Ground State Br Research Articles

Quantifying Bacteriorhodopsin Activity as a Function of its Local Environment with a Raman-Based Assay.

Bacteriorhodopsin (bR) is a transmembrane protein that functions as a light-driven proton pump in halophilic archaea. The bR photocycle has been well-characterized; however, these measurements almost exclusively measured purified bR, outside of its native membrane. To investigate what effect the cellular environment has on the bR photocycle, we have developed a Raman-based assay that can monitor the activity of the bR in a variety of conditions, including in its native membrane. The assay uses two continuous-wave lasers, one to initiate photochemistry and one to monitor bR activity. The excitation leads to the steady-state depletion of ground-state bR, which directly relates to the population of photocycle intermediate states. We have used this assay to monitor bR activity both in vitro and in vivo. Our in vitro measurements confirm that our assay is sensitive to bulk environmental changes reported in the literature. Our in vivo measurements show a decrease in bR activity with increasing extracellular pH for bR in its native membrane. The difference in activity with increasing pH indicates that the native membrane environment affects the function of bR. This assay opens the door to future measurements into understanding how the local environment of this transmembrane protein affects function.

Theoretical prediction of Gibbs free energy and specific heat capacity of gaseous molecules

• Highly efficient equations for the prediction of molar Gibbs free energy and molar specific heat capacity are presented. • The equations are based on the improved generalized Pöschl-Teller oscillator. • The analytical models are applicable to large class of diatomic and triatomic molecules. • The equations require only three molecular constants of the molecule as input data. Using the improved generalized Pöschl-Teller oscillator to model the internal vibration of a diatomic molecule, analytical equations to predict molar Gibbs free energy and specific heat capacity at constant pressure were obtained. The equations were applied to ground-state Br 2 , Cl 2 diatomic molecules, and CO gaseous molecule. Average absolute deviations of 0.0755%, 0.0787%, and 0.3583%, were obtained using the equation of reduced molar Gibbs free energy. The expression of molar specific heat capacity yielded average absolute deviations of 4.5055%, 2.5308%, and 2.3665%, respectively from experimental data for the diatomic molecules. The results are in good agreement with existing literature data on the diatomic molecules.

Systematical study on photodissociation dynamics of BrCN from 225 nm to 260 nm

The photodissociation dynamics of Br − C bond cleavage for BrCN in the wavelength region from 225 nm to 260 nm has been studied by our homebuilt time-slice velocity-map imaging setup. The images for both of the ground state Br(2P3/2) and spin-orbit excited Br*(2P1/2) channels are obtained at several photodissociation wavelengths. From the analysis of the translational energy release spectra, the detailed vibrational and rotational distributions of CN products have been measured for both of the Br and Br* channels. It is found that the internal excitation of the CN products for the Br* channel is colder than that for the Br channel. The most populated vibrational levels of the CN products are v=0 and 1 for the Br and Br* channels, respectively. For the Br channel, the photodissociation dynamics at longer wavelengths are found to be different from those at shorter wavelengths, as revealed by their dramatically different vibrational and rotational excitations of the CN products.

Photodissociation study of spatially oriented (R)-3-bromocamphor by the hexapole state selector

A photodissociation experiment of (R)-3-bromocamphor was carried out by the hexapole orientation technique by (2 + 1) REMPI detection at about 234 nm. The atomic bromine products in excited state (Br 2P1/2) and in ground state (Br 2P3/2) were observed by the velocity map imaging. The anisotropy parameters of Br (2P1/2) and Br (2P3/2) were obtained as 0.75 ± 0.04 and 0.48 ± 0.03, respectively. In addition, we also successfully controlled (R)-3-bromocamphor, the bulky and asymmetric bio-molecule’s, spatial orientation using the electrostatic hexapole. The orientational distribution was derived and its orientation efficiency was estimated as |<cos θ>|=0.35. We carried out a slice imaging experiment with the spatially oriented parent molecules to study the photodissociation dynamics of (R)-3-bromocamphor. The angular analysis of the obtained slice images suggests fast axial recoiling of the bromine atomic fragments and comparable contributions from the multiple upper states involved in A-band excitation, which results in the relatively isotropic angular distributions.

