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Rocío Uzal-Varela1, Aurora Rodríguez-Rodríguez1, Miguel Martínez-Calvo1

  • 1Centro de Investigacións Cientı́ficas Avanzadas (CICA) and Departamento de Química Fundamental, Universidade da Coruña, Campus da Zapateira-Rúa da Fraga 10, 15008 A Coruña, Spain.

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We developed novel manganese complexes with pH-responsive ligands for enhanced MRI contrast agents. The DPASAm ligand shows a significant relaxivity change within the physiological pH range.

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Area of Science:

  • Coordination Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Development of responsive MRI contrast agents is crucial for advanced diagnostics.
  • Manganese complexes offer potential as T1 contrast agents due to their paramagnetism.
  • Ligand design is key to controlling metal ion coordination and relaxivity properties.

Purpose of the Study:

  • To synthesize and characterize novel ligands (H3DPASAm and H3NO2ASAm) with a sulfonamide group for pH-dependent Mn2+ complexation.
  • To investigate the protonation and stability constants of these ligands and their Mn2+ complexes.
  • To evaluate the relaxivity of the Mn2+ complexes and their response to pH changes.

Main Methods:

  • Potentiometric titrations to determine protonation and stability constants.
  • Spectrophotometric experiments for complementary data.
  • X-ray crystallography to elucidate complex structure.
  • Relaxivity measurements (r1p) at various pH values and frequencies.

Main Results:

  • Two ligands, H3DPASAm and H3NO2ASAm, were synthesized and complexed with Mn2+.
  • Protonation constants of sulfonamide groups were determined (log KiH ≈ 10.4) and decreased upon Mn2+ coordination (log KiH ≈ 5.4-6.4).
  • [Mn(DPASAm)]- forms a stable seven-coordinate complex, exhibiting a significant relaxivity increase from 3.8 to 8.9 mM-1 s-1 as pH decreases from 9.0 to 4.0.

Conclusions:

  • The sulfonamide protonation in [Mn(DPASAm)]- occurs within the physiological pH range, enabling pH-dependent relaxivity.
  • This pH-responsive behavior makes [Mn(DPASAm)]- a promising candidate for targeted MRI contrast agents.
  • The [Mn(NO2ASAm)]- complex showed lower stability and less favorable pH-dependent relaxivity compared to [Mn(DPASAm)]-.