Sequence and Conformational Analysis of Peptide-Polymer Bioconjugates by Multidimensional Mass Spectrometry

The sequence and helical content of two alanine-rich peptides (AQK18 and GpAQK18, Gp: l-propargylglycine) and their conjugates with poly(ethylene glycol) (PEG) have been investigated by multidimensional mass spectrometry (MS), encompassing electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) interfaced with tandem mass spectrometry (MS²) fragmentation and shape-sensitive separation via ion mobility mass spectrometry (IM-MS). The composition, sequence, and molecular weight distribution of the peptides and bioconjugates were identified by MS and MS² experiments, which also confirmed the attachment of PEG at the C-terminus of the peptides. ESI coupled with IM-MS revealed the existence of random coil and α-helical conformers for the peptides in the gas phase. More importantly, the proportion of the helical conformation increased substantially after PEG attachment, suggesting that conjugation adds stability to this conformer. The conformational assemblies detected in the gas phase were largely formed in solution, as corroborated by independent circular dichroism (CD) experiments. The collision cross sections (rotationally averaged forward moving areas) of the random coil and helical conformers of the peptides and their PEG conjugates were simulated for comparison with the experimental values deduced by IM-MS in order to confirm the identity of the observed architectures and understand the stabilizing effect of the polymer chain. C-terminal PEGylation is shown to increase the positive charge density and to solvate intramolecular positive charges at the conjugation site, thereby enhancing the stability of α-helices, preserving their conformation and increasing helical propensity.