Estimated Quantum Yield Research Articles

Intramolecular rate-constant calculations based on the correlation function using temperature dependent quantum Green's functions.

A theoretical method for calculating rate constants for internal conversion (IC), intersystem crossing (ISC) and radiative (R) electronic transitions is presented. The employed method uses temperature-dependent quantum Green's functions, which give the opportunity to consider almost any nth-order polynomial perturbation operator and the influence of external electromagnetic fields on the rate constants. The rate constants of the IC, ISC and R processes are calculated for two important indocyanine molecules namely indocyanine green (ICG) and heptamethine cyanine (IR808) at the Franck-Condon level using the temperature-dependent quantum Green's function approach. Calculations at the time-dependent density functional theory level with the MN15 functional show that ICG and IR808 have only one triplet state below the S1 state. The main deactivation channel of the S1 state is the IC process with a large (kIC(S1 → S0)) rate constant of ∼109-1011 s-1. The estimated quantum yield of fluorescence (φfl) is ∼0.001-0.24 for the two studied molecules, which agrees rather well with experimental values. Thus, the present approach enables calculations of the three kinds of rate constants and the quantum yield of fluorescence using the same computational methodology.

Photodissociation of H2S: A New Pathway for the Production of Vibrationally Excited Molecular Hydrogen in the Interstellar Medium.

Hydrogen sulfide (H2S) is the most abundant S-bearing molecule in the solar nebula. Although its photochemistry has been studied for decades, the H2 fragment channel is still not well-understood. Herein, we describe the photodissociation dynamics of H2S + hv → S(1S) + H2(X1Σg+) with the excitation wavelength of 122 nm ≤ λ ≤ 136 nm. The results reveal that the H2(X) fragments formed are significantly vibrationally excited, with the quantum yields of ∼87% of H2(X) fragments populated in vibrational levels v″ = 3, 4, 5, and 6. Theoretical analysis suggest that these H2 products are formed on the H2S 41A' state surface following a nonadiabatic transition via an avoided crossing from the 31A' to 41A' state. The estimated quantum yield of the S(1S) + H2 channel is ∼0.05, implying this channel should be incorporated into the appropriate interstellar chemistry models.

Self-doped defect-mediated TiO2 with disordered surface for high-efficiency photodegradation of various pollutants

Photocatalytic technology in eliminating organic pollutants is considered to be one of the most promising technologies to solve environmental issues. However, the low catalytic activity exhibited by Titanium dioxide (TiO2) limits its further application. In order to enhance the photocatalytic activity, structural regulation of TiO2 is designed by chemical reduction method to promote the production of massive Ti3+ and oxygen vacancies (OVs), these defects can serve as inter-band level of semiconductor to enhance photon capture and transfer efficiency of photogenerated charge. The samples show strong light absorption ability, which leads to excellent photocatalytic activity for various organic pollutants degradation. Results showed robust degradation of MO, RhB, DCP and TC under UV irradiation within 60 min. Estimated quantum yields of as-synthesized TiO2 systems for removing representative pollutants are calculated, which indicates higher reactivity than commercial TiO2. The XPS, TEM, photoelectrochemical analysis and EPR results intuitive reveal the micro-morphology, band structure and active species of Ti3+ doped defective TiO2. This work can provide an essential reference for structural regulation and composition of oxide semiconductor since the methodology could be freely applicable to other systems.

