Tarábek Peter, Bonifacić Marija, Beckert Dieter
Interdisciplinary Research Group Time Resolved Spectroscopy, Faculty of Chemistry and Mineralogy, University of Leipzig, Permoserstrasse 15, Leipzig, D-04318, Germany.
J Phys Chem A. 2007 Jun 14;111(23):4958-64. doi: 10.1021/jp0709914. Epub 2007 May 22.
Laser flash photolysis with the Fourier transform electron paramagnetic resonance (FT EPR) and optical spectroscopy detection methods on the nanosecond time scale have been employed in order to investigate the oxidation mechanism of cyclic dipeptides glycine, alanine, and sarcosine anhydrides initiated by SO4*- or 9,10-anthraquinone-2,6-disulfonate (2,6-AQDS) triplet in oxygen free aqueous solutions. A direct hydrogen abstraction from the ring C-H position of an anhydride by both oxidants is proposed as the primary reaction, rather then an electron transfer from nitrogen followed by (alpha)C-H deprotonation. The overall second-order rate constants for the reaction with SO4*- were determined to be 7.2 x 10(7) M(-1) s(-1), 1.2 x 10(8) M(-1) s(-1), and 5.2 x 10(8) M(-1) s(-1) for glycine anhydride, alanine anhydride, and sarcosine anhydride, respectively. The rate constants for 2,6-AQDS triplet as oxidizing species are about two times lower. The radical intermediate products derived from cyclic dipeptides observed on the microsecond time scale were assigned to the general structure of piperazine-2,5-dione-3-yl radical. These are spin polarized by the mechanisms of chemically induced dynamic electron polarization (CIDEP). For SO4*- as the oxidant the spectra are exhibiting an E/A* polarization pattern originating partially from F-pairs of two piperazine-2,5-dione-3-yl radicals.
为了研究在无氧水溶液中由硫酸根阴离子自由基（SO4*-）或9,10-蒽醌-2,6-二磺酸盐（2,6-AQDS）三线态引发的环二肽甘氨酸酐、丙氨酸酐和肌氨酸酐的氧化机制，采用了纳秒时间尺度下的傅里叶变换电子顺磁共振（FT EPR）和光学光谱检测方法的激光闪光光解技术。提出两种氧化剂均直接从酐的环C-H位置夺取氢作为主要反应，而不是从氮进行电子转移随后发生（α）C-H去质子化。与SO4*-反应的总二级速率常数对于甘氨酸酐、丙氨酸酐和肌氨酸酐分别确定为7.2×10⁷ M⁻¹ s⁻¹、1.2×10⁸ M⁻¹ s⁻¹和5.2×10⁸ M⁻¹ s⁻¹。作为氧化物种的2,6-AQDS三线态的速率常数约低两倍。在微秒时间尺度上观察到的源自环二肽的自由基中间产物被确定为哌嗪-2,5-二酮-3-基自由基的一般结构。这些通过化学诱导动态电子极化（CIDEP）机制发生自旋极化。对于作为氧化剂的SO4*-，光谱呈现出部分源自两个哌嗪-2,5-二酮-3-基自由基的F-对的E/A*极化模式。
