Georghiou S, Gerke L S
Department of Physics, University of Tennessee, Knoxville 37996-1200, USA.
Photochem Photobiol. 1999 Jun;69(6):646-52.
Published results on synthetic polynucleotides point to T as the major emitting fluorophore in DNA. We have reported also that the bases of the nonalternating polynucleotide poly(dA).poly(dT), in which T was selectively excited, undergo large-amplitude motions on the picosecond-nanosecond time scales (S. Georghiou et al., Biophys. J. 70, 1909-1922, 1996). In that study, the fluorescence decay profile of the T bases of this polynucleotide was found to contain a number of components; these may be considered to be the result of the motions of the bases that give rise to a distribution of stacked geometries of varying rigidity as well as dispersion and polar interactions. Here, we report the results of a study that we have undertaken in order to test this hypothesis. To this effect, we have studied the photophysical properties of thymidine (1) in aqueous buffer and in a number of organic solvents and (2) in aqueous sucrose solutions of viscosity extending to 149 cP. The results suggest that the fluorescence quantum yield decreases with an increase in the polarizability of the solvent, whereas it increases with an increase in the solvent polarity (on the basis of the empirical parameter of solvent polarity ETN) or viscosity. These findings suggest the following for the photophysical properties of the T bases in DNA: (1) Base stacking results in two antagonistic effects, namely it causes a reduction in fluorescence as a result of dispersion interactions and an enhancement as a result of a reduction in the motions of the bases and (2) exposure of the bases to the aqueous environment results in fluorescence enhancement.
关于合成多核苷酸已发表的结果表明，胸腺嘧啶（T）是DNA中主要的荧光发色团。我们还报道过，在非交替多核苷酸聚（dA）·聚（dT）中，当T被选择性激发时，其碱基在皮秒至纳秒时间尺度上会发生大幅度运动（S. 乔治乌等人，《生物物理学杂志》70，1909 - 1922，1996）。在该研究中，发现这种多核苷酸中T碱基的荧光衰减曲线包含多个成分；这些成分可被认为是碱基运动的结果，碱基运动导致了不同刚性的堆积几何结构以及色散和极性相互作用的分布。在此，我们报告为检验这一假设而进行的一项研究结果。为此，我们研究了胸腺嘧啶核苷（1）在水性缓冲液和多种有机溶剂中的光物理性质，以及（2）在粘度高达149 cP的水性蔗糖溶液中的光物理性质。结果表明，荧光量子产率随溶剂极化率的增加而降低，而随溶剂极性（基于溶剂极性经验参数ETN）或粘度的增加而增加。这些发现对于DNA中T碱基的光物理性质有如下启示：（1）碱基堆积导致两种相反的效应，即由于色散相互作用导致荧光减弱，以及由于碱基运动减少导致荧光增强；（2）碱基暴露于水环境会导致荧光增强。