Durable fluorescent cotton textile by immobilization of unique tetrahydrothienoisoquinoline derivatives

Due to the sensitivity for interchanging color by exposure to UV light, designing of fluorescent textiles is highly demanded to be employed in camping, military and sensing purposes. Aromatic compounds containing tetrahydroisoquinoline are known with their biological and medicinal activity, but they are not applied as fluorescent materials. Therefore, the present study focuses on synthesis of tetrahydrothienoisoquinoline derivatives and their application in textile technology to obtain fluorescent cotton fabrics. Four tetrahydrothienoisoquinoline derivatives were synthesized in three steps, starting from 2,4-diacetyl-5-hydroxy-5-methyl-3-(4-methoxyphenyl)cyclohexanone. The chemical structures of the synthesized tetrahydrothienoisoquinoline derivatives (Ar1-Ph, Ar2-Ph-Cl, Ar3-Ph-CH3 and Ar4-Ph-OCH3) were confirmed by infrared spectra, 1HNMR and 13CNMR. The yields and melting points were both measured for all the synthesized compounds. The tetrahydrothienoisoquinoline derivatives were applied onto the cotton fabrics to obtain Ar1-Ph@cotton, Ar2-Ph-Cl@cotton, Ar3-Ph-CH3@cotton and Ar4-Ph-OCH3@cotton fabrics. All modified cotton fabrics were exhibited by green emission under UV lamp (325 nm). The excited modified cotton fabrics showed an intense fluorescence emission peak at 444–467 nm, related to the implemented compounds. The intensity of the fluorescence peak followed the order of Ar1-Ph@cotton > Ar3-Ph-CH3@cotton ≥ Ar4-Ph-OCH3@cotton >> Ar2-Ph-Cl@cotton. The estimated quantum yield (QY) were 68.79%, 23.15%, 13.83% and 13.38%, for Ar1-Ph, Ar2-Ph-Cl, Ar3-Ph-CH3 and Ar4-Ph-OCH3, respectively. Due to the water insoluble character of tetrahydrothienoisoquinoline derivatives, the modified fabrics showed quite good durability, as the the quantum yields were marginally diminished by only 10.1 – 12.3%, after 5 washings. The prepared tetrahydrothienoisoquinoline derivatives succeeded for obtaining durable fluorescent textiles, and could be further applied in advanced purposes including technical textiles, sensors/biosensors, smart labeling and anti-counterfeiting purposes.

Degradation of pesticides present in tomato rinse water by direct photolysis and UVC/H2O2: optimization of process conditions through sequential Doehlert design

The degradation of three pesticides, azoxystrobin (AZO), difenoconazole (DFZ), and imidacloprid (IMD), commonly found in the tomato rinse water, was studied through UVC (251-257 nm) and UVC/H2O2 photolysis. The results showed that direct photolysis follows pseudo-first-order kinetics, with total AZO and IMD removals within 15 min, using 21.8 and 28.6 W m-2, respectively, while the highest percentage of DFZ degradation was 51.7% at 28.6 W m-2 UVC. The estimated quantum yields were 0.572, 0.028, and 0.061 mol Einstein-1 for AZO, DFZ, and IMD, respectively. With regard to UVC/H2O2, total pesticide removal was achieved after 10 min, while optimal treatment conditions in relation to the pesticide removal rates, estimated through the sequential Doehlert design, were about [H2O2]0 = 130 mg L-1 and 26 W m-2. Cytotoxicity and genotoxicity assays carried out with Allium cepa, for real industrial tomato rinse water sampled from washing belts did not show abnormalities during cell division, with total pesticides degradation after 15 min, demonstrating the potential application of the UVC/H2O2 process as a viable localized treatment with a focus on the possible reuse of treated water.

Photoinduced Tetrazole‐Based Functionalization of Off‐Stoichiometric Clickable Microparticles

