Experimental evidence for an inverse hydrogen migration in arginine radicals
Experimental evidence for an inverse hydrogen migration in arginine radicals
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DOI：
10.1021/ja800207x
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发表时间：
2008-06-18
影响因子：
15
通讯作者：
Turecek, Frantisek
中科院分区：
文献类型：
--
作者：
Hayakawa, Shigeo;Matsubara, Hiroshi;Turecek, Frantisek
理论上预测，由电子转移到质子化精氨酸形成的自由基将经历氢原子从C-α位置到胍碳原子的反向迁移。这里报道的实验证实，一小部分精氨酸和精氨酰胺自由基经历了这样的反向氢迁移。在动能为3和50keV时，重排的精氨酸和精氨酰胺C-α自由基与前驱体阳离子Cs原子发生电荷反转后，被检测到是稳定的阴离子。在B3-PMP2/AUG-CC-pVTZ势能面上的RRKM计算表明，精氨酸自由基经历侧链的快速旋转，达到适合C-α-H转移的构象，这在电子转移形成的自由基中是快的(k>；10(9)S(-1))。相比之下，H原子从胍基转移到羧基或酰胺C=O基团的速度是C-α-H原子迁移速度的50倍。精氨酸自由基中的胍基被认为是一个很差的氢原子供体，但却是一个很好的氢原子受体，因此可以被视为一个自由基陷阱。这一性质可以解释为什么在含精氨酸多肽的电子俘获和电子转移质谱图中经常观察到非解离阳离子自由基。
Radicals formed by electron transfer to protonated arginine have been predicted by theory to undergo an inverse migration of the hydrogen atom from the C-alpha position to the guanidine carbon atom. Experiments are reported here that confirm that a fraction of arginine and arginine amide radicals undergo such an inverse hydrogen migration. The rearranged arginine and arginine amide C-alpha radicals are detected as stable anions after charge inversion by collisions with Cs atoms of precursor cations at 3 and 50 keV kinetic energies. RRKM calculations on the B3-PMP2/aug-cc-pVTZ potential energy surface indicate that arginine radicals undergo rapid rotations of the side chain to reach conformations suitable for C-alpha-H transfer, which is calculated to be fast (k > 10(9) s(-1)) in radicals formed by electron transfer. By contrast, H-atom transfer from the guanidine group onto the carboxyl or amide C = O groups is 50 times slower than the C-alpha-H atom migration. The guanidine group in arginine radicals is predicted to be a poor hydrogen-atom donor but a good H-atom acceptor and thus can be viewed as a radical trap. This property can explain the frequent observation of nondissociating cation radicals in electron capture and electron transfer mass spectra of arginine-containing peptides.