Electronic Excited-State Hydrogen Bonding Study Of The Photoactive Yellow Protein Chromophore And Dimethylamino Cinnamic Acid In Solution

In the last decades, intermolecular hydrogen bonding, as a site-specific interaction between hydrogen donor and acceptor molecules, has gained wide attention because of its significance in physical, chemical and biological processes. Moreover, electronic excited state hydrogen bond could associate with many important photochemical and photophysical phenomena have been demonstrated recently.Time-dependent density functional theory method has been carried out to investigate excited state hydrogen bonds of a series of hydrogen-bonded complexes of PYP chromophore with water molecules both in vacuum and aqueous solution. The analysis on excitation energies suggest the intermolecular hydrogen bonds formed between carbonyl oxygen and hydrogen of water are strengthened in the meaningful lowest bright state as well as one other charge transfer excited state and show spectral redshift. Whereas, a totally opposite result happens for the site of phenolate oxygen, in which hydrogen bonds are weakened and induce spectral blueshift. It can be confirmed based on the excited-state geometric optimizations by TDDFT method. In addition, the frontier molecular orbital analysis reveals the nature of the lowest bright state and the CT state as well as understanding of the hydrogen bonding dynamics. Besides, two approaches are adopted to consider the solvation effect. For our monomer in aqueous solution, the influences on excitation energy shift by site-specific hydrogen bonding and electrostatic field of the environment have something in common.In the light of analysis on excitation energies and frontier molecular orbitals, we have theoretically demonstrated that the changes about intermolecular hydrogen bonds at carboxyl terminal of dimethylamino cinnamic acid upon photoexcitation are unconspicuous, as a consequence, these hydrogen bonds are secondary for many photophysics processes. However, due to the obvious weakening of hydrogen bond at N terminal in S1state, the maximum absorption band would show hypochromatic shift. In addition, the N terminal hydrogen bond could induce geometric conformations of DMACA changed evidently in ground state as well as excited state. Last but not least, we theoretically predict a non-radiative decay route likes Sâ†'T2â†'T1is facilitated by the weakening of N terminal hydrogen bond in Si state, resulting in appreciable decrease in fluorescence quantum yield.