We report the preparation of tetrazole‐containing step‐growth microparticles and the subsequent use of photoinduced nitrile imine‐mediated tetrazole‐ene cycloaddition (NITEC) reactions on the particles with spatiotemporal control. Microparticles with an average diameter of 4.1 µm and with inherent tetrazole‐ene dual functionality are prepared by a one‐pot off‐stoichiometric thiol‐Michael addition dispersion polymerization. The NITEC reaction is performed efficiently in the solid phase by UV irradiation, leading to the formation of fluorescent pyrozoline adducts, with an estimated quantum yield of 0.7. Particle concentration‐independent reaction kinetics are observed and full conversion is reached within 10 min of UV exposure at an intensity of 8 mW cm−2. Temporal control is demonstrated with either UV or rooftop sunlight irradiation of variable duration. By using two‐photon writing with a laser centered around 700 nm wavelength, spatial control is demonstrated with micrometer‐scale resolution via surface patterning of the microparticles. Further, microparticles with exclusive tetrazole functionality are prepared by a one‐pot, two‐step thiol‐Michael addition dispersion polymerization. The NITEC reaction between tetrazole‐functional particles and acrylates in solution is examined at various tetrazole/alkene molar ratios, and a 10:1 excess of alkenes in solution is found necessary for efficient functionalization.

Photo-induced 1,3-cyclohexadiene ring opening reaction: Ab initio on-the-fly nonadiabatic molecular dynamics simulation

Abstract By a newly developed algorithm to compute global nonadiabatic switching probability only using electronic adiabatic potential energy surfaces and its gradients, we could able to perform on-the-fly trajectory surface hopping molecular dynamics at the SA-CASSCF(14, 8)/6-31G* quantum level to investigate the photo-induced 1,3-cyclohexadiene ring opening reaction from 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT). We found four conical intersection zones within three singlet low-lying electronic states, in which 1B/2A and 1A/2A are well known to be the most important conical intersection zones to govern dynamics of ring opening reaction. The present simulation gained insight into nonadiabatic dynamics in which 516 sampling trajectories out of 600 can switch back and forth between two involved potential energy surfaces not only at 1B/2A, but also at 1A/2A. This insight is the key to influence quantum yields and time scales of the ring opening reaction. We estimated time constants as 46 fs, 82 fs, and 120 fs, respectively for crossing 1B/2A and 1A/2A, and excited-state lifetime in comparison with corresponding experimental values 56 fs, 80 fs and 130 fs. We estimated quantum yield 0.47/0.53 to HT/CHD in comparison to experiment values 0.30/0.70 in the gas-phase and 0.41/0.59 in the condense-phase. Moreover, the present simulation within 600 sampling trajectories indicates that there is no five membered ring forming in agreement with experiment results and with prediction from the previous wave-packet simulation as a rare case.

The intrinsic photophysics of gaseous ethidium ions

Abstract Ethidium is a cationic dye with fluorescence that is enhanced ∼9-fold upon binding DNA. In order to better understand how the local environment modulates the behavior of this dye, we measured the photophysical properties of gaseous ethidium ions, using a quadrupole ion trap mass spectrometer that has been modified for fluorescence spectroscopy. The photodissociation maximum of gaseous ethidium measured through action spectroscopy is 485 nm and the emission maximum is 548 nm. The Stokes shift (2370 cm−1) of gaseous ethidium is marginally larger than that of ethidium in non-polar solvents, and significantly less than that in polar solvents. Time-resolved fluorescence measurements of gaseous ethidium ions show two components with lifetimes of 21.4 ± 1.5 and 5.1 ± 0.7 ns, which suggest the presence of multiple conformations in the gas phase. Both lifetimes are significantly longer than that of aqueous ethidium, while the longer of the two lifetimes is remarkably similar to that of ethidium in complex with double-stranded DNA in solution. In line with this, the estimated quantum yield of gaseous ethidium is ∼30% lower than that of ethidium in complex with DNA in solution, and ∼10-fold higher than that of aqueous ethidium. These benchmark results provide a reference from which to better understand the factors that modulate the fluorescence of phenanthridine-based dyes by the local environment.

