Davis Ian, Koto Teruaki, Terrell James R, Kozhanov Alexander, Krzystek J, Liu Aimin
Department of Chemistry , University of Texas , San Antonio , Texas 78249 , United States.
Department of Chemistry , Georgia State University , Atlanta , Georgia 30303 , United States.
J Phys Chem A. 2018 Mar 29;122(12):3170-3176. doi: 10.1021/acs.jpca.7b12434. Epub 2018 Mar 15.
Tryptophan-based free radicals have been implicated in a myriad of catalytic and electron transfer reactions in biology. However, very few of them have been trapped so that biophysical characterizations can be performed in a high-precision context. In this work, tryptophan derivative-based radicals were studied by high-frequency/high-field electron paramagnetic resonance (HFEPR) and quantum chemical calculations. Radicals were generated at liquid nitrogen temperature with a photocatalyst, sacrificial oxidant, and violet laser. The precise g-anisotropies of l- and d-tryptophan, 5-hydroxytryptophan, 5-methoxytryptophan, 5-fluorotryptophan, and 7-hydroxytryptophan were measured directly by HFEPR. Quantum chemical calculations were conducted to predict both neutral and cationic radical spectra for comparison with the experimental data. The results indicate that under the experimental conditions, all radicals formed were cationic. Spin densities of the radicals were also calculated. The various line patterns and g-anisotropies observed by HFEPR can be understood in terms of spin-density populations and the positioning of oxygen atom substitution on the tryptophan ring. The results are considered in the light of the tryptophan and 7-hydroxytryptophan diradical found in the biosynthesis of the tryptophan tryptophylquinone cofactor of methylamine dehydrogenase.
基于色氨酸的自由基在生物学中的众多催化和电子转移反应中发挥作用。然而，其中只有极少数被捕获，因此能够在高精度条件下进行生物物理表征。在这项工作中，通过高频/高场电子顺磁共振（HFEPR）和量子化学计算对基于色氨酸衍生物的自由基进行了研究。在液氮温度下，利用光催化剂、牺牲性氧化剂和紫光激光产生自由基。通过HFEPR直接测量了L-和D-色氨酸、5-羟基色氨酸、5-甲氧基色氨酸、5-氟色氨酸和7-羟基色氨酸的精确g各向异性。进行量子化学计算以预测中性和阳离子自由基光谱，以便与实验数据进行比较。结果表明，在实验条件下，形成的所有自由基都是阳离子。还计算了自由基的自旋密度。根据自旋密度分布以及色氨酸环上氧原子取代基的位置，可以理解HFEPR观察到的各种谱线模式和g各向异性。结合在甲胺脱氢酶色氨酸色氨酰醌辅因子生物合成中发现的色氨酸和7-羟基色氨酸双自由基对结果进行了探讨。
