摘要/Abstract

RNA与蛋白质相互作用是生物体进行生命活动的基础，光活化核苷酸引发的共价交联是研究其相互作用的有效手段.对其机理的研究有助于理解并调控交联的位点及氨基酸，因此探测关键的中间体来揭示机理很有必要.本工作选择光活化核苷酸4-硫代尿嘧啶（4-TU）和色氨酸（TrpH）为模型体系，通过激光闪光光解技术探测瞬态中间体并由此揭示机理.实验发现4-TU三重态与TrpH发生电子转移反应，瞬态吸收光谱上观察到4-TU^·-及TrpH^·+，以及TrpH^·+脱质子生成的Trp^·.通过测量4-TU三重态衰减动力学得到电子转移反应的速率常数为2.88×10⁹ L·mol^-1·s^-1，并研究了pH值对反应的影响.电子转移驱动力ΔG为-0.15 eV，是能量有利的过程.结果表明电子转移反应是光致共价交联过程中关键的第一步反应，引发了后续的质子转移及自由基交联过程.
关键词: 光活化核苷酸, 光致共价交联, 电子转移, 自由基中间体, 三重态衰减动力学
RNA-protein interactions are inevitably existing in many fundamental biological processes of organisms and it is an effective method to investigate the nature of RNA-protein interactions through crosslinking induced by photoactivation. Therefore, it is of great importance to detect the crucial transient intermediates to elucidate the mechanism of photo crosslinking between RNA and proteins, which will shed light on regulating the crosslinking sites as well as the favorable cross-linked amino acids. In this research, we choose the photoactivatable ribonucleotide analog, 4-thiouracil, and the aromatic amino acid, tryptophan, as a model system to study, from which the photo crosslinking is found to be initiated by the electron transfer as the first step. By means of the nanosecond time-resolved laser flash photolysis, the key intermediates of photo-induced electron transfer from tryptophan to the triplet of 4-thiouracil, 4-thiouracil anion radical (4-TU^·-)and tryptophan cation radical (TrpH^·+) are observed, as well as the deprotonated species of tryptophan neutral radical (Trp^·). By monitoring the 4-TU triplet decay kinetics, the pseudo-first order rate constant of photo-induced electron transfer is determined to be 2.88×10⁹ L·mol^-1·s^-1 and found to be diffusion-controlled. The pH-effect on the electron transfer and proton transfer have been further examined. In addition, the driving force for electron transfer from tryptophan to 4-TU triplet is estimated using the classic Rehm-Weller empirical equation to be -0.15 eV, which means the photo-induced electron transfer process is favorable thermodynamically. These results demonstrate that photo-induced electron transfer between 4-thiouracil triplet and tryptophan is the key step, which can trigger the following proton transfer and radical coupling processes and lead to the covalent photoadducts. These studies provide a basis for mechanistic understandings of photo crosslinking between RNA and proteins in more complex system.
Key words: photoactivatable ribonucleotide, photo crosslinking, electron transfer, radical intermediate, triplet decay kinetics


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