Removal of Rhodamine B with Fe-supported bentonite as heterogeneous photo-Fenton catalyst under visible irradiation

Fe-supported bentonite (Fe-B) was successfully fabricated as a low-cost heterogeneous catalyst for adsorption and visible light photo-Fenton degradation of Rhodamine B (RhB) from aqueous solution. The support of iron rendered a significant increase in specific surface area and a slight increase in interlayer spacing of bentonite, thus improving the adsorption performance of Fe-B. The maximum adsorption capacity of RhB onto Fe-B was calculated to be 227.27mgg−1 by the fitting of Langmuir adsorption isotherm. The catalytic activity of Fe-B was evaluated by the degradation of RhB (80mgL−1) in the presence of H2O2 under visible light irradiation. Light-emitting diodes (LED) lamps were used as visible light source as they are small in size, energy efficient, compact, have a relatively long life span, and can be facile to operate. The estimated quantum yield of photodegradation of RhB was calculated as (1.66±0.21)×10−2. The results showed that the Fe-B catalyst demonstrated good performance in the removal of RhB by combination of adsorption and degradation with optimum operating conditions of Fe-B 0.25gL−1, natural pH 4.2 and H2O2 12mM. The Fe-B catalyst retained high visible light photocatalytic activity toward RhB in a wide pH range from 3.0 to 9.0 and exhibited good chemical stability after five consecutive adsorption/degradation cycles.

Contactless photoconductance study on undoped and doped nanocrystalline diamond films.

Hydrogen and oxygen surface-terminated nanocrystalline diamond (NCD) films are studied by the contactless time-resolved microwave conductivity (TRMC) technique and X-ray photoelectron spectroscopy (XPS). The optoelectronic properties of undoped NCD films are strongly affected by the type of surface termination. Upon changing the surface termination from oxygen to hydrogen, the TRMC signal rises dramatically. For an estimated quantum yield of 1 for sub-bandgap optical excitation the hole mobility of the hydrogen-terminated undoped NCD was found to be ∼0.27 cm(2)/(V s) with a lifetime exceeding 1 μs. Assuming a similar mobility for the oxygen-terminated undoped NCD a lifetime of ∼100 ps was derived. Analysis of the valence band spectra obtained by XPS suggests that upon oxidation of undoped NCD the surface Fermi level shifts (toward an increased work function). This shift originates from the size and direction of the electronic dipole moment of the surface atoms, and leads to different types of band bending at the diamond/air interface in the presence of a water film. In the case of boron-doped NCD no shift of the work function is observed, which can be rationalized by pinning of the Fermi level. This is confirmed by TRMC results of boron-doped NCD, which show no dependency on the surface termination. We suggest that photoexcited electrons in boron-doped NCD occupy nonionized boron dopants, leaving relatively long-lived mobile holes in the valence band.

Rapid Photooxidation of As(III) through Surface Complexation with Nascent Colloidal Ferric Hydroxide

Contamination of water and soils with arsenic, especially inorganic arsenic, has been one of the most important topics in the fields of environmental science and technology. The interactions between iron and arsenic play a very significant role in the environmental behavior and effect of arsenic species. However, the mechanism of As(III) oxidation in the presence of iron has remained unclear because of the complicated speciation of iron and arsenic. Photooxidation of As(III) on nascent colloidal ferric hydroxide (CFH) in aqueous solutions at pH 6 was studied to reveal the transformation mechanism of arsenic species. Experiments were done by irradiation using light-emitting diodes with a central wavelength of 394 nm. Results show that photooxidation of As(III) and photoreduction of Fe(III) occurred simultaneously under oxic or anoxic conditions. Photooxidation of As(III) in the presence of nascent CFH occurred through electron transfer from As(III) to Fe(III) induced by absorption of radiation into a ligand-to-metal charge-transfer (LMCT) band. The estimated quantum yield of photooxidation of As(III) at 394 nm was (1.023 ± 0.065) × 10(-2). Sunlight-induced photooxidation of As(III) also occurred, implying that photolysis of the CFH-As(III) surface complex could be an important process in environments wherein nascent CFH exists.

