A redox-responsive Schiff-base Fe(III) complex for MRI-readable magnetic relaxation detection of glutathione and cysteine

Biothiols play important roles in maintaining cellular redox homeostasis, and abnormal biothiol levels are associated with various pathological processes. Herein, a Schiff-base Fe(III) complex, complex 1, was developed as a redox-responsive magnetic relaxation probe for the detection of glutathione (GSH) and cysteine (Cys). Complex 1 was identified as a high-spin Fe(III) species (d⁵, S = 5/2) and exhibited a pronounced T₂-shortening effect in aqueous solution. Upon reaction with GSH or Cys, it was converted to a high-spin Fe(II) species (d⁶, S = 2), accompanied by slower transverse relaxation, an increase in T₂, and a corresponding decrease in the transverse relaxation rate (R₂, R₂=1/T₂). Under optimized conditions, R₂ showed good linear relationships with GSH and Cys concentrations over the ranges of 0.3-3.0 mM and 0.5-3.5 mM, with detection limits of 11.75 μM and 9.89 μM, respectively. The method was successfully applied to spiked human serum samples, affording recoveries of 100.96-102.17% for GSH and 98.37-101.5% for Cys. The magnetic relaxation response could also be directly visualized by T₂-weighted MR imaging. Mechanistic studies using mass spectrometry, ferrozine assay, EPR, XPS, magnetic susceptibility measurements, and DFT calculations supported a biothiol-triggered Fe(III)-to-Fe(II) redox conversion, with both oxidation states adopting high-spin configurations, coupled with thiol-to-disulfide oxidation. This work provides a basis for designing redox-responsive iron complexes for magnetic relaxation sensing of biothiols. Further development should focus on distinguishing contributions from coexisting thiols and validating analytical performance at endogenous concentrations in complex biological samples.