Hardman Samantha J O, Botchway Stanley W, Thompson Katherine C
School of Biological and Chemical Sciences, Birkbeck University of London, London, UK.
Photochem Photobiol. 2008 Nov-Dec;84(6):1473-9. doi: 10.1111/j.1751-1097.2008.00368.x. Epub 2008 May 29.
Pyrrolocytosine (PC), is a highly fluorescent analog of the natural nucleobase cytosine. The fluorescence of PC is quenched upon helix formation but the origin of the quenching is not known. We investigated the effects of base stacking in the aqueous phase by following the fluorescence of dinucleotides and trinucleotides containing PC. The quantum yields and lifetimes (ns) (in parenthesis) obtained at 25 degrees C were: PC-T, 0.026 (2.0), PC-C, 0.033 (2.5), PC-A, 0.032 (2.7), PC-G, 0.021 (2.0), T-PC-T, 0.044 (3.0) and G-PC-G, 0.036 (0.65 and 2.6), compared with 0.038 (2.9) for PC and 0.028 (2.1) for the nucleoside triphosphate. The results show that base stacking does not, except in the case of guanine, quench the fluorescence of PC; indeed the increased solvent shielding can enhance the emitted fluorescence. In the case of G-PC-G the guanines do shield the fluorescent base from the solvent but a particular environment of PC between two guanines also appears to allow a rapid nonradiative pathway, suggested to be electron transfer to the excited PC, to depopulate the excited state leading to the shorter fluorescence lifetime.
吡咯胞嘧啶（PC）是天然核碱基胞嘧啶的一种高荧光类似物。PC的荧光在螺旋形成时会猝灭，但猝灭的起源尚不清楚。我们通过跟踪含PC的二核苷酸和三核苷酸的荧光，研究了水相中碱基堆积的影响。在25摄氏度下获得的量子产率和寿命（纳秒）（括号内）为：PC-T，0.026（2.0）；PC-C，0.033（2.5）；PC-A，0.032（2.7）；PC-G，0.021（2.0）；T-PC-T，0.044（3.0）；G-PC-G，0.036（0.65和2.6），而PC的量子产率和寿命分别为0.038（2.9），核苷三磷酸的为0.028（2.1）。结果表明，除了鸟嘌呤的情况外，碱基堆积不会猝灭PC的荧光；实际上，增加的溶剂屏蔽可以增强发射的荧光。在G-PC-G的情况下，鸟嘌呤确实将荧光碱基与溶剂屏蔽开，但PC在两个鸟嘌呤之间的特定环境似乎也允许一种快速的非辐射途径，推测是电子转移到激发态的PC，使激发态去极化，导致荧光寿命缩短。
