Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
J Am Soc Mass Spectrom. 2010 Oct;21(10):1737-52. doi: 10.1016/j.jasms.2010.06.010. Epub 2010 Jun 25.
Gas-phase reactivity of a positively charged aromatic σ,σ-biradical (N-methyl-6,8-didehydroquinolinium) was examined toward six aliphatic amino acids and 15 dipeptides by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) and laser-induced acoustic desorption (LIAD). While previous studies have revealed that H-atom and NH(2) abstractions dominate the reactions of related monoradicals with aliphatic amino acids and small peptides, several additional, unprecedented reaction pathways were observed for the reactions of the biradical. For amino acids, these are 2H-atom abstraction, H(2)O abstraction, addition - CO(2), addition - HCOOH, and formation of a stable adduct. The biradical reacts with aliphatic dipeptides similarly as with aliphatic amino acids, but undergoes also one additional reaction pathway, addition/C-terminal amino acid elimination (addition - CO - NHCHR(C)). These reactions are initiated by H-atom abstraction by the biradical from the amino acid or peptide, or nucleophilic addition of an NH(2) or a HO group of the amino acid or peptide at the radical site at C-6 in the biradical. Reactions of the unquenched C-8 radical site then yield the products not observed for related monoradicals. The biradical reacts with aromatic dipeptides with an aromatic ring in N-terminus (i.e., Tyr-Leu, Phe-Val, and Phe-Pro) similarly as with aliphatic dipeptides. However, for those aromatic dipeptides that contain an aromatic ring in the C-terminus (i.e., Leu-Tyr and Ala-Phe), one additional pathway, addition/N-terminal amino acid elimination (addition - CO - NHCHR(N)), was observed. This reaction is likely initiated by radical addition of the biradical at the aromatic ring in the C-terminus. Related monoradicals add to aromatic amino acids and small peptides, which is followed by Cα-Cβ bond cleavage, resulting in side-chain abstraction by the radical. For biradicals, with one unquenched radical site after the initial addition, the reaction ultimately results in the loss of the N-terminal amino acid. Similar to monoradicals, the C-S bond in amino acids and dipeptides was found to be especially susceptible to biradical attack.
采用傅里叶变换离子回旋共振质谱（FT-ICR）和激光诱导声解吸（LIAD）技术，研究了带正电荷的芳香 σ,σ-双自由基（N-甲基-6,8-二去氢喹啉鎓）与六种脂肪族氨基酸和 15 种二肽的气相反应活性。虽然先前的研究表明，H 原子和 NH2 夺取是相关单自由基与脂肪族氨基酸和小肽反应的主要途径，但对于双自由基的反应，观察到了几个额外的、前所未有的反应途径。对于氨基酸，这些反应途径包括 2 个 H 原子夺取、H2O 夺取、加成-CO2、加成-HCOOH 和稳定加合物的形成。双自由基与脂肪族二肽的反应与脂肪族氨基酸类似，但也经历了另外一种反应途径，加成/C-末端氨基酸消除（加成-CO-NHCHR(C)）。这些反应是由双自由基从氨基酸或肽中夺取 H 原子，或氨基酸或肽中的 NH2 或 HO 基团在双自由基的 C-6 位上进行亲核加成而引发的。然后，未猝灭的 C-8 自由基位的反应生成了与相关单自由基反应中观察到的不同的产物。双自由基与 N-末端含有芳香环的芳香族二肽（即 Tyr-Leu、Phe-Val 和 Phe-Pro）的反应与脂肪族二肽类似。然而，对于那些 C-末端含有芳香环的芳香族二肽（即 Leu-Tyr 和 Ala-Phe），观察到了另外一种反应途径，加成/N-末端氨基酸消除（加成-CO-NHCHR(N)）。该反应可能是由双自由基在 C-末端的芳香环上的自由基加成引发的。相关的单自由基加成到芳香族氨基酸和小肽上，随后 Cα-Cβ 键断裂，导致侧链由自由基夺取。对于双自由基，在初始加成后，有一个未猝灭的自由基位，反应最终导致 N-末端氨基酸的丢失。与单自由基类似，氨基酸和二肽中的 C-S 键也容易受到双自由基的攻击。
