Figure A) Principle of q-based CuII-responsive MRI CAs, green: GdIII-complex, blue: CuII-binding site, red: switching arm. B) Structure of GdIII-DO3A-pyrGH. C) Characterisation of GdIII-DO3A-pyrGH: 1) Relaxometric titration of GdIII-DO3A-pyrGH (2 mM) by addition of CuII in HEPES buffer pH 7.4, measured at 60 MHz, 25°C. 2) T1-weighted images of phantoms of GdIII-DO3A-pyrGH in absence and presence of metal ions (0.3 mM GdIII-DO3A-pyrGH, 1 eq. CuII, 10 eq. ZnII, 10 eq. CaII, HEPES buffer pH 7.4, 9.4 T, 25°C). 3) Structures and CuII-affinities of GdIII-DO3A-pyrGH and its isolated CuII-binding site.