Initial Excited-State Structural Dynamics of dT and dA Oligonucleotide Homopentamers Using Resonance Raman Spectroscopy.

Abstract

Photochemical damage of DNA is initiated by absorption of ultraviolet light, and the photoproducts are formed as a result of excited-state structural and electronic dynamics. We have used UV resonance Raman spectroscopy to measure the initial excited-state structural dynamics of homopentamers of adenosine monophosphate (3'-dApdApdApdApdAp-5') and thymidine monophosphate (3'-dTpdTpdTpdTpdTp-5') and compare them to those of the monomeric nucleobases. The resonance Raman spectra of the homopentamers are similar to those of the corresponding monomers. Initial excited-state slopes, homogeneous and inhomogeneous broadening, and other excited-state parameters were extracted by self-consistent simulation of the resonance Raman excitation profiles and absorption spectra with a time-dependent formalism and are also similar to the initial excited-state slopes and broadening in the nucleotide monomers. The lack of differences between the initial excited-state structural dynamics of the nucleotides within the pentamer and the isolated nucleobases is consistent with a model in which the formation of photochemical products in oligonucleotides and DNA is dependent on the formation of the transition-state structure within these polymers, dictated by their large-scale dynamics. These results are discussed in light of the known photochemistry of DNA and the nucleobases.