Singlet Oxygen Generating Activity of an Electron Donor Connecting Porphyrin Photosensitizer Can Be Controlled by DNA

To control the activity of singlet oxygen ((1)O2) generation by photosensitizer through interaction with DNA, the electron- donor-connecting water-soluble porphyrin, meso-(9-anthryl)tris(N-methyl-p-pyridinio)porphyrin (AnTMPyP), was designed and synthesized. Molecular orbital calculation speculated that the photoexcited state of AnTMPyP can be deactivated via intramolecular electron transfer from the anthracene moiety to the porphyrin moiety, forming a charge-transfer (CT) state. The electrostatic interaction between the cationic porphyrin and anionic DNA predicts a rise in the CT state energy, leading to the inhibition of the electron transfer quenching. AnTMPyP showed almost no fluorescence in an aqueous solution, and the fluorescence lifetime was very short (<0.04 ns). Furthermore, this porphyrin did not demonstrate (1)O2 generating activity under photoirradiation. The fluorescence intensity and lifetime of AnTMPyP were markedly increased in the presence of DNA. The photosensitized (1)O2 generation by this porphyrin was also enhanced through interaction with DNA. The estimated quantum yield of (1)O2 generation by AnTMPyP interacting with DNA without guanine sequence was 0.22. The molecular design to control the photosensitized (1)O2 generation is possible based on the regulation of electron transition and steric hindrance of photosensitizing molecule.

Dicyano-Substituted Poly(phenylenevinylene) (DiCN–PPV) and the Effect of Cyano Substitution on Photochemical Stability

A didecyloxy-substituted poly(phenylenedicyanovinylene), DiCN–PPV, has been synthesized. The dicyano-substituted vinylene units exist in both trans and cis (∼65:35) conformations as determined by 1H NMR analysis, and cannot be converted to all trans due to the presence of a thermodynamic equilibrium of the two conformations, in contrast to the vinylene units in regular PPVs. The unusually high cis content makes this polymer highly amorphous, very soluble in organic solvent, and highly fluorescent in the solid state with an estimated quantum yield up to 0.34, four times more fluorescent than its chloroform solution. The LUMO and HOMO energies of the new polymer were measured by cyclovoltammetry. The cyano groups in DiCN–PPV brings a decrease in LUMO energy by 0.79 eV, and makes the polymer more stable to intense white light (>20 times as strong as the sunlight) than poly(2,5-didecyloxy-1,4-phenylenevinylene), C10O–PPV, by more than 2 orders of magnitude. The excellent photochemical stability and high fluor...

Near-Infrared Quantum Cutting Material Er3+/Yb3+ Doped La2O2S with an External Quantum Yield Higher than 100%

Near-infrared quantum cutting is widely studied for its potential application in photovoltaic solar cells. By using a fluorometer equipped with an integrating sphere, we measured the external quantum yield of near-infrared quantum cutting in Er3+/Yb3+ doped La2O2S. It is found that by optimizing the synthesis process and the dopant concentration, the external quantum yield higher than 100% can be achieved. Additionally, by comprehensively considering the emission efficiency of the dopants as well as the energy transfer efficiency between them, the estimated quantum yield could be in good accordance with the measured one.

Temporal patterns of net CO2exchange for a tropical semideciduous forest of the southern Amazon Basin

