Self-association of nucleotides. Effects of protonation and metal ion coordination.
The self-association of nucleosides decreases within the series adenosine greater than guanosine greater than inosine greater than cytidine approximately uridine. The same trend is observed for the corresponding nucleotides, though less pronounced, as the charge effect governs series like adenosine much greater than AMP2- greater than ADP3- approximately greater than ATP4-. Protonation of adenosine considerably reduces its self-stacking tendency: this is different with ATP4-, where a maximum is reached for H2(ATP)2- caused by additional ionic interactions in the [H2(ATP)]2(4-) dimer. Metal ion coordination may promote self-association, e.g., of ATP4- via (mainly) charge neutralization (Mg2+) and the formation of intermolecular bridges in dimeric stacks (Zn2+, Cd2+). These results allow definition of conditions with negligible self-association and thus the determination of the stability and structure of monomeric nucleotide complexes in aqueous solution, e.g., quantification of macrochelate formation in M(ATP)2- complexes. Some biological implications of the results are indicated.