Catechol incubated in DMEM at 37 °C exhibited rapid, spontaneous, and concentrationdependent reactions in physiologically-relevant media that could be observed visually and monitored spectrophotometrically in real-time (Fig. ). Given that there were no live cells in this medium, nor was there any serum supplementation, these reactions therefore occurred entirely in the absence of biomacromolecular catalysts or cellular biotransformation (i.e. reactions were enzyme-and cell-free) and resulted in significant browning of the medium, which for catechol concentrations ranging from 0.2 to 15.6 mM was visually and spectrophotometrically evident at 340 nm within 1 h or less (Fig. ). Browning rates followed a hyperbolic trend: linear at concentrations below ~1 mM, and plateauing at higher concentrations (Fig. ). Moreover, at catechol concentrations <1 mM, a lag phase preceded detectable changes at 340 nm. For example, 0.2 mM catechol exhibited a ~1-h lag phase before showing linear absorbance increases over the final 120 min of the 190-min assay (r² = 0.9250; n = 3 independent kinetic assays). Higher catechol concentrations shortened lag phases and improved linearity (r² = 0.9600 at 0.5 mM, r² = 0.9800 at 1.0 mM; n = 3 independent kinetic assays), with no lag phase observed at ≥ 2.0 mM, where browning commenced immediately upon addition of catechol. The reason