Khamis M I, Casas-Finet J R, Maki A H
J Biol Chem. 1987 Feb 5;262(4):1725-33.
The complexes formed between Escherichia coli single-stranded DNA binding protein (SSBP) and the heavy atom-modified single-stranded polynucleotides poly(5-BrU) and poly(5-HgU) are investigated using optically detected magnetic resonance (ODMR) methods. In these complexes the triplet state properties of the tryptophan residues are subjected to the external heavy atom effect generated by bromine and mercury atoms and are characterized by a shortened triplet state lifetime and the appearance of the otherwise dark [D] + [E] slow passage ODMR signal. These features provide direct evidence for close range interactions between tryptophan residue(s) and the nucleotide bases in the complexes. The extent of the triplet state lifetime reduction in the case of the SSBP-poly(5-HgU) complex together with steric considerations of the complex structure is consistent only with a van der Waals contact between the perturbed molecule and the heavy atom perturber by means of a stacking interaction. Fast passage ODMR measurements show a lifetime for a sublevel of the perturbed tryptophan chromophore(s) in this complex on the order of 1 ms. The amplitude-modulated phosphorescence microwave double resonance technique captures selectively the broadened and red-shifted phosphorescence spectrum of the heavy atom-perturbed tryptophan residue(s). This work supports a model for the binding of SSBP to single-stranded polynucleotides in which the bases are inserted into hydrophobic regions of the protein, where they are likely to undergo stacking interactions with the indole moiety of buried tryptophan residues.
利用光探测磁共振（ODMR）方法研究了大肠杆菌单链DNA结合蛋白（SSBP）与重原子修饰的单链多核苷酸聚（5-溴尿嘧啶）（poly(5-BrU)）和聚（5-汞尿嘧啶）（poly(5-HgU)）形成的复合物。在这些复合物中，色氨酸残基的三重态性质受到溴和汞原子产生的外部重原子效应的影响，其特征是三重态寿命缩短，以及原本暗的[D]+[E]慢通过ODMR信号的出现。这些特征为复合物中色氨酸残基与核苷酸碱基之间的近距离相互作用提供了直接证据。SSBP-聚（5-汞尿嘧啶）复合物中三重态寿命降低的程度，连同复合物结构的空间考虑，仅与受扰分子和重原子扰动剂之间通过堆积相互作用的范德华接触一致。快速通过ODMR测量表明，该复合物中受扰色氨酸发色团亚能级的寿命约为1毫秒。振幅调制磷光微波双共振技术选择性地捕获了重原子扰动的色氨酸残基的加宽和红移磷光光谱。这项工作支持了一个SSBP与单链多核苷酸结合的模型，其中碱基插入蛋白质的疏水区域，在那里它们可能与埋藏的色氨酸残基的吲哚部分发生堆积相互作用。
