Nascent NO Research Articles

Hemoglobin’s β-subunit is primed to synergize oxygen delivery with nitric oxide-mediated increased blood flow

Hemoglobin (Hb) undergoes a well-documented R[Formula: see text]T quaternary transition from a high [Formula: see text] affinity R state to a low [Formula: see text] affinity T state to optimize [Formula: see text] delivery to tissues. Also, red blood cells (RBCs) release a nitrovasodilator that increases blood flow to boost [Formula: see text] delivery. Hb’s R[Formula: see text]T transition coordinates its [Formula: see text] desaturation and RBC nitrovasodilator release by much-debated mechanisms. Here we investigate the allosterically controlled [Formula: see text]-nitrosation of Hb at its conserved βCys93 (i.e., SNO-Hb formation) for nitrovasodilator release. First, we examined NO[Formula: see text]/deoxyHb (HbFeII) incubations following aeration to mimic RBC NO production by the nitrite reductase activity of HbFeII and trapping of the nascent NO by βCys93 to give SNO-Hb on HbFeII conversion to oxyHb (HbFeO2). We confirmed SNO-Hb formation in the incubations with yields modulated by RBC antioxidant enzymes and [Formula: see text] but not CO. Since FeO2 hemes scavenge free NO, we hypothesized NO channeling within Hb and found by molecular dynamics simulations that most unligated NO molecules placed in the β-distal heme pocket (βDP) rapidly diffuse into a wide β-tunnel connecting the βDP to Hb’s central cavity and βCys93. Contraction of the central cavity brings NO closer to βCys93 in R-state plus βPhe71 and βTyr145 adopt conformations favorable to thiol access and SNO-Hb formation. In T-state, the SNO group is surface-exposed and destabilized to extrude NO. Thus, its structure, dynamics and conserved reactive thiol (βCys93) suggest that the β-subunit evolved to synergize [Formula: see text] and nitrovasoactivity delivery to tissues as a function of Hb [Formula: see text] saturation.

Experimental and Theoretical Studies of Hyperthermal N + O2 Collisions.

The dynamics of hyperthermal N(4S) + O2 collisions were investigated both experimentally and theoretically. Crossed molecular beams experiments were performed at an average center-of-mass (c.m.) collision energy of ⟨Ecoll⟩ = 77.5 kcal mol-1, with velocity- and angle-resolved product detection by a rotatable mass spectrometer detector. Nonreactive (N + O2) and reactive (NO + O) product channels were identified. In the c.m. reference frame, the nonreactively scattered N atoms and reactively scattered NO molecules were both directed into the forward direction with respect to the initial direction of the reagent N atoms. On average, more than 90% of the available energy (⟨Eavl⟩ = 77.5 kcal mol-1) was retained in translation of the nonreactive products (N + O2), whereas a much smaller fraction of the available energy for the reactive pathway (⟨Eavl⟩ = 109.5 kcal mol-1) went into translation of the NO + O products, and the distribution of translational energies for this channel was broad, indicating extensive internal excitation in the nascent NO molecules. The experimentally derived c.m. translational energy and angular distributions of the reactive products suggested at least two dynamical pathways to the formation of NO + O. Quasiclassical trajectory (QCT) calculations were performed with a collision energy of Ecoll = 77 kcal mol-1 using two sets of potential energy surfaces, denoted as PES-I and PES-II, and these theoretical results were compared to each other and to the experimental results. PES-I is a reproducing kernel Hilbert space (RKHS) representation of multireference configurational interaction (MRCI) energies, while PES-II is a many-body permutation invariant polynomial (MB-PIP) fit of complete active space second order perturbation (CASPT2) points. The theoretical investigations were both consistent with the experimental suggestion of two dynamical pathways to produce NO + O, where reactive collisions may proceed on the doublet (12A') and quartet (14A') surfaces. When analyzed with this theoretical insight, the experimental c.m. translational energy and angular distributions were in reasonably good agreement with those predicted by the QCT calculations, although minor differences were observed which are discussed. Theoretical translational energy and angular distributions for the nonreactive N + O2 products matched the experimental translational energy and angular distributions almost quantitatively. Finally, relative yields for the nonreactive and reactive scattering channels were determined from the experiment and from both theoretical methods, and all results are in reasonable agreement.

