Resolving the ultrafast dynamics of the anionic green fluorescent protein chromophore in water

The chromophore of the green fluorescent protein (GFP) is critical for probing environmental influences on fluorescent protein behavior. Using the aqueous system as a bridge between the unconfined vacuum system and a constricting protein scaffold, we investigate the steric and electronic effects of the environment on the photodynamical behavior of the chromophore. Specifically, we apply ab initio multiple spawning to simulate five picoseconds of nonadiabatic dynamics after photoexcitation, resolving the excited-state pathways responsible for internal conversion in the aqueous chromophore. We identify an ultrafast pathway that proceeds through a short-lived (sub-picosecond) imidazolinone-twisted (I-twisted) species and a slower (several picoseconds) channel that proceeds through a long-lived phenolate-twisted (P-twisted) intermediate. The molecule navigates the non-equilibrium energy landscape via an aborted hula-twist-like motion toward the one-bond-flip dominated conical intersection seams, as opposed to following the pure one-bond-flip paths proposed by the excited-state equilibrium picture. We interpret our simulations in the context of time-resolved fluorescence experiments, which use short- and long-time components to describe the fluorescence decay of the aqueous GFP chromophore. Our results suggest that the longer time component is caused by an energetically uphill approach to the P-twisted intersection seam rather than an excited-state barrier to reach the twisted intramolecular charge-transfer species. Irrespective of the location of the nonadiabatic population events, the twisted intersection seams are inefficient at facilitating isomerization in aqueous solution. The disordered and homogeneous nature of the aqueous solvent environment facilitates non-selective stabilization with respect to I- and P-twisted species, offering an important foundation for understanding the consequences of selective stabilization in heterogeneous and rigid protein environments.

Excited state relaxation dynamics of model green fluorescent protein chromophore analogs: evidence for cis-trans isomerism

Two green fluorescent protein (GFP) chromophore analogs (4Z)-4-(N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DMPI) and (4Z)-4-(N,N-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ(1)) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ(2)) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of Z and E isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to p-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP.

Photophysics and dihedral freedom of the chromophore in yellow, blue, and green fluorescent protein

Green fluorescent protein (GFP) and GFP-like fluorescent proteins owe their photophysical properties to an autocatalytically formed intrinsic chromophore. According to quantum mechanical calculations, the excited state of chromophore model systems has significant dihedral freedom, which may lead to fluorescence quenching intersystem crossing. Molecular dynamics simulations with freely rotating chromophoric dihedrals were performed on green, yellow, and blue fluorescent proteins in order to model the dihedral freedom available to the chromophore in the excited state. Most current theories suggest that a restriction in the rotational freedom of the fluorescent protein chromophore will lead to an increase in fluorescence brightness and/or quantum yield. According to our calculations, the dihedral freedom of the systems studied (BFP > A5 > YFP > GFP) increases in the inverse order to the quantum yield. In all simulations, the chromophore undergoes a negatively correlated hula twist (also known as a bottom hula twist mechanism).

Ultrafast Electronic and Vibrational Dynamics of Stabilized A State Mutants of the Green Fluorescent Protein (GFP): Snipping the Proton Wire

Two blue absorbing and emitting mutants (S65G/T203V/E222Q and S65T at pH 5.5) of the green fluorescent protein (GFP) have been investigated through ultrafast time resolved infra-red (TRIR) and fluorescence spectroscopy. In these mutants, in which the excited state proton transfer reaction observed in wild type GFP has been blocked, the photophysics are dominated by the neutral A state. It was found that the A* excited state lifetime is short, indicating that it is relatively less stabilised in the protein matrix than the anionic form. However, the lifetime of the A* state can be increased through modifications to the protein structure. The TRIR spectra show that a large shifts in protein vibrational modes on excitation of the A* state occurs in both these GFP mutants. This is ascribed to a change in H-bonding interactions between the protein matrix and the excited state.

Meta and para effects in the ultrafast excited-state dynamics of the green fluorescent protein chromophores

Femtosecond transient absorption and fluorescence upconversion experiments have been performed to investigate the photoinduced dynamics of the meta isomer of the green fluorescent protein chromophore, m-HBDI, and its O-methylated derivative, m-MeOBDI, in various solvent mixtures at neutral, acidic, and basic pH. The para isomer, p-HBDI, and its O- and N-methylated derivatives, p-MeOBDI and p-HBDIMe(+), were also studied for comparison. In all cases, fast quenching of the excited S1 state by internal conversion (IC) to the ground state was observed. In the para compounds, IC, presumably promoted by the internal twisting, arises in <1 ps. A similar process takes place in the meta compounds in nonaqueous solvents but with notably slower kinetics. In aqueous solutions, the meta compounds undergo ultrafast intermolecular excited-state proton transfer that competes with isomerization.

Mechanistic aspects of proton chain transfer in the green fluorescent protein : Part II. A comparison of minimal quantum chemical models

In this paper we report the results of extensive quantum chemical reaction pathway calculations for the electronic ground state of several different cluster models that mimic the proton chain transfer path within the green fluorescent protein (GFP). Our principal objective is to establish the robustness with respect to variations in the model of our recent mechanistic inferences for the ground state proton chain transfer [S. Wang and S. C. Smith, J. Phys. Chem. B, 2006, 110, 5084]. Additionally, comparison of our ground state results with the excited state proton transfer (ESPT) study by Vendrell et al. [O. Vendrell, R. Gelabert, M. Moreno and J. M. Lluch, J. Am. Chem. Soc., 2006, 128, 3564] leads to the conclusion that the mechanism of proton chain transfer may be expected to be analogous in ground and excited states, principally because in both cases the loss of the chromophore's phenolic proton contributes strongly to the reaction coordinate only late in the reaction path.

Excitation Wavelength Dependence of the Proton-Transfer Reaction of the Green Fluorescent Protein

Picosecond time-correlated single-photon counting was used to measure the proton-transfer rate of green fluorescent protein (GFP) excited by several wavelengths between 266 and 405 nm. When samples of GFP in water and D2O are excited at short wavelengths, lambda(ex) < 295 nm, the fluorescence properties are largely modified with respect to excitation at a wavelength around 400 nm, the peak of the absorption band of the S0 --> S1 transition of the ROH form of the chromophore. The shorter the excitation wavelength, the longer the decay time of the ROH emission band at 450 nm and the longer the rise time of the RO- emission band at 512 nm. The proton transfer is slower by an order of magnitude and about a factor of 3 when GFP in water and D2O are excited by 266 nm, respectively.

Electronic excitations of the green fluorescent protein chromophore in its protonation states: SAC/SAC-CI study

Two ground-state protonation forms causing different absorption peaks of the green fluorescent protein chromophore were investigated by the quantum mechanical SAC/SAC-CI method with regard to the excitation energy, fluorescence energy, and ground-state stability. The environmental effect was taken into account by a continuum spherical cavity model. The first excited state, HOMO-LUMO excitation, has the largest transition moment and thus is thought to be the source of the absorption. The neutral and anionic forms were assigned to the protonation states for the experimental A- and B-forms, respectively. The present results support the previous experimental observations.