Dynamics of Bacteriorhodopsin in the Dark‐Adapted State from Solution Nuclear Magnetic Resonance Spectroscopy

To achieve efficient proton pumping in the light-driven proton pump bacteriorhodopsin (bR), the protein must be tightly coupled to the retinal to rapidly convert retinal isomerization into protein structural rearrangements. Methyl group dynamics of bR embedded in lipid nanodiscs were determined in the dark-adapted state, and were found to be mostly well ordered at the cytosolic side. Methyl groups in the M145A mutant of bR, which displays only 10 % residual proton pumping activity, are less well ordered, suggesting a link between side-chain dynamics on the cytosolic side of the bR cavity and proton pumping activity. In addition, slow conformational exchange, attributed to low frequency motions of aromatic rings, was indirectly observed for residues on the extracellular side of the bR cavity. This may be related to reorganization of the water network. These observations provide a detailed picture of previously undescribed equilibrium dynamics on different time scales for ground-state bR.

Bacteriorhodopsin‐Based Biophotovoltaic Devices Driven by Chemiluminescence as Endogenous Light Source

AbstractAs a light‐driven proton pump, bacteriorhodopsin (bR) captures light and transports protons directionally. Here, a new bR‐based bioelectronic device that employs the chemiluminescence from luminol as an endogenous light source to generate steady current is designed. Purple membrane embedded with bR is modified on indium tin oxide (ITO) electrode followed by electrostatically binding of cationic oligo (p‐phenylene vinylene) derivative (OPV). The device is fabricated by coupling bR/OPV electrode and bare ITO electrode. As a suitable luminescence acceptor of luminol, OPV emits green light through chemiluminescence resonance energy transfer (CRET), which is capable of exciting ground state bR (bR570) nearby to initiate photocycle. Simultaneously, luminol emits blue chemiluminescent light which can be absorbed by M410 intermediate in photocycle, accelerating its reconversion to bR570 by shortcuts. As a result, the photocycle is accelerated and stabilized to generate a stable photocurrent. Furthermore, this study achieves to use oxygen and glucose as the initial source of hydrogen peroxide and produce stable photocurrent by taking advantage of the cascade reaction of glucose oxidase and horseradish peroxide. This provides a proof‐of‐concept for successfully using the CRET system as endogenous light source and provides new ideas for energy utilization and conversion taking advantage of natural bR‐based photosynthetic systems.

Ultraviolet photodissociation dynamics of m-bromofluorobenzene at around 240 nm

The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(2P3/2) and Br(2P1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying 1ππ* excited states dominate the production process of the ground state Br(2P3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(2P1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.

Molecular elimination of Br2 in photodissociation of CH2BrC(O)Br at 248 nm using cavity ring-down absorption spectroscopy

The primary elimination channel of bromine molecule in one-photon dissociation of CH(2)BrC(O)Br at 248 nm is investigated using cavity ring-down absorption spectroscopy. By means of spectral simulation, the ratio of nascent vibrational population in v = 0, 1, and 2 levels is evaluated to be 1:(0.5 ± 0.1):(0.2 ± 0.1), corresponding to a Boltzmann vibrational temperature of 581 ± 45 K. The quantum yield of the ground state Br(2) elimination reaction is determined to be 0.24 ± 0.08. With the aid of ab initio potential energy calculations, the obtained Br(2) fragments are anticipated to dissociate on the electronic ground state, yielding vibrationally hot Br(2) products. The temperature-dependence measurements support the proposed pathway via internal conversion. For comparison, the Br(2) yields are obtained analogously from CH(3)CHBrC(O)Br and (CH(3))(2)CBrC(O)Br to be 0.03 and 0.06, respectively. The trend of Br(2) yields among the three compounds is consistent with the branching ratio evaluation by Rice-Ramsperger-Kassel-Marcus method. However, the latter result for each molecule is smaller by an order of magnitude than the yield findings. A non-statistical pathway so-called roaming process might be an alternative to the Br(2) production, and its contribution might account for the underestimate of the branching ratio calculations.