Vector Correlations in the 225 nm Photodissociation of Co(CO)3NO

Metal nitrosyls are fascinating compounds because they undergo significant geometry changes in the excited state. The volatile compound, Co(CO)3NO, is a model for understanding the excited-state behavior. In this experiment, Co(CO)3NO was photodissociated in a DC-sliced velocity mapping ion imaging apparatus with 1 + 1' resonance-enhanced multiphoton ionization (REMPI) detection of the nascent NO. Ion images were collected for different linear polarization of the dissociation and probe lasers to determine the vector correlations in the photodissociation. The fastest NO products arise from an excitation parallel to the dissociating Co-NO bond. The Co-NO bond bends in the excited state, producing an NO photoproduct with angular momentum that is also aligned in the laboratory frame. The μ-v-J vector correlations were measured and are consistent with the orientation μ||v⊥J caused by an excited-state Co-NO bend prior to dissociation. Slower NO photoproducts emerge with smaller vector correlations stemming from fragmentation, parent or fragment rotation, or intersystem crossing.

Quantum-state-resolved studies of aqueous evaporation dynamics: NO ejection from a liquid water microjet.

This work presents the first fully quantum-state-resolved measurements of a solute molecule evaporating from the gas-liquid interface in vacuum. Specifically, laser-induced fluorescence detection of NO(2Π1/2, 3/2, v = 0, J) evaporating from an ∼5 mM NO-water solution provides a detailed characterization of the rotational and spin-orbit distributions emerging from a ⌀4-5 μm liquid microjet into vacuum. The internal-quantum-state populations are found to be well described by Boltzmann distributions, but corresponding to temperatures substantially colder (up to 50 K for rotational and 30 K for spin-orbit) than the water surface. The results therefore raise the intriguing possibility of non-equilibrium dynamics in the evaporation of dissolved gases at the vacuum-liquid-water interface. In order to best interpret these data, we use a model for evaporative cooling of the liquid microjet and develop a model for collisional cooling of the nascent NO evaporant in the expanding water vapor. In particular, the collisional-cooling model illustrates that, despite the 1/r drop-off in density near the microjet greatly reducing the probability of collisions in the expanding water vapor, even small inelastic cross sections (≲ 20 Å2) could account for the experimentally observed temperature differences. The current results do not rule out the possibility of non-equilibrium evaporation dynamics, but certainly suggest that correct interpretation of liquid-microjet studies, even under conditions previously considered as "collision-free," may require more careful consideration of residual collisional dynamics.

Vibrationally excited NO tagging by NO(A²∑⁺) fluorescence and quenching for simultaneous velocimetry and thermometry in gaseous flows.

We present measurements demonstrating simultaneous determination of velocity and temperature using a variant of the Vibrationally Excited Nitric Oxide Monitoring (VENOM) technique that does not employ NO2. The variant is based on tagging by electronic excitation of NO in the A²∑(1/2)⁺ (v'=0)←X2Π1/2(v''=0) band and subsequent formation of vibrationally excited NO(X2Π) by spontaneous emission and collisional quenching. Sequential planar laser-induced fluorescence imaging of the nascent NO(X2Π, v''=1) was used to obtain spatially resolved average streamwise velocity and rotational/translational temperature. The temperature determination using this approach extends the applicability of the VENOM technique to low-density, high-speed flows, where slow thermalization of the tagged molecules represents a limiting factor.

