Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.
J Phys Chem A. 2011 Oct 6;115(39):10679-88. doi: 10.1021/jp203909y. Epub 2011 Sep 14.
The radical cationic reactivity of the peptide analogue molecule CH(3)CO-Gly-NH(2) is addressed both experimentally and theoretically. The radical cation intermediate of CH(3)CO-Gly-NH(2) is created by single-photon ionization of this molecule at 118.22 nm (~10.5 eV). The two most stable conformers (C(7) and C(5)) of this molecule exhibit different folds along the backbone: the C(7) conformer has a γ-turn structure, and the C(5) conformer has a β-strand structure. The experimental results show that the radical cation intermediate of CH(3)CO-Gly-NH(2) dissociates and generates a fragment-ion signal at 73 amu that is observed through TOFMS. Theoretical results show how the fragment-ion signal at 73 amu is generated by only one conformer of CH(3)CO-Gly-NH(2) (C(7)) and how local charge and specific hydrogen bonding in the molecule influence fragmentation of the radical cation intermediate of CH(3)CO-Gly-NH(2). The specific fold of the molecule controls fragmentation of this reactive radical cation intermediate. Whereas the radical cation of the C(7) conformer dissociates through a hydrogen-transfer mechanism followed by HNCO elimination, the radical cation of the C(5) conformer does not dissociate at all. CASSCF calculations show that positive charge in the radical cationic C(7) conformer is localized at the NH(2)CO moiety of the molecular ion. This site-specific localization of the positive charge enhances the acidity of the terminal NH(2) group, facilitating hydrogen transfer from the NH(2) to the COCH(3) end of the molecular ion. Positive charge in the C(5) conformer of the CH(3)CO-Gly-NH(2) radical cation is, however, localized at the COCH(3) end of the molecular ion, and this conformer does not have enough energy to surmount the energy barrier to dissociation on the ion potential energy surface. CASSCF results show that conformation-specific localization of charge in the CH(3)CO-Gly-NH(2) molecular ion occurs as a result of the different hydrogen-bonding interactions involved in the different molecular conformers.
本文从实验和理论两方面研究了肽类似物分子 CH(3)CO-Gly-NH(2)的自由基阳离子反应性。通过 118.22nm（~10.5eV）的单光子电离，生成了 CH(3)CO-Gly-NH(2)的自由基阳离子中间体。该分子的两种最稳定构象（C(7)和 C(5)）在骨架上具有不同的折叠方式：C(7)构象具有γ-转角结构，C(5)构象具有β-链结构。实验结果表明，CH(3)CO-Gly-NH(2)的自由基阳离子中间体解离，并通过 TOFMS 观察到在 73amu 处产生碎片离子信号。理论结果表明，只有 CH(3)CO-Gly-NH(2)的一种构象（C(7)）如何生成 73amu 处的碎片离子信号，以及分子中的局部电荷和特定氢键如何影响 CH(3)CO-Gly-NH(2)的自由基阳离子中间体的碎裂。分子的特定折叠控制着这种反应性自由基阳离子中间体的碎裂。C(7)构象的自由基阳离子通过氢转移机制随后发生 HNCO 消除而解离，而 C(5)构象的自由基阳离子则根本不发生解离。CASSCF 计算表明，在 C(7)构象的正离子中，正电荷定位于分子离子的 NH(2)CO 部分。这种特定位置的正电荷增强了末端 NH(2)基团的酸性，促进了氢从 NH(2)向分子离子的 COCH(3)端的转移。然而，CH(3)CO-Gly-NH(2)自由基阳离子的 C(5)构象中的正电荷定位于分子离子的 COCH(3)端，并且该构象没有足够的能量克服离子势能面上的解离能垒。CASSCF 结果表明，CH(3)CO-Gly-NH(2)分子离子中电荷的构象特异性定位是由于不同分子构象中涉及的不同氢键相互作用所致。
