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：将核苷酸转化为 脱氧核苷酸。RnR由两个单元组成：大单元(R1)和 R2单位，它是小的(ER)。后一单位含有酪氨酸 激进的。该自由基通过以下方式启动核苷酸还原 长程耦合电子质子转移反应。当R2为 用铁处理后，生成二亚铁络合物，它迅速地被 转化为二铁中心，而稳定的酪氨酸根 正在形成。此转换涉及到一个步骤，在该步骤中 二铁络合物转变为中间体(称为 中间X)。正如共振拉曼实验所表明的那样， R2中的Di-Iron中心在两个铁之间包含一个氧桥。 有兴趣了解中间体X中的氧桥 由一个或两个氧原子组成。这个问题的答案可能是 揭示参与形成的化学反应的洞察 酪氨酸根。为了澄清这个问题， 进行了以下实验。S波段连续波EPR谱 同时采集了一份未标记的和一份17O标记的样本 中间X。我们期望标记的样本的EPR行具有 形状和宽度与未标记的样品不同，原因是 超精细加宽。1996年获得的光谱可以模拟，但 最好的模拟使用了75%的同位素氧，17O，而85% 浓缩是意料之中的事。1997年春，低频S频段 在新的样品和新的环隙谐振器上获得了光谱。 获得了较好的信噪比，并且可以模拟光谱 使用85%17O。显然不是样品就是谐振器(或者 两者)都有不太激进的信号，这在 模拟。得出结论：中间体X由一座桥梁组成 从一个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.