ESR ON LABELED & UNLABELED "INTERMEDIATE X"

该项目是1996年启动的一个项目的延续（见 突出显示#91）。 核糖核苷酸还原酶（RNRs）催化第一个 DNA生物合成的步骤：核苷酸转化为 脱氧核苷酸 RNR由两个单元组成：大单元（R1）和 R2单位，这是小（er）。 后一个单元含有酪氨酰 激进 该自由基通过以下方式引发核苷酸还原： 一种长程耦合电子质子转移反应。 当R2是 用铁处理，生成二铁络合物，其迅速地 转化为二铁中心，而稳定的酪氨酰基自由基， 正在形成。 这种转换涉及一个步骤， 二铁络合物变成中间体（称为 中间体X）。 如共振拉曼实验所示， R2中的二铁中心在两个铁之间含有氧桥。 有趣的是，了解中间体X中的氧桥是否 由一个或两个氧原子组成。 这个问题的答案可能 揭示了参与形成的化学反应， 酪氨酰自由基。 为了解释这个问题， 进行了以下实验。 S波段CW EPR谱具有 一个未标记的和一个17O标记的样品， 中间体X.我们期望标记样品的EPR线具有 与未标记样品的形状和宽度不同 超精细增宽 可以模拟1996年获得的光谱， 最好的模拟使用75%的同位素氧17O，而85%的同位素氧17O。 预计会有所增加。 1997年春天，低频S波段 在新的样品和新的环隙谐振器上获得光谱。 获得了较好的信噪比，并可模拟光谱 使用85%的17O。 显然，样品或谐振器（或 两者）具有较少的激进信号，这在 模拟 得出结论，中间体X由桥组成 一个17O原子

This project is a continuation of a project initiated in 1996 (see Highlight #91). Ribonucleotide reductases (RNRs) catalyze the first step of the DNA biosynthesis: the conversion of nucleotides to deoxynucleotides. RNR consists of two units: a large unit (R1) and a R2 unit which is small(er). The latter unit contains a tyrosyl radical. This radical initiates the nucleotide reduction by means of a long range coupled electron proton transfer reaction. When R2 is treated with iron, a di-ferrous complex is generated, which rapidly is transformed into a di-ferric center while a stable tyrosyl radical is being formed. This transformation involves a step in which the di-ferrous complex is changed into an intermediate (called Intermediate X). As has been shown by resonant Raman experiments, the di-iron center in R2 contains an oxygen bridge between the two irons. It is of interest to learn if the oxygen bridge in Intermediate X consists of one or two oxygen atoms. The answer to this question may reveal insight in the chemical reactions involved in the formation of the tyrosyl radical. In an attempt to elucidate the problem, the following experiments have been performed. S-band CW EPR spectra have been taken of both an unlabeled and a 17O labeled-sample of Intermediate X. We expect the EPR line of the labeled sample to have a different shape and width than that of the unlabeled sample due to the hyperfine broadening. Spectra obtained in 1996 could be simulated but the best simulations used 75% isotopic oxgyen, 17O, while 85% enrichment was expected. In the spring of 1997, low-frequency S-band spectra were obtained on new samples and a new loop-gap resonator. Better signal to noise was obtained and the spectra could be simulated using 85% 17O. Apparently either the sample or the resonator (or both) had less radical signal which was not considered in the simulations. It is concluded that Intermediate X consists of a bridge from one 17O atom.