Photodissociation of ClNO in the 2 1A′ State: Computational and Experimental NO Product State Distributions

Ultrafast photodissociation of the 2 1A′ state of ClNO, which has an absorption spectrum peaking at 335 nm, is studied by computational and experimental methods. New potential-energy surfaces are calculated for the 1 and 2 1A′ states at the multireference configuration interaction (MRCI) level. Wavepacket dynamics simulations performed both exactly and by using the multiconfiguration time-dependent Hartree method yield essentially identical results. Transition dipole moments at a range of geometries are included in these calculations to correctly model the excitation. Vibrational and rotational state distributions of the NO product are obtained both computationally by analysing the quantum flux on the 2 1A′ surface and experimentally by use of 3D resonant multiphoton ionisation (REMPI), a variant of the velocity map imaging technique. The nascent NO is found to be only marginally vibrationally excited, with 91 % formed in v=0. The calculated NO rotational distribution peaks in the j=45–55 region, which compares favourably to experiment.

Determination of the v-j vector correlation in the photodissociation of nitrosobenzene at 305 nm

The 305 nm photofragmentation dynamics of nitrosobenzene was measured using velocity-mapped ion imaging with polarized 1+1' resonance-enhanced multiphoton ionization probing of the NO (X, v"=0) fragment. The product recoil energy and angular distributions of selected rotational states of NO (X (2)Pi(1/2,3/2)) products have been measured. The recoil anisotropy of the NO photofragment is very small with beta(0)(2)(20)=0.03. The v-j vector correlation of NO shows a preference for v//j with an average value of beta(0)(0)(22)=+0.15 over the transitions studied. A mechanism involving dissociation from the ground state is used to explain the unusual propellerlike trajectory of the nascent NO.

The photodissociation dynamics of N-nitrosopyrrolidine from the first and second excited singlet states studied by velocity map imaging

The photodissociation reaction of N-nitrosopyrrolidine isolated and cooled in a supersonic jet has been studied following excitation to the S(1) and S(2) electronic states. The nascent NO (X[combining tilde] (2)Pi((1/2),3/2), v, j) radicals were ionized by state-selective (1 + 1)-REMPI via the A(2)Sigma(+) state. The angularly resolved velocity distribution of these ions was measured with the velocity-map imaging (VMI) technique. Photodissociation from S(1) produces NO in the vibrational ground state and the pyrrolidine radical in the electronic ground state 1 (2)B. About 73% of the excess energy is converted into kinetic energy of the fragments. The velocity distribution shows a strong negative anisotropy (beta = -0.9) in accordance with the npi*-character of the S(0)--> S(1) transition. An upper limit for the N-NO dissociation energy of (14 640 +/- 340) cm(-1) is determined. We conclude that photodissociation from S(1) occurs very fast on a completely repulsive potential energy surface. Excitation into the S(2)pipi*-state leads to a bimodal velocity distribution. Two dissociation channels can be distinguished which show both positive anisotropy (beta = 1.3 and 1.6) but differ considerably in the total kinetic energy and the rotational energy of the NO fragment. We assign one channel to the direct dissociation on the S(2) potential energy surface, leading to pyrrolidine radicals in the excited electronic state 1 (2)A. The second channel leads to pyrrolidine in the electronic ground state 1 (2)B, presumably after crossing to the S(1) state via a conical intersection.

Photolysis of n-butyl nitrite and isoamyl nitrite at 355 nm: A time-resolved Fourier transform infrared emission spectroscopy and ab initio study

We report on the photodissociation dynamics study of n-butyl nitrite and isoamyl nitrite by means of time-resolved Fourier transform infrared (TR-FTIR) emission spectroscopy. The obtained TR-FTIR emission spectra of the nascent NO fragments produced in the 355 nm laser photolysis of the two alkyl nitrite species showed an almost identical rotational temperature and vibrational distributions of NO. In addition, a close resemblance between the two species was also found in the measured temporal profiles of the IR emission of NO and the recorded UV absorption spectra. The experimental results are consistent with our ab initio calculations using the time-dependent density functional theory at the B3LYP/6-311G(d,p) level, which indicate that the substitution of one of the two gamma-H atoms in n-C(4)H(9)ONO with a methyl group to form (CH(3))(2)C(3)H(5)ONO has only a minor effect on the photodissociation dynamics of the two molecules.

