The concentration dependence of the chemical shifts of the protons H-2, H-8 and H-1' for 2'-, 3'-and 5'-AMP2-and of the protons H-2, H-7, H-8 and H-1' for tubercidin 5'-monophosphate (= 7-deaza-AMP2-; TuMP2-) has been measured in D 2 0 at 27°C to elucidate the self-association of the nucleoside monophosphates (NMPs). The results are consistent with the isodesmic model of indefinite non-cooperative stacking; the association constants for all four NMPs are very similar: K w 2 M -l . These 'H-NMR measurements and those on the dependence of the chemical shifts on the pD of the solutions indicate that the NMP2-species exist predominately in the anti conformation. Comparison of the shift data for 5'-TuMP and 5'-AMP shows that no hydrogen bonding between N-7 and -P03H-occurs; hence, the previously observed and confirmed 'wrongway' chemical shift [Martin, R. B. (1985) Acc. Chem. Res 18, 321 connected with the deprotonation of the -P03Hp group most probably results from the anisotropic properties of the phosphate group which is in the anti conformation close to N-7. From the dependence between the chemical shift and the pD of the solutions the acidity constants were calculated for the four protonated NMPs, and for adenosine and D-ribose 5'-monophosphate. The measurements also allow an estimation of the first acidity constant of H3(5'-AMP)+ = 0.9 and pKHH,(AMp) = 0.4). The values for pK&(NMP) and pK:(NMP) were also determined from potentiometric pH titrations in aqueous solution ( I = 0.1 M, NaN03; 25°C). The agreement of the results obtained by the two methods is excellent. The position of the phosphate group at the ribose moiety and the presence of N-7 in the base moiety influence somewhat the acid-base properties of the mentioned NMPs. Measurements with 5'-AMP in 50% (v/v) aqueous dioxane show that lowering of the solvent polarity facilitates removal of the proton from the H+(N-I) site while the -PO:-group becomes more basic; this increases the pH range in which the monoprotonated H(S'-AMP)-species is stable and which is now also extended into the physiological pH region. Some consequences of this observation for biological systems are indicated.About one sixth of all enzyme systems need ATP or a related adenine cofactor [l]. With this fact in mind, it is not surprising that the properties of adenine nucleotides are receiving much attention. One of their most fascinating qualities is the self-association via base-stacking [2] which, for example, is thought to be important in the storage of some neurotransmitters [3 -51, possibly playing a role in stabilizing the aggregates.The self-stacking of adenine nucleotides occurs clearly beyond the dimer stage, so that oligomers are formed [6 -131. 'Head-to-tail' stacking, with the five-membered and sixmembered rings alternating in the stack, has been suggested [14], but other geometries are also possible [2, 10, 151. Due to electrostatic repulsion of the negatively charged phosphate residues the tendency for self-association decreases within the series, adenosine > 5'-AMPZ-> 5-...

070ChemInform Abstract The stability constants for the divalent metals Mg, Ca, Mn, Co, Ni, Cu, Zn, and Cd are determined by potentiometric pH titrations at 25 rc C. The results show that the stability of most of the M(ATP)2-complexes is significantly larger than that of the corresponding complexes formed with the pyrimidine-nucleoside 5'-triphosphates (PNTP). This increased stability is attributed to the formationof outer-sphere macrochelates. Inner-and outer-sphere forms of M(ATP)2-occur for Mn, Co, Ni, Zn, and Cd in comparable amounts, Cu forms no outer-sphere species to any significant extent and Ca(ATP)2-exists only in the open, phosphate-coordinated form. Of all the M(PNTP)2-complexes, only Cu(CTP)2-forms a base-back-bound species.

The interaction of 2-amino-2(hydroxymethyl)-1,3-propanediol (Tris) with the metal ions (M2+) Mg2+, Ca2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ was studied by potentiometry and spectrophotometry in aqueous solution (I = 0.1 or 1.0 M, KNO3, 25 degrees C). Stability constants of the M(Tris)2+ complexes were determined; those constants which were measured by both methods agreed well. Ternary complexes containing ATP4- as a second ligand were also investigated and it is shown that in the presence of Tris, mixed-ligand complexes of the type M(ATP)(Tris)2- are formed. The values for delta log KM, where delta log KM = log KM(ATP)M(ATP)Tris--log KMM(Tris), are all negative, thus indicating that the interaction of Tris with M(ATP)2- is somewhat less pronounced than with M2+. However, it should be noted that even in mixed-ligand systems complex formation with Tris may still be considerable, hence great reservations should be exercised in employing Tris as a buffer in systems which also contain metal ions. Distributions of the complex species in dependence on pH are shown for several systems, and the structures of the binary M(Tris)2- and the ternary M(ATP)(Tris)2- complexes are discussed. The participation of a Tris-hydroxo group in complex formation is, at least for the M(Tris)2- species, quite evident.

No abstract