[1] The carbon cycling of tropical ecosystems has received considerable attention over the last 1–2 decades; however, interactions between climate variation and tropical forest net ecosystem CO2 exchange (NEE) are still uncertain. To reduce this uncertainty, and assess the biophysical controls on NEE, we used the eddy covariance method over a 3 year period (2005–2008) to measure the CO2 flux and energy balance for a 25–28 m tall, mature tropical semideciduous forest located near Sinop Mato Grosso, Brazil. The study period encompassed warm-dry, cool-wet, and cool-dry climate conditions, and based on previous research, we hypothesized that the net CO2 accumulation of the semideciduous forest would be lower during periods of drought. Using time series of the enhanced vegetation index (EVI), a NEE-light-use model, and path analysis, we found that the estimated quantum yield (a′, μmol CO2μmol photons−1) was directly affected by temporal variations in the EVI, precipitation, and photosynthetically active radiation (PAR), while the optimal rate of gross primary production (FGPP,opt, μmol m−2 s−1) was directly affected by the EVI and PAR. However, indirect effects of precipitation on the a′ and FGPP,opt were stronger than direct effects because variations in precipitation also lead to variations in the EVI and the atmospheric vapor pressure deficit (VPD). Daytime ecosystem respiration (FRE,day, μmol m−2 s−1) was directly affected by temporal variations in temperature and VPD and indirect effects of other variables were of lesser importance. Net ecosystem CO2 uptake was often higher in the dry season than the wet season, not because of a dry season “green-up” but because rates of ecosystem respiration declined relatively more than rates of canopy photosynthesis. Over interannual timescales, average daily NEE increased over the 3 year study period and was highest in 2007–2008, which was also the driest year in terms of rainfall. However, 2007–2008 was also the coolest year during the 3 year study period, and the low temperature appeared to compensate for the low rainfall. Overall, our data suggest that the NEE of tropical semideciduous forests is sensitive to temporal variations in surface water availability but that indirect effects of other variables, such as temperature and VPD, are important in controlling CO2 gain and loss. Such interactions will be important for the future NEE under warmer and drier conditions that are anticipated with anthropogenic climate change.

Quantification of Photosensitized Singlet Oxygen Production by a Fluorescent Protein

Fluorescent proteins are increasingly becoming actuators, rather than just sensors, in a range of cell biology techniques. One of those techniques is chromophore-assisted laser inactivation (CALI), which is employed to specifically inactivate the function of target proteins or organelles by producing photochemical damage [1]. CALI is achieved by the irradiation of dyes that are able to produce reactive oxygen species (ROS). The combination of CALI and the labelling specificity that fluorescent proteins provide is very useful to avoid uncontrolled photodamage. Indeed, fluorescent proteins have been successfully used in CALI, although of the inactivation mechanisms by ROS are dependent on the fluorescent protein used and are not fully understood [2,3]. Here, we present a quantitative study of the ability of TagRFP to produce ROS, in particular singlet oxygen. TagRFP is able to photosensitize singlet oxygen with an estimated quantum yield of 0.004 [4]. This is the first estimation of a quantum yield of singlet oxygen production value for a GFP-like protein. We also find that TagRFP has a short triplet lifetime, which reflects relatively high oxygen accessibility to the chromophore compared to EGFP. Our results provide photophysical insight that allows the understanding of the mechanism behind CALI. Moreover, it has implications in improving photobleaching in fluorescent proteins. [1] K. Jacobson, Z. Rajfur, E. Vitriol, K. Hahn, Trends Cell Biol.2008, 18, 443. [2] M. A. McLean, Z. Rajfur, Z. Z. Chen, D. Humphrey, B. Yang, S. G. Sligar, K. Jacobson, Anal. Chem. 2009, 81, 1755. [3] M. E. Bulina et al, Nat .Biotechnol.2006, 24, 95..[4] X. Ragas, L. P. Cooper, J. H. White, S. Nonell, C. Flors, submitted.

Quantification of Photosensitized Singlet Oxygen Production by a Fluorescent Protein

Fluorescent proteins are increasingly becoming actuators in a range of cell biology techniques. One of those techniques is chromophore-assisted laser inactivation (CALI), which is employed to specifically inactivate the function of target proteins or organelles by producing photochemical damage. CALI is achieved by the irradiation of dyes that are able to produce reactive oxygen species (ROS). The combination of CALI and the labelling specificity that fluorescent proteins provide is useful to avoid uncontrolled photodamage, although the inactivation mechanisms by ROS are dependent on the fluorescent protein and are not fully understood. Herein, we present a quantitative study of the ability of the red fluorescent protein TagRFP to produce ROS, in particular singlet oxygen ((1)O(2)). TagRFP is able to photosensitize (1)O(2) with an estimated quantum yield of 0.004. This is the first estimation of a quantum yield of (1)O(2) production value for a GFP-like protein. We also find that TagRFP has a short triplet lifetime compared to EGFP, which reflects relatively high oxygen accessibility to the chromophore. The insight into the structural and photophysical properties of TagRFP has implications in improving fluorescent proteins for fluorescence microscopy and CALI.