Photodissociation dynamics of n-butyl nitrite at 266 nm: Internal state distributions of nascent NO fragments

Measurements of the nascent NO fragments from the photodissociation of gaseous n-butyl nitrite at 266 nm were performed using the one-photon excited laser induced fluorescence technique. The rotational state distributions of the photofragment NO are found to be well characterized by a Gaussian distribution, which indicates that the photolysis process in the S 2 state is a direct and fast dissociation. The measured relative population of NO v″ = 0:1:2:3 is 0.65:0.19:0.11:0.05, respectively. Preferential populations of the NO Π + Λ-doublet state and F 1( 2Π 1/2) spin-orbit state are observed. The TD-DFT calculations of n-butyl nitrite are in good agreement with the experimental results.

Photodissociation of nitrobenzene at 266 nm: Experimental and theoretical approach

The 266 nm photodissociation of gaseous nitrobenzene has been studied by monitoring the NO X 2Π product using the single-photon laser-induced fluorescence technique. The rotational population and internal energy distributions of fragment NO product were determined. In the experiment, we only observed the rotational state distributions of the NO fragment for v″=0. The rotational state distributions of the nascent NO photofragment can be characterized by a Boltzmann temperature of 3300±300 K. Ab initio calculations were performed to characterize the transition state and to determine the barrier height for the rearrangement of nitrobenzene to phenylnitrite. A clear photodissociation picture has been proposed theoretically.

Photodissociation of NO2 in the Region 217−237 nm: Nascent NO Energy Distribution and Mechanism

Photodissociation of NO2 in the region of 217−237 nm is investigated by probing the nascent NO generated using a one-laser photofragmentation/fragment-detection technique. By mixing O2 in the sample (20%−100%) and by setting proper detection timing, only mass-resolved excitation spectra (MRES) of photofragmented NO are obtained and examined. The nascent NO spectrum changes depending on the intensity (W/cm2) of the laser beam. For low laser intensity, the dissociation of NO2 produces rotationally and vibrationally cold NO through the NO2 22B2 (B̃) excited state. Excess energy for this photofragmentation is ∼0.4 eV at 226 nm. This excess energy is distributed almost entirely to the kinetic energy of NO(X 2A2) and O(1D) products. The spectrum of photofragmented NO becomes very crowded as the laser intensity is increased because higher rotational and vibrational levels of NO become populated through multiphoton excitation and eventual photofragmentation of NO2 from higher-energy electronic states. The rotational temperature of NO is ca. 200 K for high laser intensity and less than 30 K for low laser intensity. Near 230 nm, a rovibronic spectrum is observed that cannot be attributed to NO even though it is detected in the NO mass channel. The time-of-flight mass spectrum line width for photofragmented NO in this photolysis region increases by a factor of 2 to about 40 ns (estimated NO kinetic energy is 0.40 ± 0.04 eV) for high laser intensity. These observations lead to the conclusion that a higher electronic state of NO2 has been accessed to generate highly excited NO through multiphoton absorption processes for the high laser intensity experiments. Such intermediate states can be Rydberg, ion pair, or other high-energy states of NO2 accessed by multiphoton absorption.

