Gas-Phase Acidities of Cysteine-Polyglycine Peptides: The Effect of the Cysteine Position

Abstract

The sequence and conformational effects on the gas-phase acidities of peptides have been studied by using two pairs of isomeric cysteine-polyglycine peptides, CysGly3,4NH2 and Gly3,4CysNH2. The extended Cooks kinetic method was employed to determine the gas-phase acidities using a triple quadrupole mass spectrometer with an electrospray ionization source. The ion activation was achieved via collision-induced dissociation experiments. The deprotonation enthalpies (δacidH) were determined to be 323.9 ±2.5 kcal/mol (CysGly3NH2), 319.2 ±2.3 kcal/mol (CysGly4NH2), 333.8 ±2.1 kcal/mol (Gly3CysNH2), and 321.9 ±2.8 kcal/mol (Gly4CysNH2), respectively. The corresponding deprotonation entropies (δacidS) of the peptides were estimated. The gas-phase acidities (δacidG) were derived to be 318.4 ±2.5 kcal/mol (CysGly3NH2), 314.9 ±2.3 kcal/mol (CysGly4NH2), 327.5 ±2.1 kcal/mol (Gly3CysNH2), and 317.4 ±2.8 kcal/mol (Gly4CysNH2), respectively. Conformations and energetic information of the neutral and anionic peptides were calculated through simulated annealing (Tripos), geometry optimization (AM1), and single point energy calculations (B3LYP/6-31+G(d)), respectively. Both neutral and deprotonated peptides adopt many possible conformations of similar energies. All neutral peptides are mainly random coils. The two C-cysteine anionic peptides, Gly3,4(Cys-H)-NH2, are also random coils. The two N-cysteine anionic peptides, (Cys-H)-Gly3,4NH2, may exist in both random coils and stretched helices. The two N-cysteine peptides, CysGly3NH2 and CysGly4NH2, are significantly more acidic than the corresponding C-terminal cysteine ones, Gly3CysNH2 and Gly4CysNH2. The stronger acidities of the former may come from the greater stability of the thiolate anion resulting from the interaction with the helix-macrodipole, in addition to the hydrogen bonding interactions. © 2010 American Society for Mass Spectrometry.