Homogeneous Light-Driven Water Oxidation Catalyzed by a Tetraruthenium Complex with All Inorganic Ligands

A totally homogeneous, molecular, visible-light-driven water oxidation system is reported. The three system components are (i) a water oxidation catalyst, 1 (a Ru(IV)(4)O(4) cluster stabilized by oxidatively resistant [SiW(10)O(32)](8-) ligands); (ii) a photosensitizer, [Ru(bpy)(3)](2+); and (iii) a sacrificial electron acceptor, S(2)O(8)(2-). Dioxygen is formed rapidly with an initial turnover frequency of approximately 8 x 10(-2) s(-1) and an estimated quantum yield (defined as the number of O(2) molecules formed per two photons absorbed) of approximately 9%.

Concentration quenching and migration of excitations in a bulk Cd0.5Mn0.5Te crystal

The curves of intracenter luminescence decay for Mn2+ ions in the Cd0.5Mn0.5Te semiconductor solid solution, obtained in a low-temperature experiment, have been simulated by the Monte Carlo method. The features of the kinetics of the 2-eV band in the time interval where significant nonexponentiality of relaxation at different points of the emission band profile manifests itself, as well the integral kinetics and energy relaxation, have been considered. Migration of ion excitations and concentration quenching (which was previously disregarded) are considered to be the main mechanisms determining the kinetic curve formation. It was established that excitation by 2.34-eV photons leads to both selective (intracenter) and band excitation of Mn2+ ions. Comparison of the results of numerical simulation and experiment showed that the characteristic values of the migration and quenching rates (Wm and Wq, respectively) are close in magnitude and Wq, m ≈ 0.1/τ, where τ is the lifetime at the long-wavelength band wing with the exponential kinetics. The estimated quantum yield (0.56) indicates significant influence of the concentration quenching on the 2-eV luminescence quantum yield in Cd1 − xMnxTe and Zn1 − xMnxS crystals with a high concentration of Mn2+ ions.

Characterization of the Primary Photochemistry of Proteorhodopsin with Femtosecond Spectroscopy

Proteorhodopsin is an ion-translocating member of the microbial rhodopsin family. Light absorption by its retinal chromophore initiates a photocycle, driven by trans/cis isomerization, leading to transmembrane translocation of a proton toward the extracellular side of the cytoplasmic membrane. Here we report a study on the photoisomerization dynamics of the retinal chromophore of proteorhodopsin, using femtosecond time-resolved spectroscopy, by probing in the visible- and in the midinfrared spectral regions. Experiments were performed both at pH 9.5 (a physiologically relevant pH value in which the primary proton acceptor of the protonated Schiff base, Asp97, is deprotonated) and at pH 6.5 (with Asp97 protonated). Simultaneous analysis of the data sets recorded in the two spectral regions and at both pH values reveals a multiexponential excited state decay, with time constants of ∼0.2ps, ∼2ps, and ∼20ps. From the difference spectra associated with these dynamics, we conclude that there are two chromophore-isomerizaton pathways that lead to the K-state: one with an effective rate of ∼(2ps)−1 and the other with a rate of ∼(20ps)−1. At high pH, both pathways are equally effective, with an estimated quantum yield for K-formation of ∼0.7. At pH 6.5, the slower pathway is less productive, which results in an isomerization quantum yield of 0.5. We further observe an ultrafast response of residue Asp227, which forms part of the counterion complex, corresponding to a strengthening of its hydrogen bond with the Schiff base on K-state formation; and a feature that develops on the 0.2ps and 2ps timescale and probably reflects a response of an amide II band in reaction to the isomerization process.