Nucleobase carbonyl groups are poor Mg2+ inner-sphere binders but excellent monovalent ion binders-a critical PDB survey

Precise knowledge of Mg2+ inner-sphere binding site properties is vital for understanding the structure and function of nucleic acid systems. Unfortunately, the PDB, which represents the main source of Mg2+ binding sites, contains a substantial number of assignment issues that blur our understanding of the functions of these ions. Here, following a previous study devoted to Mg2+ binding to nucleobase nitrogens, we surveyed nucleic acid X-ray structures from the PDB with resolutions <= 2.9 A to classify theMg(2+) inner-sphere binding patterns to nucleotide carbonyl, ribose hydroxyl, cyclic ether, and phosphodiester oxygen atoms. From this classification, we derived a set of prior-knowledge nucleobase Mg2+ binding sites. We report that crystallographic examples of trustworthy nucleobase Mg2+ binding sites are fewer than expected since many of those are associated with misidentified Na+ or K+. We also emphasize that binding of Na+ and K+ to nucleic acids is much more frequent than anticipated. Overall, we provide evidence derived from X-ray structures that nucleobases are poor inner-sphere binders for Mg2+ but good binders for monovalent ions. Based on strict stereochemical criteria, we propose an extended set of guidelines designed to help in the assignment and validation of ions directly contacting nucleobase and ribose atoms. These guidelines should help in the interpretation of X-ray and cryo-EM solvent density maps. When borderline Mg2+ stereochemistry is observed, alternative placement of Na+, K+, or Ca2+ must be considered. We also critically examine the use of lanthanides (Yb3+, Tb3+) as Mg2+ substitutes in crystallography experiments.