Steric Effects on the Chelation of Mn2+ and Zn2+ by Hexadentate Polyimidazole Ligands: Modeling Metal Binding by Calprotectin Site 2

Abstract

Recently, there has been increasing interest in the design of ligands that bind Mn²⁺ with high affinity and selectivity, but this remains a difficult challenge. It has been proposed that the cavity size of the binding pocket is a critical factor in most synthetic and biological examples of selective Mn²⁺ binding. Here, we use a bioinspired approach adapted from the hexahistidine binding site of the manganese-sequestering protein calprotectin to systematically study the effect of cavity size on Mn²⁺ and Zn²⁺ binding. We have designed a hexadentate, trisimidazole ligand whose cavity size can be tuned through peripheral modification of the steric bulk of the imidazole substituents. Conformational dynamics and redox potentials of the complexes are dependent on ligand steric bulk. Stability constants are consistent with the hypothesis that larger ligand cavities are relatively favorable for Mn²⁺ over Zn²⁺ , but this effect alone may not be sufficient to achieve Mn²⁺ selectivity.

Keywords: bioinorganic chemistry; chelates; manganese; metalloprotein mimcs; steric effects.

Figure 1.

Crystal structure of the two metal coordination sites for calprotectin, A. apo-Site 1 (His₃Asp, or N₃O) and B. site 2 (His₆, or N₆) with manganese bound. (PDB code: 4XJK)^[17]

Figure 2.

A. Octadentate ligand by Cieslik et al. showing high affinity for manganese(II).^[7] B. Heptadentate manganese(II)-activated fluorescent probe by Bakthavatsalam et al.^[3] C. Generalized ligand from this work (^R,iPrL).

Figure 3.

Histograms of Mn-N and Zn-N bond distances for coordinated imidazoles from the Cambridge Structural Database. The dashed vertical line in each case shows the (average) experimentally determined metal-N_his bond lengths for Mn²⁺ and Zn²⁺ in calprotectin’s site 2.

Figure 4.

Crystal structure of [^tBu,iPrLMn](ClO₄)₂.^[41] Thermal ellipsoids are shown at 50%; perchlorate counteranions and hydrogen atoms are omitted for clarity. (Color code: blue = N, gray = C, magenta = Mn).

Figure 5.

Space filling models for A) [^H,iPrLMn]²⁺, B) [^iPr,iPrLMn]²⁺, and C) [^tBu,iPrLMn]²⁺. Perchlorate counter anions and cocrystallized acetonitrile solvent are omitted for clarity.

Figure 6.

M²⁺-N distances for the imidazole nitrogens and for the tacn nitrogens are plotted. Solid red circles (, Mn²⁺) and blue diamonds (, Zn²⁺) correspond to crystallographically determined bond distances.

Figure 7.

A. ¹H NMR of [^H,iPrLZn](ClO₄)₂ in CD₃CN at room temperature. B. Hydrogen labeling of ligand, with [^iPr,iPrLZn]²⁺ shown as example. C. ¹H NMR of [^iPr,iPrLZn](ClO₄)₂ in CD₃CN at room temperature. D. Variable temperature ¹H NMR of [^tBu,iPrLZn](ClO₄)₂ in CD₃CN (*EtOAc)

Figure 8.

Cyclic voltammograms for manganese compounds with R = H, ⁱPr, ^tBu. Data was collected in 0.1 M tetrabutylammonium hexafluorophosphate in degassed acetonitrile under an argon atmosphere. The concentration of the compounds was 1 mM.

Figure 9.

Species distribution curves for binding of ^H,iPrL with Mn²⁺ (A) and Zn²⁺ (B, calculated using the lower limit value of K_Zn) and for binding of ^tBu,iPrL with Mn²⁺ (C) and Zn²⁺ (D). Note: coordinated waters are omitted from formulae in species labels. Distribution curves were simulated for [L] = [Mn²⁺] = [Zn²⁺] = 3 mM.

Figure 10.

Steric energy vs. bond lengths for ^H,iPrL, ^iPr,iPrL, and ^tBu,iPrL.

Scheme 1.

Synthetic approach to substitution of imidazoles and formation of the ligand ^R,iPrL.

Scheme 2.

Equilibrium water binding and imidazole hemilability on [^tBu,iPrLZn]²⁺, showing proposed structure of water-bound complex.