Multiphoton dissociative ionization of tert-pentyl bromide near 265 nm

We report on the photodissociation dynamics of tert-pentyl bromide near 265 nm investigated by time-sliced velocity map imaging. The speed and angular distributions have been analyzed for both the ground-state Br((2)P(3∕2)) atom (denoted Br) and the spin-orbit excited-state Br((2)P(1∕2)) atom (denoted Br*). The speed distributions of Br and Br* atoms are all found to consist of three Gaussian components, which correlate to three independent dissociation pathways on the excited potential energy surfaces: (1) the high translational energy (E(T)) component from the prompt dissociation along the C-Br stretching mode, (2) the middle E(T) component from the repulsive mode along the C-Br stretching coupled with some bending motions, and (3) the low E(T) component from the repulsive mode along the C-Br stretching coupled with more bending motions. More interestingly, we have also observed the tert-C(5)H(11)(+) ions in 263-267 nm. The near-zero kinetic energy distributions extracted from the three tert-C(5)H(11)(+) images near 265 nm show the typical characteristics that are attributable to multiphoton dissociative ionization, suggesting the existence of a neutral superexcited state of the parent tert-pentyl bromide molecule. The contribution of bromine atoms formed in this dissociative ionization channel adds in the total relative distribution of low E(T) component in the Br*(Br) formation channel, which reasonably explains the abnormal distributions observed in between the middle and low E(T) components in the Br*(Br) formation channel.

Br2 molecular elimination in photolysis of (COBr)2 at 248 nm by using cavity ring-down absorption spectroscopy: A photodissociation channel being ignored

A primary dissociation channel of Br(2) elimination is detected following a single-photon absorption of (COBr)(2) at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br(2) fragment in the B(3)Π(ou)(+)-X(1)Σ(g)(+) transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br(2) contribution from a secondary reaction. By means of spectral simulation, the ratio of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 ± 0.09):(0.34 ± 0.07), corresponding to a Boltzmann vibrational temperature of 893 ± 31 K. The quantum yield of the ground state Br(2) elimination reaction is determined to be 0.11 ± 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr)(2) via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br(2) + 2CO. The resulting Br(2) is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism.

Plasmonic Field Enhancement of the Bacteriorhodopsin Photocurrent during Its Proton Pump Photocycle

The proton pump photocycle of bacteriorhodopsin (bR) produces photocurrent on a microsecond time scale which is assigned to the deprotonation step forming the M(412) intermediate. The return of the M(412) intermediate to the bR ground state (bR(570)) has two pathways: (1) thermally via multiple intermediates (which takes 15 ms) or (2) by a more rapid and direct process by absorbing blue light (which takes hundreds of nanoseconds). By using nanoparticles (Ag, Ag-Au, and Au NPs) having different surface plasmon resonance extinction spectra, it is found that Ag NPs whose spectrum overlaps best with the M(412) absorption regions enhance the stationary photocurrent 15 times. This large enhancement is proposed to be due to the accelerated photoexcitation rate of the M(412) (in the presence of the plasmon field of the light in this region) as well as short-circuiting of the photocycle, increasing its duty cycles.

Photodissociation Dynamics of &lt;em&gt;o&lt;/em&gt;-Bromotoluene at 234 and 267 nm

The photodissociation of o-bromotoluene was studied at 234 and 267 nm using velocity map imaging combined with a resonance-enhanced multiphoton ionization (REMPI) technique. Translational energy distributions suggested that ground state Br (2P(subscript 3/2)) and spin-orbit excited state Br(superscript *) (2P(subscript 1/2)) fragments were all generated via two dissociation channels: a fast channel and a slow channel. The anisotropy parameters of the fast channels were determined to be 1.15 (Br) and 0.55 (Br(superscript *)) at 234 nm, 0.90 (Br) and 0.60 (Br(superscript *)) at 267 nm. The anisotropy parameters of the slow channels were 0.12 (Br) and 0.14 (Br(superscript *)) at 234 nm, 0.11 (Br) and 0.10 (Br(superscript *)) at 267 nm. The Br and Br(superscript *) fragments of the slow channel were less anisotropic than those of the fast channel. The total relative quantum yields of Br (Φ (Br)) were 0.67 at 234 nm and 0.70 at 267 nm. Ground state Br (2P(subscript 3/2)) was the main product from the photolysis of o-bromotoluene at 234 and 267 nm. We propose that the fast channel originates from excitation of bound excited singlet (π, π(superscript *)) states followed by predissociation along repulsive (n, σ(superscript *)) states. The anisotropy parameters of the slow channels were close to zero indicating a hot dissociation mechanism on a highly vibrational ground state followed the internal conversion of the excited singlet state.

