Hydroquinones are known for their redox activity. For this reason, the HGA-derived pigment was investigated by EPR spectroscopy (Fig. ), which confirmed the presence of an airand water-stable radical. The signal is notably narrow, without hyperfine coupling, and relatively easy to saturate even at low microwave power. At least two distinct radical species are present within the HGA-derived pigment. The presence of the radicals in synthetic pigment suggested considerable reactivity of the pigment within the cartilage environment in AKU.