title: Photoinduced C-I bond homolysis of 5-iodouracil: A singlet predissociation pathway.
abstract: 5-Iodouracil (5-IU) can be integrated into DNA and acts as a UV sensitive chromophore suitable for probing DNA structure and DNA-protein interactions based on the photochemical reactions of 5-IU. Here, we perform joint studies of time-resolved Fourier transform infrared (TR-FTIR) spectroscopy and ab initio calculations to examine the state-specific photochemical reaction mechanisms of the 5-IU. The fact that uracil (U) is observed in TR-FTIR spectra after 266 nm irradiation of 5-IU in acetonitrile and ascribed to the product of hydrogen abstraction by the uracil-5-yl radical (U·) provides experimental evidence for the C-I bond homolysis of 5-IU. The excited state potential energy curves are calculated with the complete active space second-order perturbation//complete active space self-consistent field method, from which a singlet predissociation mechanism is elucidated. It is shown that the initially populated1(ππ*) state crosses with the repulsive1(πσ*) or1(nIσ*) state, through which 5-IU undergoes dissociation to the fragments of (U·) radical and iodine atom. In addition, the possibility of intersystem crossing (ISC) is evaluated based on the calculated vertical excitation energies. Although a probable ISC from1(ππ*) state to3(nOπ*) and then to the lowest triplet3(ππ*) could occur in principal, there is little possibility for the excited state populations bifurcating to triplet manifold, given that the singlet state predissociation follows repulsive potential and should occur within dozens to hundreds of femtoseconds. Such low population of triplet states means that the contribution of triplet state to photoreactions of 5-IU should be quite minor. These results demonstrate clearly a physical picture of C-I bond homolysis of 5-IU and provide mechanistic illuminations to the interesting applications of 5-IU as photoprobes and in radiotherapy of cancer.
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Xiaojuan
authors.initials: X
authors.lastname: Dai
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Di
authors.initials: D
authors.lastname: Song
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Kunhui
authors.initials: K
authors.lastname: Liu
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Hongmei
authors.initials: H
authors.lastname: Su
mesh_terms: D008956:Models, Chemical; D017550:Spectroscopy, Fourier Transform Infrared; D014466:Ultraviolet Rays; D014498:Uracil / Q000031:analogs & derivatives* / Q000737:chemistry / Q000528:radiation effects
chemical_list: D014498:Uracil; C048747:5-iodouracil