Halogen Effect on the Photodissociation Mechanism for Gas‐Phase Bromobenzene and Iodobenzene

The velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the halogen fragments in the photodissociation of bromobenzene and iodobenzene at 266 nm. With the aid of potential energy curve calculations by Lunell (Y. J. Liu, P. Persson, S. Lunell, J. Phys. Chem. A 2004, 108, 2339-2345.), the Br fragmentation is proposed to stem from excitation of the lowest excited singlet (pi-pi*) state followed by predissociation along a repulsive triplet (n-sigma*) state. The slowed dissociation rate leads to production of the isotropic Br fragments and 93 % internal energy deposition. Only the ground state Br((2)P(3/2)) is detectable. In contrast, when iodine is substituted, the iodine effect stabilizes the repulsive states associated with the I-C6H5 bond rupture and the subsequent dissociation channels become more complicated. 84 % of the iodobenzene molecules obtained follow a direct dissociation channel, while the remaining undergo a predissociative process. Both routes result in rapid dissociation with anisotropy parameters of 0.7+/-0.2 and 0.9+/-0.2 as well as 70 % and 26 % in the fractions of translational energy deposition, respectively. The relative quantum yields of I* and I are 0.35 and 0.65 and their related photodissociation pathways are discussed in detail.

Interactions between anionic and neutral bromine and rare gas atoms

High-quality, ab initio potential energy functions are obtained for the interaction of bromine atoms and anions with atoms of the six rare gases (Rg) from He to Rn. The potentials of the nonrelativistic (2)Sigma(+) and (2)Pi electronic states arising from the ground-state Br((2)P)-Rg interactions are computed over a wide range of internuclear separations using a spin-restricted version of the coupled cluster method with single and double excitations and noniterative correction to triple excitations [RCCSD(T)] with an extrapolation to the complete basis set limit, from basis sets of d-aug-cc-pVQZ and d-aug-cc-pV5Z quality. These are compared with potentials derived previously from experimental measurements and ab initio calculations. The same approach is used also to refine the potentials of the Br(-)-Rg anions obtained previously [Buchachenko et al., J. Chem. Phys. 125, 064305 (2006)]. Spin-orbit coupling in the neutral species is included both ab initio and via an atomic approximation; deviations between two approaches that are large enough to affect the results significantly are observed only in the Br-Xe and Br-Rn systems. The resulting relativistic potentials are used to compute anion zero electron kinetic energy photoelectron spectra, differential scattering cross sections, and the transport coefficients of trace amounts of both anionic and neutral bromine in the rare gases. Comparison with available experimental data for all systems considered proves a very high precision of the present potentials.

Photodissociation Dynamics of Bromofluorobenzenes Using Velocity Imaging Technique

Velocity imaging technique combined with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) has been used to detect the Br fragment in photodissociation of o-, m-, and p-bromofluorobenzene at 266 nm. The branching ratio of ground state Br(2P3/2) is found to be larger than 96%. Its translational energy distributions suggest that the Br fragments are generated via two dissociation channels for all the molecules. The fast route, which is missing in p-bromofluorobenzene detected previously by femtosecond laser spectroscopy, giving rise to an anisotropy parameter of 0.50-0.65, is attributed to a direct dissociation from a repulsive triplet T1(A' ') or T1(B1) state. The slow one with anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*)state followed by predissociation along a repulsive triplet (pi,sigma*) state localized on the C-Br bond. For the minor product of spin-orbit excited state Br(2P1/2), the dissociating features are similar to those found in Br(2P3/2). Our kinetic and anisotropic features of decomposition obtained in m- and p-bromofluorobenzene are opposed to those by photofragment translational spectroscopy. Discrepancy between different methods is discussed in detail.

