Alkaptonuria is a rare disease that leads to striking pigmentation and mechanical failure of cartilage tissue. DNP-enhanced solid-state NMR enabled us to observe disruption of interstrand hydrogen bonding in collagen proteins from alkaptonuria patient tissue. We propose that a glycyl radical mechanism underlies the degradation and pigmentation of cartilage tissue in alkaptonuria and can also be a contributing mechanism in common osteoarthritis.