A model of odd nitrogen in the aurorally dosed nighttime terrestrial thermosphere

A model of the production and loss of odd nitrogen species (mainly, N(4S), N(2D), NO, and NO+) and the enhanced vibration‐rotation band emission from NO in the infrared from the aurorally dosed terrestrial nighttime thermosphere is described. This model is assessed by analyzing the observations of the fundamental (Δv = −1, ∼5.3 μm) and overtone (Δv = −2, ∼2.7 μm) NO vibration‐rotation band limb emissions made by a Michelson interferometer and radiometer, respectively, aboard the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS‐1A) experiment on the space shuttle Discovery during an auroral event. The auroral dosing along the line of sight is inferred from the 2.7‐μm radiance profiles using a self‐consistent procedure. The dosing obtained is then used in the model to predict the spectrally resolved emission in the 5.3‐μm region. The calculated overtone radiance profiles agree with the measured ones at lower tangent altitudes, where the signal‐to‐noise ratio is large. The calculation, however, underpredicts the observed radiance, both fundamental and overtone, at higher tangent altitudes by a factor of ∼3 and gives a colder rotational temperature for the rotationally hot component of the spectrally resolved emission from nascent NO. The causes of this discrepancy and its impact on the densities of the odd N species are discussed.

Photodissociation of methyl nitrite on Ag(111): Simulation

The photodissociation dynamics of methyl nitrite, CH3ONO, on Ag(111) have been simulated using a description that models 61 cis–methyl nitrite molecules adsorbed on a three-layer block of Ag(111). Based on classical intra- and intermolecular potentials and periodic boundary conditions, molecular dynamics (MD) simulation led to two domain structures at 100 K: those with CONO planes oriented nearly parallel and nearly perpendicular to the Ag(111) surface. To simulate photodissociation dynamics of NO, many NO trajectories were determined, each carried out as follows. At some instant of the MD simulation, a CH3ONO molecule was randomly selected from within the group of 61 and its internal CH3O–NO bond was stretched to a defined dissociation transition state. The nascent NO was given momentum along the direction of the bond broken and NO translational and internal energies were chosen to match those determined experimentally in collision-free gas phase photodissociation. The motion of the whole adsorbate–substrate system was then calculated while following the trajectory of NO. Analyzing the ensemble of NO trajectories, we conclude that, while the initial orientation of the dissociating CH3ONO influences the number of subsequent collisions, the exit direction, and the final translational and internal energy of NO, it does not fully account for the properties of ejected NO. Furthermore, for those molecules lying nearly parallel to the surface, a transition state prepared by simply stretching the O–N bond is often located away from the lowest potential energy exit path due to interactions with nearest neighbor species. As a result, coordinates, e.g., internal twisting, other than the internal CH3O–NO stretching mode are intimately involved in the dissociation channel.

Suicide inactivation of xanthine oxidoreductase during reduction of inorganic nitrite to nitric oxide

Xanthine oxidoreductase (XOR) is progressively inactivated while catalysing the reduction of inorganic nitrite to NO by xanthine. Inactivation results from conversion of the enzyme into its desulpho-form. The rate of inactivation increases with nitrite concentration. Similar behaviour was shown when NADH replaced xanthine as reducing substrate. A kinetic model is proposed incorporating a ‘suicide’ inactivation involving an enzyme–substrate (product) complex, rather than inactivation by free NO. The model provides a good fit to progress curves of the reaction of xanthine or NADH with nitrite in the presence of the oxidase or dehydrogenase forms of the enzyme. Inorganic nitrate, like nitrite, was shown to be reduced at the molybdenum site of XOR. With xanthine as reducing substrate, nitrite was produced in essentially a 1:1 stoichiometric ratio with respect to urate. Unlike the case of nitrite, the enzyme was not significantly inactivated, implying that inactivation during nitrite reduction depends on the presence of nascent NO in its enzyme complex.

Suicide inactivation of xanthine oxidoreductase during reduction of inorganic nitrite to nitric oxide.

