Biophysical studies of anhydrous peptide structure



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Defining the intrinsic properties of amino acids which dictate the formation of helices, the most common protein secondary structure element, is an essential part of understanding protein folding. Pauling and co-workers initially predicted helical peptide folding motifs in the absence of solvent, suggesting that in vacuo studies may potentially discern the role of solvation in protein structure. Ion mobility-mass spectrometry (IMMS) combines a gas-phase ion separation based on collision cross-section (apparent surface area) with time-of-flight MS. The result is a correlation of collision cross-section with mass-to-charge, allowing detection of multiple conformations of the same ion. Most gas-phase peptide ions assume a compact, globular state that minimizes exposure to the low dielectric environment and maximizes intramolecular charge solvation. Conversely, a small number of peptides adopt a more extended (?-sheet or ?-helix) conformation and exhibit a larger than predicted collision cross-section. Collision cross-sections measured using IM-MS are correlated with theoretical models generated using simulated annealing and allow for assignment of the overall ion structural motif (e.g. helix vs. chargesolvated globule). Here, two series of model peptides having known solution-phase helical propensities, namely Ac-(AAKAA)nY-NH2 (n = 3, 4, 5, 6 and 7) and Ac-Y(AEAAKA)nF-NH2 (n = 2, 3, 4, and 5), are investigated using IM-MS. Both protonated ([M + H]+) and metalcoordinated ([M + X]+ where X = Li, Na, K, Rb or Cs) species were analyzed to better understand the interplay of forces involved in gas-phase helical structure and stability. The data are analyzed using computational methods to examine the influence of peptide length, primary sequence, and number of basic (Lys, K) and acidic (Glu, E) residues on anhydrous ion structure.