We determined an empirical correlation that relates the amide I vibrational band frequencies of the glutamine (Q) side chain to the strength of hydrogen bonding, van der Waals, and Lewis acid-base interactions of its primary amide carbonyl. We used this correlation to determine the Q side chain carbonyl interaction enthalpy (Δ H) in monomeric and amyloid-like fibril conformations of DQK (Q10). We independently verified these Δ H values through molecular dynamics simulations that showed excellent agreement with experiments. We found that side chain-side chain and side chain-peptide backbone interactions in fibrils and monomers are more enthalpically favorable than are Q side chain-water interactions. Q10 fibrils also showed a more favorable Δ H for side chain-side chain interactions compared to backbone-backbone interactions. This work experimentally demonstrates that interamide side chain interactions are important in the formation and stabilization of polyQ fibrils.
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