Enzyme-responsive MRIcontrast agents containing a "self-immolative" benzylcarbamate moiety that links the MRI-reporter lanthanide complex to a specific enzyme substrate have been developed. The enzymatic cleavage initiates an electronic cascade reaction that leads to a structural change in the Ln III complex, with a concomitant response in its MRI-contrast-enhancing properties. We synthesized and investigated a series of Gd 3+ and Yb 3+ complexes, including those bearing a self-immolative arm and a sugar unit as selective substrates for bgalactosidase; we synthesized complex LnL 1 , its NH 2 amine derivatives formed after enzymatic cleavage, LnL 2 , and two model compounds, LnL 3 and LnL 4 . All of the Gd 3+ complexes synthesized have a single inner-sphere water molecule. The relaxivity change upon enzymatic cleavage is limited (3.68 vs. 3.15 mm À1 s À1 for complexes GdL 1 and GdL 2 , respectively; 37 8C, 60 MHz), which prevents application of this system as an enzyme-responsive T 1 relaxation agent. Variable-temperature 17 O NMR spectroscopy and 1 H NMRD (nuclear magnetic relaxation dispersion) analysis were used to assess the parameters that determine proton relaxivity for the Gd 3+ complexes, including the water-exchange rate (k ex 298 , varies in the range 1.5-3.9 10 6 s À1 ). Following the enzymatic reaction, the chelates contain an exocyclic amine that is not protonated at physiological pH, as deduced from pH-potentiomet-ric measurements (log K H = 5.