On the metal‐ion coordinating properties of the 5′‐monophosphates of 1, N⁶‐ethenoadenosine (ɛ‐AMP), adenosine and uridine

Abstract

The concentration dependence of the chemical shifts of the protons H‐2, H‐8, H‐10, H‐11, and H‐1′ of 1,N6‐ethenoadenosine 5′‐monophosphate (ɛ‐AMP2−) has been measured. The results are consistent with the isodesmic model of indefinite noncooperative stacking; the association constant, K= 2.5 ± 0.3 M−1, is within experimental error identical to the value determined earlier for AMP2−,K= 2.1 ± 0.4 M−1. The conditions for the potentiometric pH titrations, used to determine the acidity constants of H2(ɛ‐AMP), H2(AMP), and H(UMP)− and the stability constants of the metal ion (M2+) complexes of the corresponding nucleoside 5′‐monophosphates (NMP), were chosen so that the ligands were present in the monomeric form. The stabilities of Mg(ɛ‐AMP) and Mg(AMP) are similar; however, the stabilities of the Mn2+, Cu2+ and Zn2+ complexes of ɛ‐AMP2− are much larger (in the case of Cu2+ by a factor of 700) than those of AMP2−. This is due to the much larger metal ion affinity of the ɛ‐adenosine moiety compared to that of the parent adenosine residue. As the uridine moiety does not participate in complex formation, the stability constants of M(UMP) have been used to evaluate the extent of macrochelation (i.e. the simultaneous coordination of M2+ to the base moiety and the phosphate group) in the ɛ‐AMP and AMP complexes: the concentration of the macrochelated isomer is considerably larger for M(ɛ‐AMP) than for M(AMP). A comparison with previous results for the complexes with ADP3− and ATP4− indicates the order, M(AMP)cl < M(ADP)−cl > M(ATP)2−cl for the tendency to form macrochelates (cl). Due to the relatively high affinity of the ɛ‐adenosine moiety towards Mn2+, Cu2+ and Zn2+, the phosphate‐monoprotonated complexes M(H ·ɛ‐AMP)+ also become important; the corresponding complexes play only a minor role in the M2+/AMP systems. Intramolecular aromatic‐ring stacking occurs in the ternary Cu(2,2′‐bipyridyl)(NMP) complexes: about 80% of Cu(Bpy)(AMP) and Cu(Bpy)(ɛ‐AMP) exist as the stacked isomer in aqueous solution; for the former system it has been shown in a previous X‐ray study that the intramolecular ligand‐ligand interaction occurs also in the solid state [Aoki, K. (1978) J. Am. Chem. Soc. 100, 7106]. Overall, the results emphasize that great care should be exercised in drawing conclusions based on studies of metal‐ion‐containing enzymic systems in which the natural adenine nucleotide cofactors have been replaced by the corresponding 1,N6‐etheno derivatives.

Complex Formation of Cadmium with Sugar Residues, Nucleobases, Phosphates, Nucleotides, and Nucleic Acids

“…In accord with the above are the acid-base properties of H 2 (ε-AMP) ± (for the structure see [136,178]. Hence, in the 48 neutral pH range complexes of the type M(ε-AMP) are expected and actually observed [136,178] though M(H;ε-AMP) + species are also known [178].…”

“…Hence, in the 48 neutral pH range complexes of the type M(ε-AMP) are expected and actually observed [136,178] though M(H;ε-AMP) + species are also known [178]. The stability enhancement for the Zn(ε-AMP) complex, compared with a sole Zn 2+ -phosphate binding (eq.…”

Complex Formation of Nickel(II) with Sugar Residues, Nucleobases, Phosphates, Nucleotides, and Nucleic Acids

“…In accord with this are the acid-base properties of H 2 (ε-AMP) ± (for the structure see Figure 12, top [155,156]) which releases the proton from (N6)H ϩ with pK H H2(ε-AMP) ϭ 4.23 ± 0.02 and the one from P(O) 2 (OH) Ϫ with pK H H(ε-AMP) ϭ 6.23 ± 0.01 [109,157]. Hence, in the neutral pH range complexes of the type M(ε-AMP) are expected and actually observed [109,157] though M(H;ε-AMP) ϩ species are also known [157].…”

“…Here more work needs to be done. Another diffi culty refers to the binding of Ni 2ϩ to the triphosphate chain of a NTP 4Ϫ [68,107,157]: Is Ni 2ϩ always α,β,γ-coordinated or does the interaction with the α-phosphate group vary in dependence on macrochelate formation with N7 of a purine residue? In other words, does N7-backbinding initiate a change from an innersphere to an outersphere coordination of Ni 2ϩ to the α-phosphate group?…”

“…Hence, in the neutral pH range complexes of the type M(ε-AMP) are expected and actually observed [109,157] though M(H;ε-AMP) ϩ species are also known [157]. The stability enhancement of the Ni(ε-AMP) complex, compared with a sole Ni 2ϩ -phosphate binding, amounts to approximately 2 log units, meaning that the macrochelated species (Equilibrium 24) dominate with a formation degree of about 99% [109].…”