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

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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 (.epsilon.-AMP2-) was 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 (.epsilon.-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(.epsilon.-AMP) and Mg(AMP) are similar; however, the stabilities of the Mn2+, Cu2+ and Zn2+ complexes of .epsilon.-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 .epsilon.-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) were used to evaluate the extent of macrochelation (i.e., the simultaneous coordination of M2+ to the base of moiety and the phosphate group) in the .epsilon.-AMP and AMP complexes: the concentration of the macrochelated isomer is considerably larger for M(.epsilon.-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)cl2- > M(ATP)cl2- for the tendency to form macrochelates (cl). Due to the relatively high affinity of the .epsilon.-adenosine moiety towards Mn2+, Cu2+ and Zn2+, the phosphare-monoprotonated complexes M(H .cntdot. .epsilon.-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: .apprx. 80% of Cu(Bpy)(AMP) and Cu(Bpy)(.epsilon.-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. 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.

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