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为了探讨电荷转移机理,详细研究了咖啡酸(DHPAA)及其甲酯(MDHPA)类保护剂的保护效应。DHPAA和MDHPA属于苯基取代的不饱和羧酸,和马来酸相似,它们具有较强的亲电子性能,因而其最低空轨道能量(1.79eV,1.75eV)都比胸腺嘧啶(2.60eV),半胱氨酸(2.02eV)低,此外,咖啡酸与碱基还会形成长链分子堆积,有利于电子从嘧啶碱阴离子向DHPAA或MDHPA的长程转移。ESR研究有力地证实了上述的预测;即使dTMP与DHPAA或MDHPA的克分子比为10:1时,dTMP-DHPAA或dTMP-MDHPA 77K的ESR谱显示与DHPAA或MDHPA十分相似的特征而与dTMP不同,室温退火后便转化为DHPAA、MDHPA室温下的单峰,均不见胸腺嘧啶5位自由基的痕迹,而DNA-DHPAA、DNA-MDHPA当核苷酸与保护剂的克分子比为4:1时,DNA-DHPAA、DNA-MDHPA的ESR谱也显示与DHPAA或MDHPA相似的特征,室温下也不见胸腺嘧啶5位自由基的痕迹。
In order to investigate the mechanism of charge transfer, the protective effect of caffeic acid (DHPAA) and its methyl ester (MDHPA) protective agent was studied in detail. DHPAA and MDHPA belong to phenyl-substituted unsaturated carboxylic acids, which are similar to maleic acid and possess strong electrophilic properties. Therefore, their lowest empty orbital energies (1.79eV, 1.75eV) are higher than that of thymine (2.60eV) Cysteine (2.02eV) low, in addition, caffeic acid and base will form long-chain molecular accumulation, is conducive to the electron from the pyrimidine base anion to DHPAA or MDHPA long-range transfer. The ESR study strongly confirmed the above prediction; the ESR spectrum of dTMP-DHPAA or dTMP-MDHPA 77K shows a feature that is very similar to DHPAA or MDHPA and is different from dTMP even if the molar ratio of dTMP to DHPAA or MDHPA is 10: 1 , Annealed at room temperature and then converted to DHPAA, MDHPA single peak at room temperature, were not visible traces of thymine 5 free radicals, and DNA-DHPAA, DNA-MDHPA when the nucleotide and the protective agent molar ratio of 4: 1 , The ESR spectra of DNA-DHPAA, DNA-MDHPA also showed similar characteristics to DHPAA or MDHPA with no traces of thymine at position 5 at room temperature.