Figure 2.

A) ¹³C aromatic region 110–140 ppm sections of ssNMR ¹H–¹³C HETCOR spectra of: synthetic HGA‐derived pigment (brown, top row), pigmented (red, second row) and non‐pigmented cartilage (green, third row) from AKU patient, and cartilage (purple, bottom row) from OA patient. DNP enhancement was not obtained on the synthetic pigment. Grey stripes highlight the signals attributed to the AKU pigment, also in extracted slices (right) which we also present as an overlay (50 μs, Figure S5). B) EPR spectra of two aqueous solutions of the synthetic HGA‐derived pigment. Sample 1 was recovered from the solid‐state NMR rotor on which DNP enhancement was attempted, on which a room‐temperature X‐band EPR continuous‐wave spectrum was acquired. Sample 2 did not have other radical species added. Here, a W‐band EPR field swept echo spectrum (brown) was acquired at 80 K, and the pseudo field modulation spectrum (magenta) calculated. Two components with different g‐matrices, (1) narrow and (2) broad, can be distinguished. Asterisks (*) indicate Mn²⁺ impurities.