Pouwels P J, Hartman R F, Rose S D, Kaptein R
Bijvoet Center for Biomolecular Research, Utrecht University, The Netherlands.
Photochem Photobiol. 1995 Jun;61(6):575-83. doi: 10.1111/j.1751-1097.1995.tb09872.x.
A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor N alpha-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1'-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.
使用电子供体Nα-乙酰色氨酸（AcTrp）作为光敏剂，对嘧啶单体和二聚体进行了一系列光化学诱导动态核极化（photo-CIDNP）实验。CIDNP光谱证明了二聚体自由基阴离子及其解离产物单体自由基阴离子的存在，二聚体自由基阴离子是由激发态的AcTrp*向二聚体转移电子形成的。由于嘧啶自由基阴离子和阳离子中未成对电子自旋密度分布不同，AcTrp光谱与用蒽醌-2-磺酸盐等氧化敏化剂获得的光谱完全不同。在反式（1,3-二甲基尿嘧啶）二聚体的光谱中，检测到了源于二聚体自由基对中自旋分选过程的极化，这表明所涉及的二聚体自由基阴离子具有相对较长的寿命。尽管1,1'-三亚甲基桥连嘧啶的二聚体自由基阴离子可能也有相对较长的寿命，但其质子的超精细相互作用非常弱，这解释了为什么没有检测到源于二聚体自由基对的极化。在桥连嘧啶的光谱中，由于单体自由基对中的自旋分选，观察到了极化的二聚体质子，由此可知二聚体自由基阴离子解离为单体自由基阴离子是可逆的。对CIDNP强度随pH值变化的研究表明，pH值在3到4之间最适合观察源于单体自由基对中自旋分选的单体极化。在较高pH值下，同一产物中产生的逃逸极化部分抵消了双生复合极化。
