Marek Aleš, Tureček František
Department of Chemistry, University of Washington, Bagley Hall, Seattle, WA, 98195-1700, USA.
J Am Soc Mass Spectrom. 2014 May;25(5):778-89. doi: 10.1007/s13361-014-0832-0. Epub 2014 Feb 19.
Gas-phase dissociations were investigated for several peptide ions containing the Gly-Leu* N-terminal motif where Leu* was a modified norleucine residue containing the photolabile diazirine ring. Collisional activation of gas-phase peptide cations resulted in facile N₂ elimination that competed with backbone dissociations. A free lysine ammonium group can act as a Brønsted acid to facilitate N₂ elimination. This dissociation was accompanied by insertion of a lysine proton in the side chain of the photoleucine residue, as established by deuterium labeling and gas-phase sequencing of the products. Electron structure calculations were used to provide structures and energies of reactants, intermediates, and transition states for Gly-Leu*-Gly-Gly-Lys amide ions that were combined with RRKM calculations of unimolecular rate constants. The calculations indicated that Brønsted acid-catalyzed eliminations were kinetically preferred over direct loss of N₂ from the diazirine ring. Mechanisms are proposed to explain the proton-initiated reactions and discuss the reaction products. The non-catalyzed diazirine ring cleavage and N₂ loss is proposed as a thermometer dissociation for peptide ion dissociations.
对几种含有Gly-Leu* N端基序的肽离子进行了气相解离研究，其中Leu是一个含有光不稳定重氮环的修饰正亮氨酸残基。气相肽阳离子的碰撞活化导致了与主链解离相竞争的N₂的轻松消除。一个游离的赖氨酸铵基团可以作为布朗斯特酸来促进N₂的消除。通过氘标记和产物的气相测序证实，这种解离伴随着赖氨酸质子插入到光亮氨酸残基的侧链中。电子结构计算用于提供Gly-Leu-Gly-Gly-Lys酰胺离子的反应物、中间体和过渡态的结构和能量，并结合RRKM单分子速率常数计算。计算表明，布朗斯特酸催化的消除在动力学上比直接从重氮环上损失N₂更有利。提出了一些机制来解释质子引发的反应并讨论反应产物。非催化的重氮环裂解和N₂损失被认为是肽离子解离的一种温度计解离。
