BIOPHYSICAL THEORY AND MODELING |
Site-Specific Solvation of the Photoexcited Protochlorophyllide a in Methanol: Formation of the Hydrogen-Bonded Intermediate State Induced by Hydrogen Bond Strengthening
Guangjiu Zhao 1 and Keli Han 1*
1 Dalian Institute of Chemical Physics
* To whom correspondence should be addressed. E-mail: klhan{at}dicp.ac.cn.
Submitted on June 11, 2007
Revised on July 18, 2007
Accepted on 3 August 2007
 |
Abstract |
|---|
The site-specific solvation of the photoexcited protochlorophyllide a (Pchlide a) in methanol solvent has been investigated using the time-dependent density functional theory (TDDFT) method for the first time. 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 around 690 nm of Phclide 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 bonds strengthening upon photoexcitation. Then the hydrogen-bonded intermediate state is formed in 22-27 ps time scales 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 bonds strengthening.
Key Words:
coordination bonding, hydrogen bond strengthening, hydrogen bonding, photosynthesis, site-specific solvation, time-resolved spectroscopy
