Unveiling Charge-Pair Salt-Bridge Interaction Between GAGs and Collagen Protein in Cartilage: Atomic Evidence from DNP-Enhanced ssNMR at Natural Isotopic Abundance.

The interactions between glycosaminoglycans (GAGs) and proteins are essential in numerous biochemical processes that involve ion-pair interactions. However, there is no evidence of direct and specific interactions between GAGs and collagen proteins in native cartilage. The resolution of solid-state NMR (ssNMR) can offer such information but the detection of GAG interactions in cartilage is limited by the sensitivity of the experiments when C and N isotopes are at natural abundance. In this communication, this limitation is overcome by taking advantage of dynamic nuclear polarization (DNP)-enhanced magic-angle spinning (MAS) experiments to obtain two-dimensional (2D) N-C and C-C correlations on native samples at natural abundance. These experiments unveiled inter-residue correlations in the aliphatic regions of the collagen protein previously unobserved. Additionally, our findings provide direct evidence of charge-pair salt-bridge interactions between negatively charged GAGs and positively charged arginine (Arg) residues of collagen protein. We also identified potential hydrogen bonding interactions between hydroxyproline (Hyp) and GAGs, offering atomic insights into the biochemical interactions within the extracellular matrix of native cartilage. Our approach may provide a new avenue for the structural characterization of other native systems.