der ivat ives4 For example, D N A polymerase contains tightly bound Zn2+, and there is evidence that this metal ion binds the enzyme to DNA.5s6 To fulfill its function the enzyme must also be activated by a divalent cation such as Mg2+ or Mn2+, and these metal ions bind the nucleoside triphosphate substrates to the enzyme. ’,* Thus the interplay between metal ions and nucleotides, or their derivatives, is receiving much attention at