Helical Peptoid Ions in the Gas Phase: Thwarting the Charge Solvation Effect by H-Bond Compensation

Folding and unfolding processes are key aspects that should be mastered for the design of foldamer molecules for targeted applications. In contrast to the solution phase, in vacuo conditions represent a well-defined environment to analyze the intramolecular interactions that largely control the folding/unfolding dynamics. Ion mobility mass spectrometry coupled to theoretical modeling represents an efficient method to decipher the spatial structures of gaseous ions, including foldamers. However, charge solvation typically compacts the ion structure in the absence of strong stabilizing secondary interactions. This is the case in peptoids that are synthetic peptide regioisomers whose side chains are connected to the nitrogen atoms of the backbone instead of α-carbon as in peptides, thus implying the absence of H-bonds among the core units of the backbone. A recent work indeed reported that helical peptoids based on Nspe units formed in solution do not retain their secondary structure when transferred to the gas phase upon electrospray ionization (ESI). In this context, we demonstrate here that the helical structure of peptoids bearing (S)-N-(1-carboxy-2-phenylethyl) bulky side chains (Nscp) is largely preserved in the gas phase by the creation of a hydrogen bond network, induced by the presence of carboxylic moieties, that compensates for the charge solvation process.