No 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.

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA 3 and HSCA 4 , using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (⌬ acid H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be ⌬ acid H(HSCA 3 ) ϭ 317.3 Ϯ 2.4 kcal/mol and ⌬ acid H (HSCA 4 ) ϭ 316.2 Ϯ 3.9 kcal/mol. Large entropy effects (⌬(⌬S) ϭ 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (⌬ acid G app ) of 322.1 Ϯ 2.4 kcal/mol (HSCA 3 ) and 320.1 Ϯ 3.9 kcal/mol (HSCA 4 ), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA 3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation. (J Am Soc Mass Spectrom 2007, 18, 188 -194)