Alanine radicals. 2. The composite polycrystalline alanine EPR spectrum studied by ENDOR, thermal annealing, and spectrum simulations
Alanine radicals. 2. The composite polycrystalline alanine EPR spectrum studied by ENDOR, thermal annealing, and spectrum simulations
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DOI：
10.1021/jp026023c
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发表时间：
2002-09-26
影响因子：
2.9
通讯作者：
Sagstuen, E
中科院分区：
文献类型：
--
作者：
Heydari, MZ;Malinen, E;Sagstuen, E
用粉末和单晶X、K和Q波段电子顺磁共振光谱、X波段粉末电子-核双共振、热退火和电子顺磁共振谱模拟研究了氨基酸L-α-丙氨酸辐射诱导自由基的形成。室温照射后得到的光谱是复合的，主要由三个表示为R1、R2和R3的基团的共振组成。R1是已知的、稳定的室温物种，由质子化的丙氨酸阴离子自由基脱胺而成。根据在X、K和Q波段获得的EPR谱的模拟，室温EPR谱似乎由约55%的R1组成。在热退火后，RI共振消失的速度比其他两个组分快。R2物种可能是通过从中心丙氨酸碳原子中净抽氢而在辐射诱导事件的氧化链中形成的。Q-频段EPR用于确定R2的g张量。本种对在室温下记录的共振贡献了大约35%。在热退火时，该自由基的衰变速度慢于R1，导致在480K长期升温后获得的光谱中R2占优势。用粉末Endor验证了在此温度下热退火后剩余的主要物种确实是R2，而不是这两个室温自由基的后继物种。R3物种先前被指定为在羧基上额外质子化的N-去质子化版本的R2。这种共振的详细光谱数据缺失，但基于现有数据和使用类似产品的文献值估计的一组参数已被构建。模拟表明，5%-10%的室温共振可以归因于R3。R3比R1和R2自由基更耐热，在480K长期热处理后，估计所产生的共振由大约51%的R2和43%的R3组成。其余部分(约6%)的共振是由于R1。然而，这些数字必须被认为是暂定的，因为缺乏关于R3的精确光谱数据。
Radiation-induced free radical formation in the amino acid l-alpha-alanine has been studied using powder and single-crystal X-, K-, and Q-band electron paramagnetic resonance (EPR) spectroscopy, X-band powder electron-nuclear double resonance (ENDOR), thermal annealing, and EPR spectrum simulations. The spectra obtained after room temperature irradiations are composite, consisting of resonances from mainly three radicals denoted R1, R2, and R3. R1 is the well-known, stable room-temperature species formed by deamination from a protonated alanine anion radical. On the basis of simulations of EPR spectra, obtained at X-, K-, and Q-bands, the room-temperature EPR spectrum seems to consist of about 55% of R1. Upon thermal annealing, the RI resonance disappears faster than those of the other two components. The R2 species is presumably formed in the oxidative chain of radiation-induced events by net H-abstraction from the central alanine carbon atom. Q-band EPR was used to determine the g-tensor of R2. This species contributes about 35% to the resonance recorded at room temperature. Upon thermal annealing this radical decays slower than R1, resulting in the predominance of R2 in spectra obtained after prolonged warming at 480 K. Powder ENDOR was used to verify that the dominating species remaining after thermal annealing at this temperature indeed is R2 and not a successor species of either of the room-temperature radicals. The R3 species was previously assigned to an N-deprotonated version of R2 being additionally protonated at the carboxyl group. Detailed spectral data for this resonance are missing but a set of parameters based on available data and otherwise estimated using literature values for similar products was constructed. Simulations indicated that 5-10% of the room-temperature resonance could be ascribed to R3. R3 is more heat-resistant than the R1 and R2 radicals, and after prolonged annealing at 480K it was estimated that the resulting resonance consisted of about 51% R2 and 43% R3. The remaining part (about 6%) of the resonance was due to R1. These numbers must, however, be considered as tentative because of the lack of precise spectral data for R3.