Cell-free screening. In vitro assays determining the PQ probes' redox properties are based on reduction-induced fluorophore release, quantified using a BMG FluoStar Omega plate reader (ex 355bp20, em 520lp). Black 96-well plates with black bottom were charged with probe and subjected to chemo-and bioreductant challenge (assay medium 1% DMSO in TE buffer). PQ release was monitored at set times (1 min, 15 min, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 15 h), in between which the plates were held in a thermostatted oven at 37 °C in H2O-saturated atmosphere. Raw data were processed using Microsoft Excel and GraphPad Prism. Fluorescence values F(t) were typically normalised per timepoint to the TCEP control F TCEP (t) (as a reference for fast, quantitative probe reduction and activation, so as to eliminate the delay of cyclisation kinetics from analysis). Data plotting was conducted using GraphPad Prism. Human recombinant thioredoxin 1 (Trx 1) (lyophilized), human recombinant glutaredoxin 1 (Grx 1) (lyophilized from 10 µL TE-buffer, pH 7.5), human thioredoxin reductase (TrxR) (1.5 mg/mL in 50% glycerol/TE-buffer, pH 7.5) and baker's yeast glutathione reductase (GR) (100 µM in 50% glycerol/TE-buffer, pH 7.5) were obtained from IMCO Corp., Stockholm (Sweden).