10.1021/acs.jpclett.8b00348.s002 David Punihaole David Punihaole Ryan S. Jakubek Ryan S. Jakubek Riley J. Workman Riley J. Workman Sanford A. Asher Sanford A. Asher Interaction Enthalpy of Side Chain and Backbone Amides in Polyglutamine Solution Monomers and Fibrils American Chemical Society 2018 Q 10 fibrils D 2 Q 10 K 2 side chain van der Waals Δ H int interamide side chain interactions Backbone Amides amide carbonyl dynamics simulations Δ H int values Polyglutamine Solution Monomers Side Chain amyloid-like fibril conformations polyQ fibrils Interaction Enthalpy vibrational band frequencies 2018-03-23 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Interaction_Enthalpy_of_Side_Chain_and_Backbone_Amides_in_Polyglutamine_Solution_Monomers_and_Fibrils/6083078 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<i>H</i><sub>int</sub>) in monomeric and amyloid-like fibril conformations of D<sub>2</sub>Q<sub>10</sub>K<sub>2</sub> (Q10). We independently verified these Δ<i>H</i><sub>int</sub> 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<i>H</i><sub>int</sub> 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.