In Fig. , the HETCOR spectra of all cartilage samples had a broad C signal at 130 ppm that corresponds to aromatic residues (tyrosine, phenylalanine, histidine) and a narrower signal at 115 ppm corresponding to tyrosine. In pigmented cartilage (Fig. , second row, red), there is an additional C signal at 120 ppm that is absent in the non-pigmented AKU cartilage and the non-AKU OA cartilage. This signal coincides with the spectrum of the synthetic HGA-derived pigment (Fig. , top row, brown). Thus, we assigned the 120 ppm signal in pigmented cartilage to the aromatic carbons in the HGA-derived pigment.