Enhanced selectivity for Mg²⁺ with a phosphinate-based chelate: APDAP versus APTRA.

Area of Science:

• Coordination Chemistry
• Bioinorganic Chemistry
• Chelation Therapy

Background:

• o-Aminophenol-N,N,O-triacetate (APTRA) is a well-established ligand for magnesium (Mg²⁺) but exhibits higher affinity for calcium (Ca²⁺).
• Aminocarboxylate ligands often show poor selectivity between divalent metal ions, particularly Mg²⁺ and Ca²⁺.

Purpose of the Study:

• To synthesize and characterize an O-phosphinate analogue of APTRA, named o-aminophenol-N,N-diacetate-O-methylene-methylphosphinate (APDAP).
• To evaluate the metal-binding properties of APDAP, focusing on its selectivity for Mg²⁺ over Ca²⁺ and Zn²⁺.
• To investigate the structural basis for APDAP's improved metal selectivity using computational methods.

Main Methods:

• Synthesis of the O-phosphinate analogue APDAP.
• Metal binding studies monitored by UV-visible spectroscopy.
• Density Functional Theory (DFT) calculations to analyze metal-ligand interactions.

Main Results:

• APDAP exhibits significantly reduced affinity for Ca²⁺ (over two orders of magnitude) and Zn²⁺ (over three orders of magnitude) compared to APTRA.
• APDAP shows a much smaller decrease in Mg²⁺ affinity (factor of ~7) compared to APTRA.
• The O-phosphinate modification substantially improves Mg²⁺ selectivity over Ca²⁺ and Zn²⁺.
• DFT calculations suggest that longer P-O and P-C bonds in APDAP favor larger metal-ligand angles, which are less unfavorable for smaller ions like Mg²⁺.

Conclusions:

• APDAP represents a promising scaffold for developing selective Mg²⁺ sensors.
• The O-phosphinate modification is an effective strategy to enhance Mg²⁺ selectivity in aminocarboxylate-type ligands.
• Future derivatives of APDAP could enable accurate Mg²⁺ sensing in physiologically relevant conditions with high Ca²⁺ concentrations.