¹H Parashift Probes for Ratiometric Detection of Nitroreductase

Spectroscopic detection of paramagnetically shifted ¹H signals of trivalent lanthanide complexes provide a means for ratiometric detection of enzymatic activity. Herein, we report on the synthesis and characterization of TbL1, DyL1, and TbL2 complexes for ratiometric detection of the enzyme nitroreductase (NTR). Ln(III) coordination environments with distinct off- and on- probe states provide unique paramagnetic contributions to the chemical shift of a 5-(^tBu)-pyridine-based reporter group for quantitative detection of enzyme activity. The paramagnetic contribution to chemical shift from Tb(III) and Dy(III) shift the ¹H chemical signal beyond the background region to low frequencies (δ^PCS < -15) for zero background signal. Paramagnetic relaxation enhancement of reporter group protons enables the development of probe-specific pulse sequences with short acquisition and delay times. A probe-specific pulse sequence for TbL2 acquired 256 scans in under 1 min and enabled ratiometric detection of NTR activity.