Gas-Phase Acidities of Cysteine-Polyalanine Peptides I: A_3,4CSH and HSCA_3,4

Abstract

The gas-phase acidities of four cysteine-polyalanine peptides, A(3,4)CSH and HSCA(3,4), were determined using the extended Cooks kinetic method with full entropy analysis. A triple-quadrupole mass spectrometer with an electrospray interface was employed for the experimental study. The ion activation was achieved via collision-induced dissociation (CID) experiments. The deprotonation enthalpies (Delta(acid)H) of the peptides were determined to be 332.2 +/- 2.0 kcal/mol (A(3)CSH), 325.9 +/- 2.0 kcal/mol (A(4)CSH), 319.3 +/- 3.0 kcal/mol (HSCA(3)), and 319.2 +/- 4.0 kcal/mol (HSCA(4)). The deprotonation entropies (Delta(acid)S) of the peptides were estimated based on the entropy term (Delta(DeltaS)) and the deprotonation entropies of the reference acids. By using the deprotonation enthalpies and entropies, the gas-phase acidities (Delta(acid)G) of the peptides were derived: 325.0 +/- 2.0 kcal/mol (A(3)CSH), 320.2 +/- 2.0 kcal/mol (A(4)CSH), 316.3 +/- 3.0 kcal/mol (HSCA(3)), and 315.4 +/- 4.0 kcal/mol (HSCA(4)). Conformations and energetic information of the peptides were calculated through simulated annealing (Tripos), geometry optimization (AM1), and single-point energy calculations (B3LYP/6-31+G(d)), respectively. The calculated theoretical deprotonation enthalpies (Delta(acid)H) of 334.2 kcal/mol (A(3)CSH), 327.7 kcal/mol (A(4)CSH), 320.6 kcal/mol (HSCA(3)), and 318.6 kcal/mol (HSCA(4)) are in good agreement with the experimentally determined values. Both the experimental and computational studies suggest that the two N-terminal cysteine peptides, HSCA(3,4), are significantly more acidic than the corresponding C-terminal ones, A(3,4)CSH. The high acidities of the former are likely due to the helical conformational effects for which the thiolate anion may be strongly stabilized by the interaction with the helix macrodipole.

Gas-phase acidities of cysteine-polyglycine peptides: The effect of the cysteine position

“…This similar behavior has been observed in the cysteine-polyalanine peptides [57]. The N-cysteine peptides, CA 3 H and CA 4 H, are more acidic than the Ccysteine ones, A 3 CH and A 4 CH, by 10 and 5 kcal/mol, respectively.…”

“…In recent years, many of the gas-phase studies have focused on the determination of the proton affinities (and gas-phase basicities) of amino acids and peptides [33][34][35][36][37][38][39][40][41][42][43][44][45]. Although the gas-phase acidities of isolated amino acids have been studied extensively [46 -54], and the apparent acidities of multiply charged peptides have been reported [55], the information on the acidities of gas-phase neutral peptides is very limited [56,57].…”

“…In recent years, we have started to investigate the helix conformational effects on the gas-phase acidities of peptides by using a series of polyalanine-and polyglycine-based model peptides with cysteine as the acidic probe [6,56,57]. All of the peptides are amidated at the C-terminus, so that the thiol group (SH) of the cysteine residue is expected to be the only acidic site.…”

Proton affinity of dipeptides containing alanine and diaminobutyric acid

“…Our previous studies have demonstrated that conformations could significantly influence the gas-phase acidity of oligopeptides. For the series of cysteine (Cys)-containing peptides, the N-Cys peptides were significantly more acidic than the C-Cys peptides [34][35][36]. Computational studies suggested that the deprotonated N-Cys peptides with four residues and longer existed in a helical conformation, while the deprotonated C-Cys peptides preferred a globular conformation [35,36].…”

“…For the series of cysteine (Cys)-containing peptides, the N-Cys peptides were significantly more acidic than the C-Cys peptides [34][35][36]. Computational studies suggested that the deprotonated N-Cys peptides with four residues and longer existed in a helical conformation, while the deprotonated C-Cys peptides preferred a globular conformation [35,36]. The stronger acidity of the N-Cys peptides was mainly due to the favored interaction with the helix macro-dipole that stabilized the N-terminal thiolate anion.…”