Evidence for ditopic coordination of phosphate diesters to [Mg(15-crown-5)]2+. Implications for magnesium biocoordination chemistry.

The interaction of a series of phosphate diesters and triesters (1=diphenyl phosphate,2=dimethyl phosphate,3=bis(2-ethylhexyl) phosphate,4=trimethyl phosphate,5=methyldiphenyl phosphate,6=triphenyl phosphate) with [Mg(15-crown-5)](2+) (15-crown-5=1,4,7,10,13-pentaoxocyclopentadecane) was studied as a simplified model for the interaction of aqueous Mg(2+) ion with phosphate-containing biomolecules such as RNA. Using electrospray mass spectrometry, we confirm the formation of 1:1 adducts in the gas phase. Proton and (31)P NMR titration data were used to construct binding isotherms, and a 1:1 binding equilibrium was fit to the isotherms at room temperature to estimate the binding affinities. The binding affinity data are consistent with ditopic coordination of neutral dialkyl phosphate ligands to the [Mg(15-crown-5)](2+) unit. This involves inner-sphere coordination to the Mg(2+) via an oxygen atom, which is complemented by a weak hydrogen-bonding interaction with the crown ether ligand. Ditopic interaction is consistent with low-temperature NMR spectra showing four different configurations for1 coordinated to [Mg(15-crown-5)](2+), which are interpreted in terms of hindered rotation around the Mg-O(phos) bond. Thermochemical analysis of the binding affinity data suggests that the second-shell interaction contributes only about 1 kcal/mol to the binding free energy, so additional factors, such as steric constraints, must be operative to give a preferred phosphate orientation in this system. However, the experimental data do suggest that second-shell interactions contribute as much as 40% of the total binding energy, consistent with the pronounced ability of aqueous Mg(2+) to form salt-bridges linking secondary and tertiary elements of RNA structure.

Interaction of biotin with Mg-O bonds: bifunctional binding and recognition of biotin and related ligands by the Mg(15-crown-5)2+ unit.

The interaction between biotin and the macrocyclic magnesium complex Mg(15-crown-5)(Otf)2 (15-crown-5 is 1,4,7,10,13-pentaoxacyclopentadecane, Otf(-) is trifluoromethanesulfonate anion) in solution was studied as a model for metal-biotin interactions that may be important in its speciation and function. Shifts in the solution IR spectrum establish that the interaction is dominated by ligation between the carbonyl oxygen of the ureido ring of biotin and the Mg2+ cation. However, comparative binding studies using NMR spectroscopy and conductivity reveal a substantial enthalpic contribution to binding that arises from interactions between the ureido -NH moiety and the macrocyclic ring. This is interpreted in terms of a weak-to-moderate hydrogen bond formed between the -NH group and an oxygen from the crown, which is simultaneously coordinated to Mg2+. This hypothesis is reinforced by quantitative examination of the binding of N-methylated derivatives of 2-imidazolidone, which shows that N,N'-dimethylation decreases the affinity of Mg(15-crown-5)(Otf)2 for the ligand by 2 orders of magnitude. This can be understood in terms of the structure of Mg(15-crown-5)(Otf)2. It shows a pentagonal bipyramidal coordination geometry where the five equatorial positions are occupied by the macrocyclic oxygen donors. The axial positions are occupied by weakly coordinating Otf(-) anions, which are readily displaced by biotin and related derivatives. The M-O(crown) bond distance ranges from 2.1 to 2.3 A, providing structural complementarity for the 2.2 A C=O...HN- bite distance in the ureido group, which leads to strong interaction. The contribution from hydrogen bonding illustrates the importance of second-shell interactions in the biocoordination chemistry of Mg2+. These can serve to organize cofactor interactions with biomolecules, as was recently demonstrated for a biotin-selective RNA aptamer that depends on a direct biotin-magnesium interaction for recognition of biotin (Nix, J.; Sussman, D.; Wilson, C. J. Mol. Biol. 2000, 296, 1235-1244). These results are significant in the context of the observed magnesium requirement in biotin-dependent carboxylase enzymes, where noncovalent interactions with biotin may be important in its activation toward carboxylation in the first step of biotin-dependent CO2 transfer. The synthetic system presented here also suggests that the Mg-O bond may be considered a constituent design element in the rational preparation of complexes to bind and recognize biotin.