Are Carboxyl Groups the Most Acidic Sites in Amino Acids? Gas-Phase Acidity, H/D Exchange Experiments, and Computations on Cysteine and Its Conjugate Base

详细信息    查看全文

• 作者：Zhixin Tian ; Anna Pawlow ; John C. Poutsma ; Steven R. Kass
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：May 2, 2007
• 年：2007
• 卷：129
• 期：17
• 页码：5403 - 5407
• 全文大小：165K
• 年卷期：v.129,no.17(May 2, 2007)
• ISSN：1520-5126

文摘

Hydrogen-deuterium exchange experiments were carried out on the conjugate base of cysteinewith four different deuterated alcohols. Three H/D exchanges are observed to take place in each case,and a relay mechanism which requires the SH and CO₂H groups to have similar acidities and subsequentlyproceeds through a zwitterionic intermediate is proposed. Gas-phase acidity measurements also were carriedout in a quadrupole ion trap using the extended kinetic method and in a Fourier transform mass spectrometerby an equilibrium determination. The results are in excellent accord with each other and high-level ab initioand density functional theory calculations and indicate that the side-chain thiol in cysteine is more acidicthan the carboxyl group by 3.1 kcal mol^-1. Deprotonated cysteine is thus predicted to be a thiolate ion. Azwitterionic species also was located on the potential energy surface, but it is energetically unfavorable(+10.1 kcal mol^-1).