H Parashift Probes for Ratiometric Detection of Nitroreductase.

Spectroscopic detection of paramagnetically shifted H signals of trivalent lanthanide complexes provide a means for ratiometric detection of enzymatic activity. Herein, we report on the synthesis and characterization of , , and complexes for ratiometric detection of the enzyme nitroreductase (NTR). Ln(III) coordination environments with distinct - and - probe states provide unique paramagnetic contributions to the chemical shift of a 5-(Bu)-pyridine-based reporter group for quantitative detection of enzyme activity. The paramagnetic contribution to chemical shift from Tb(III) and Dy(III) shift the H chemical signal beyond the background region to low frequencies (δ < -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 acquired 256 scans in under 1 min and enabled ratiometric detection of NTR activity.