On the mechanism of non-radiative decay of blue fluorescent protein chromophore: New insight from the excited-state molecular dynamics simulations and potential energy calculations.

A detailed theoretical investigation based on the ab initio on-the-fly surface hopping dynamics simulations and potential energy surfaces calculations has been performed to unveil the mechanism of the photoinduced non-adiabatic relaxation process of the isolated blue fluorescent protein (BFP) chromophore in gas phase. The data analysis presents that the dominant reaction coordinate of the BFP chromophore is driven by a rotation motion around the CC double bridging bond, which is in remarkable difference with a previous result which supports a Hula-Twist rotation pattern. Such behavior is consistent with the double bond rotation pattern of the GFP neutral chromophore. In addition, the dynamics simulations give an estimated decay time of 1.1ps for the S1 state, which is agrees well with the experimental values measured in proteins. The present work offers a straightforward understanding for the decay mechanism of the BFP chromophore and suggestions of the photochemical properties of analogous protein chromophores. We hope the current work would be helpful for further exploration of the BFP photochemical and photophysical properties in various environments, and can provide guidance and prediction for rational design of the fluorescent proteins catering for different demands.

Non-adiabatic dynamics investigation of the radiationless decay mechanism of trans-urocanic acid in the S2 state.

The trans-urocanic acid, a UV chromophore in the epidermis of human skin, was found to exhibit a wavelength dependent isomerization property. The isomerization quantum yield to cis-urocanic is greatest when being excited to the S1 state, whereas exciting the molecule to the S2 state causes almost no isomerization. The comparative photochemical behavior of the trans-urocanic on the S1 and S2 states continues to be the subject of intense research effort. This study is concerned with the unique photo-behavior of this interesting molecule on the S2 state. Combining the on-the-fly surface hopping dynamics simulations and static electronic structure calculations, three decay channels were observed following excitation to the S2 state. An overwhelming majority of the molecules decay to the S1 state through a planar or pucker characterized minimum energy conical intersection (MECI), and then decay to the ground state along a relaxation coordinate driven by a pucker deformation of the ring. A very small fraction of molecules decay to the S1 state by a MECI characterized by a twisting motion around the CC double bond, which continues to drive the molecule to deactivate to the ground state. The latter channel is related with the photoisomerization process, whereas the former one will only generate the original trans-form products. The present work provides a novel S2 state decay mechanism of this molecule, which offers useful information to explain the wavelength dependent isomerization behavior.

Effect of the Hydrogen Bond on Photochemical Synthesis of Silver Nanoparticles.

The effect of a hydrogen bond on the photochemical synthesis of silver nanoparticles has been investigated via experimental and theoretical methods. In a benzophenone system, the photochemical synthesis process includes two steps, which are that hydrogen abstraction reaction and the following reduction reaction. We found that for the first step, an intermolecular hydrogen bond enhances the proton transfer. The efficiency of hydrogen abstraction increases with the hydrogen bond strength. For the second step, the hydrogen-bonded ketyl radical complex shows higher reducibility than the ketyl radical. The inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurement exhibits a 2.49 times higher yield of silver nanoparticles in the hydrogen bond ketyl radical complex system than that for the ketyl radical system. Theoretical calculations show that the hydrogen bond accelerates electron transfer from the ketyl radical to the silver ion by raising the SOMO energy of the ketyl radical; thus, the SOMO-LUMO interaction is more favorable.

Non-adiabatic dynamics of isolated green fluorescent protein chromophore anion.

On-the-fly ab initio molecular dynamics calculations have been performed to investigate the relaxation mechanism of green fluorescent protein chromophore anion under vacuum. The CASSCF surface hopping simulation method based on Zhu-Nakamura theory is applied to present the real-time conformational changes of the target molecule. The static calculations and dynamics simulation results suggest that not only the twisting motion around bridging bonds between imidazolinone and phenoxy groups but the strength mode of C=O and pyramidalization character of bridging atom are major factors on the ultrafast fluorescence quenching process of the isolated chromophore anion. The abovementioned factors bring the molecule to the vicinity of conical intersections on its potential energy surface and to finish the internal conversion process. A Hula-like twisting pattern is displayed during the relaxation process and the entire decay process disfavors a photoswitching pattern which corresponds to cis-trans photoisomerization.

Sensing mechanism for a fluoride chemosensor: invalidity of excited-state proton transfer mechanism.

Our density functional theory (DFT)/time-dependent DFT calculations for the fluoride anion sensor, 5,7-dibromo-8-tert-butyldimethylsilyloxy-2-methylquinoline (DBM), suggested a different sensing mechanism from the experimentally proposed one (Chem. Commun., 2011, 47, 7098). Instead of the formation of fluoride-hydrogen-bond complex (DBMOHF) and excited-state proton transfer mechanism, the theoretical results predicted a sensing mechanism based on desilylation reaction and intramolecular charge transfer (ICT). The fluoride anion reacted with DBM and formed an anion (DBMO), with the ICT causing a red shift in the absorbance and emission spectra of the latter. The calculated vertical excitation energies in the ground and first excited states of both DBM and DBMO, as well as the calculated (1)H NMR spectra, significantly reproduced the experimental measurements, providing additional proofs for our proposed sensing mechanism for DBM.

Fluoride anion sensing mechanism of 2-ureido-4[1H]-pyrimidinone quadruple hydrogen-bonded supramolecular assembly: photoinduced electron transfer and partial configuration change.

The fluoride anion sensing mechanism of 6-methyl-5-(9-methylene-anthracene)-(2-butylureido-4[1H]-pyrimidinone) (AnUP) has been investigated using the DFT/TDDFT method. The theoretical results indicate that the proton of the N3-H3 group in pyrimidine moiety is captured by the added fluoride anion and then deprotonated. The calculated vertical excitation energies of AnUP-dimer and its deprotonated form agree well with the experimental results. The molecular orbital analysis demonstrates that the first excited state (S1) of AnUP-dimer is a local excited state with a π-π* transition, whereas for the deprotonated form, S1 is a completely charge-separation state and is responsible for the photoinduced electron transfer (PET) process. The PET process from anthracene to the pyrimidine moiety leads to the fluorescence quenching.

Theoretical study on photoisomerization effect with a reversible nonlinear optical switch for dithiazolylarylene.

DFT and TDDFT methods have been performed to investigate the photoisomerization effect for dithiazolylarylene on solution. The weak S···N interaction and CH···N hydrogen bond restrain the rotation of the side-chain thiazolyl ring in open-isomer 1a, the higher stability of which prefers to show a high quantum yield of photoisomerization. The calculated UV-Vis spectrum at around 320 nm for open-isomer 1a is bathochromically shifted to 647 nm for closed-isomer 1b, in excellent agreement with the experimental photochromic phenomenon. The electron transition in ECD (electron circular dichroism) spectra for closed-isomer 1b with two chiral carbon atoms is dominated by ICT (intramolecular charge transition) and LE (local excitation) corresponding to one positive (440 nm) and one negative Cotton effect (650 nm), respectively, where the two chiral carbon atoms play a slight role in these transitions. The PES in the S(1) and S(0) states, respectively, indicates that the cyclization reaction from open-isomer 1a to closed-isomer 1b is allowed in the photoexcited state with high-conversion quantum efficiency, while it is forbidden in the thermodynamic process. In addition, the second-order nonlinear optical response for closed-isomer 1b is nearly six times larger than that for open-isomer 1a. It is also confirmed that the photoirradiation evokes the photoisomerization character to show dramatic difference in the second-order NLO response, which can be applied to designing photochromic materials and reversible NLO switches.