Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
J Phys Chem B. 2009 Oct 29;113(43):14336-42. doi: 10.1021/jp9053378.
To explore the origin of the differences in UV photochemistry of uracil (RNA) and thymine (DNA) nucleobases, we have measured the UV resonance Raman spectra of uracil in aqueous solution at wavelengths throughout the lowest-energy absorption band and analyzed the resulting resonance Raman excitation profiles and absorption spectra using a time-dependent wave-packet formalism to obtain the initial excited-state structural changes. In contrast to thymine, which differs from uracil only by the presence of a methyl group at C(5), most of the resonance Raman intensity and resulting initial excited-state structural dynamics for uracil occur along in-plane hydrogen-bond angle deformation, ring stretching, and carbonyl vibrational modes. Weaker intensities and less significant structural dynamics are observed along the C=C stretching mode. These results suggest that the initial excited-state structural dynamics of uracil occur along a carbon pyramidalization coordinate. These dynamics are different from those of thymine, which distorts primarily along a C(5)=C(6) bond lengthening coordinate. These differences in initial excited-state structural dynamics can explain the different primary photoproducts observed for these two pyrimidine nucleobases.
为了探究尿嘧啶（RNA）和胸腺嘧啶（DNA）碱基在紫外光化学反应上的差异的起源，我们在整个最低能量吸收带波长范围内测量了水溶液中尿嘧啶的紫外共振拉曼光谱，并使用含时波包理论分析了得到的共振拉曼激发轮廓和吸收光谱，以获得初始激发态的结构变化。与仅在 C(5)位上存在甲基的胸腺嘧啶不同，尿嘧啶的大部分共振拉曼强度和初始激发态结构动力学都沿着平面内氢键角度变形、环拉伸和羰基振动模式发生。在 C=C 伸缩模式上观察到的强度较弱，结构动力学也不那么显著。这些结果表明，尿嘧啶的初始激发态结构动力学沿着一个碳金字塔化坐标发生。这些动力学与胸腺嘧啶的不同，胸腺嘧啶主要沿着 C(5)=C(6)键伸长坐标发生畸变。这些初始激发态结构动力学的差异可以解释这两种嘧啶碱基观察到的不同的初始光产物。