High-resolution slice imaging of quantum state-to-state photodissociation of methyl bromide

The photodissociation of rotationally state-selected methyl bromide is studied in the wavelength region between 213 and 235 nm using slice imaging. A hexapole state selector is used to focus a single (JK=11) rotational quantum state of the parent molecule, and a high speed slice imaging detector measures directly the three-dimensional recoil distribution of the methyl fragment. Experiments were performed on both normal (CH(3)Br) and deuterated (CD(3)Br) parent molecules. The velocity distribution of the methyl fragment shows a rich structure, especially for the CD(3) photofragment, assigned to the formation of vibrationally excited methyl fragments in the nu(1) and nu(4) vibrational modes. The CH(3) fragment formed with ground state Br((2)P(3/2)) is observed to be rotationally more excited, by some 230-340 cm(-1), compared to the methyl fragment formed with spin-orbit excited Br((2)P(1/2)). Branching ratios and angular distributions are obtained for various methyl product states and they are observed to vary with photodissociation energy. The nonadiabatic transition probability for the (3)Q(0+)-->(1)Q(1) transition is calculated from the images and differences between the isotopes are observed. Comparison with previous non-state-selected experiments indicates an enhanced nonadiabatic transition probability for state-selected K=1 methyl bromide parent molecules. From the state-to-state photodissociation experiments the dissociationenergy for both isotopes was determined, D(0)(CH(3)Br)=23 400+/-133 cm(-1) and D(0)(CD(3)Br)=23 827+/-94 cm(-1).

Photodissociation dynamics of CF 3Br at 225 nm: Direct dissociation via pseudo-linear geometry

Photodissociation dynamics of CF 3Br has been investigated at 225 nm, where the 3Q 0 state is accessed exclusively. It has been revealed that the ground state Br( 2 P 3/2) is hardly formed with a relative quantum yield less than 5%. The recoil anisotropy of Br ∗( 2 P 1/2) has been found to have a limiting value, β Br ∗ = 1.90 ± 0.05. The total translational energy distribution for the Br ∗ channel was well fitted by a Gaussian function with an average of 155 kJ/mol and a width of 30 kJ/mol. It has been proposed that the 3Q 0 state is not bent and the dissociation on this surface proceeds in a collinear geometry.

Photodissociation of propargyl bromide and photoionization of the propargyl radical

Velocity map imaging was used to study the 193 nm photodissociation of propargyl bromide C(3)H(3)Br as well as the photoionization dynamics of the resulting propargyl radical C(3)H(3). Images were recorded by using single-photon vacuum ultraviolet ionization of the propargyl radical and by using two-photon resonant, three-photon ionization of the ground state Br((2)P(32)) and spin-orbit excited Br(*)((2)P(12)) atoms. Analysis of these data allowed the determination of the branching ratio Br:Br(*) as well as the photofragment angular distributions. Images of C(3)H(3) produced by the photodissociation of both C(3)H(3)Br and C(3)H(3)Cl were recorded at several energies between 8.97 and 9.12 eV, as well as at 9.86 eV, and showed no obvious internal energy dependence of the relative photoionization cross sections.

Photodissociation of state selected BrCl+ cations: branching ratios and angular anisotropies of the Br+ product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370–420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3 P J ) + Cl(2 P J ) product channels. Ground state Br+(3 P 2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin–orbit excited Cl(2 P 1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π ← X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin–orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin–orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.

Photodissociation of state selected BrCl cations: Branching ratios and angular anisotropies of the Br product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, v=0) cations following excitation in the wavelength range 370-420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3PJ) + Cl(2PJ) product channels. Ground state Br+(3P2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin-orbit excited Cl(2P1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π←X2Π (ΔΩ=0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted (but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin-orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin-orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A2Π state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.