Many details pertaining to the formation and interactions of protein aggregates associated with neurodegenerative diseases are invisible to conventional biophysical techniques. We recently introduced 15N dark-state exchange saturation transfer (DEST) and 15N lifetime line-broadening to study solution backbone dynamics and position-specific binding probabilities for amyloid β (Aβ) monomers in exchange with large (2–80 MDa) protofibrillar Aβ aggregates. Here we use 13Cmethyl DEST and lifetime line-broadening to probe the interactions and dynamics of methyl-bearing side chains in the Aβ-protofibril-bound state. We show that all methyl groups of Aβ40 populate direct-contact bound states with a very fast effective transverse relaxation rate, indicative of side-chain-mediated direct binding to the protofibril surface. The data are consistent with position-specific enhancements of C13methyl-R2tethered values in tethered states, providing further insights into the structural ensemble of the protofibril-bound state.