Gas phase acidity of a cysteine residue in small oligopeptides

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• 作者：Jialin Shen ; Jianhua Ren ; ^{jren@pacific.edu}
• 关键词：Gas-phase acidity ; Deprotonation enthalpy ; Cysteine ; Oligopeptide ; Kinetic method
• 刊名：International Journal of Mass Spectrometry
• 出版年：2012
• 期刊代码：43_13873806
• 类别：ch
• 出版时间：15 April, 2012
• 卷：316-318
• 期：Complete
• 页码：147-156
• 文件大小：801 K

摘要

The conformational effects on the gas-phase acidities of small cysteine-containing peptides were examined using ten oligopeptides. The gas-phase deprotonation enthalpies were measured using the extended Cooks kinetic method with full entropy analysis. The experiments were carried out using a triple quadrupole mass spectrometer. The values of 螖_acidH were determined to be 335.6 卤 1.7 kcal/mol (AlaCysNH₂), 334.6 卤 1.8 kcal/mol (Ala₂CysNH₂), 331.2 卤 1.8 kcal/mol (CysAlaNH₂), 330.5 卤 2.0 kcal/mol (CysAla₂NH₂), 329.7 卤 1.8 kcal/mol (AlaCysAlaNH₂), 335.3 卤 1.8 kcal/mol (GlyCysNH₂), 334.6 卤 1.7 kcal/mol (Gly₂CysNH₂), 330.4 卤 1.8 kcal/mol (CysGlyNH₂), 329.7 卤 1.8 kcal/mol (CysGly₂NH₂), and 327.3 卤 2.0 kcal/mol (GlyCysGlyNH₂). The gas-phase acidities (螖_acidG) and the deprotonation entropies (螖_acidS) for these peptides were determined accordingly. These results suggested that the tripeptides were more acidic than the corresponding dipeptides by about 1 kcal/mol, and the N-cysteine peptides were more acidity than the isomeric C-cysteine peptides by about 4 kcal/mol. The initial conformations of the peptides were modeled via a conformational search using the MMFF method. The final geometries and energies were calculated at the B3LYP/6-31++G(d,p) level of theory. The calculated enthalpies of deprotonation agreed reasonably well with the experimental results. The conformations of the deprotonated N-cysteine peptides were more compact than those of the C-cysteine analogues. The more compact conformations allowed more efficient multiple hydrogen-bonding interactions between the thiolate anion and the nearby NH bonds. The greater acidities of the N-cysteine peptides were likely the results of the more favorable hydrogen-bonding and charge-amide dipole interactions that stabilized the thiolate anions more efficiently.