title: Site-specific solvation of the photoexcited protochlorophyllide a in methanol: formation of the hydrogen-bonded intermediate state induced by hydrogen-bond strengthening.
abstract: The site-specific solvation of the photoexcited protochlorophyllide a (Pchlide a) in methanol solvent was investigated using the time-dependent density functional theory method for the first time to our knowledge. The intermolecular site-specific coordination and hydrogen-bonding interactions between Pchlide a and methanol molecules play a very important role in the steady-state and time-resolved spectra. All the calculated absorption and fluorescence spectra of the isolated Pchlide a and its coordinated and hydrogen-bonded complexes with methanol demonstrate that the novel fluorescence shoulder at approximately 690 nm of Pchlide a in methanol should be ascribed to the coordinated and hydrogen-bonded Pchlide a-(MeOH)(4) complex. This coordinated and hydrogen-bonded complex can also account for the intermediate state found in the time-resolved spectroscopic studies. Herein, we have theoretically confirmed that the intermolecular coordination and hydrogen bonds between Pchlide a and methanol molecules can be strengthened in the electronically excited state of Pchlide a. Furthermore, the site-specific solvation of the photoexcited Pchlide a can be induced by the intermolecular coordination and hydrogen-bond strengthening upon photoexcitation. Then the hydrogen-bonded intermediate state is formed in 22-27 ps timescales after the site-specific solvation. All the steady-state and time-resolved spectral features of Pchlide a in different solvents can be explained by the formation of this hydrogen-bonded intermediate state after the site-specific solvation, which is induced by the coordination and hydrogen-bond strengthening.
authors.affiliation: State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China. gjzhao@dicp.ac.cn
authors.forename: Guang-Jiu
authors.initials: GJ
authors.lastname: Zhao
authors.forename: Ke-Li
authors.initials: KL
authors.lastname: Han
mesh_terms: D001665:Binding Sites / Q000528:radiation effects; D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000737:chemistry / Q000528:radiation effects; D003198:Computer Simulation; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding / Q000528:radiation effects; D008027:Light; D000432:Methanol / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D010777:Photochemistry / Q000379:methods; D012997:Solvents / Q000737:chemistry