There is ongoing debate about the extent to which protein structure is retained after transfer into the gas phase. Here, using ion-mobility spectrometry, we investigated the impact of side-chain–backbone interactions on the structure of gas-phase protein ions by noncovalent attachment of crown ethers (CEs). Our results indicate that in the absence of solvent, secondary interactions between charged lysine side chains and backbone carbonyls can significantly influence the structure of a protein. Once the charged residues are capped with CEs, certain charge states of the protein are found to undergo significant structural compaction.