Xanthine oxidoreductase (XOR) is progressively inactivated while catalysing the reduction of inorganic nitrite to NO by xanthine. Inactivation results from conversion of the enzyme into its desulpho-form. The rate of inactivation increases with nitrite concentration. Similar behaviour was shown when NADH replaced xanthine as reducing substrate. A kinetic model is proposed incorporating a 'suicide' inactivation involving an enzyme-substrate (product) complex, rather than inactivation by free NO. The model provides a good fit to progress curves of the reaction of xanthine or NADH with nitrite in the presence of the oxidase or dehydrogenase forms of the enzyme. Inorganic nitrate, like nitrite, was shown to be reduced at the molybdenum site of XOR. With xanthine as reducing substrate, nitrite was produced in essentially a 1:1 stoichiometric ratio with respect to urate. Unlike the case of nitrite, the enzyme was not significantly inactivated, implying that inactivation during nitrite reduction depends on the presence of nascent NO in its enzyme complex.

Internal-state distribution and polarization in nascent NO [formula omitted] from the photodissociation of C 6H 5NO

The 266 nm photodissociation of gaseous C 6H 5NO has been studied by monitoring the nascent NO ( X 2Π v″=0–3 ) product using the single-photon laser-induced fluorescence technique. The rotational-state distributions of the nascent NO photofragment have been measured and can be characterized by Bolzmann temperatures. The vibrational-state and Λ-doublet populations were also determined. Energy disposal into the internal degrees of freedom of NO ( X 2Π ) photofragment is about 18% of the total available energy. Some planarity in the fragmentation process was observed.

On the initial steps in the decomposition of energetic materials from excited electronic states

Decomposition studies of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX-C3H6N6O6, see Fig. 1) isolated in the gas phase and cooled in a supersonic expansion are reported for the excited electronic state near 225 nm. The RDX is handled safely and effectively through matrix-assisted laser desorption (MALD) of a thin film of RDX/R6G laser dye (1:1) adsorbed on an aluminum oxide coating on an aluminum drum. The aluminum oxide coating is generated by plasma electrolytic oxidation of aluminum. The combination of MALD and supersonic molecular beam techniques generates intact and cold RDX molecules isolated in the gas phase. Two basic conclusions are reached in this study. (1) Photodissociation of RDX at ≈225 nm generates NO as an initial product. (2) Nascent NO thus generated is vibrationally hot (Tvib∼1800 K) and rotationally cold (Trot∼20 K).

Photodissociation of methyl nitrite on Ag(111): Nitric oxide ejection dynamics

The thermal and photochemical properties of CH3ONO adsorbed on Ag(111), with and without a thick spacer layer of hexane, C6H14, are described. Angle-resolved time-of-flight measurements of NO ejected during photolysis at 248 and 351 nm exhibit trimodal velocity distributions. Two of the components are wavelength dependent and nonthermal, and are modeled as NO ejection with and without partially thermalizing collisions with surrounding species. The third component of the distribution is wavelength independent and characterized by the bulk temperature of the adsorbate–substrate system. These three components are present for all coverages of CH3ONO, submonolayer to multilayer, and persist even when there is a spacer of C6H14. The photochemistry is dominated by direct excitation of CH3ONO; there is no evidence for NO ejection as the result of substrate excitation. Comparing NO time-of-flight distributions for 1 monolayer (ML) of CH3ONO on clean Ag(111) to those for 1 ML on a thick layer of C6H14, evidences substrate involvement in the dynamics of nascent NO.

Nitric oxide chemiluminescence enhancement by substituted benzenes

Enhancement of NO( A 2 Σ + → X 2 Π ) chemiluminescence is reported for the 532-nm irradiation of an HC/NO 2 mixture, where HC is toluene, ethylbenzene, or xylene. Evidence is presented for the HC triplet state playing a key role in enhancing the fluorescence. The rate constants for production of NO( A 2 Σ + ) were determined as well as the quenching rate constants for each hydrocarbon. Using the latter, the NO( A 2 Σ + ) depletion rate constants were calculated for each reaction mixture. From this data, the nascent NO( A 2 Σ + ) yields were determined and compared to the observed chemiluminescence yields. Little difference was observed between the chemiluminescence yields for each of the hydrocarbons. However, comparison of the nascent NO( A 2 Σ + ) yields showed an increase for the xylenes that was twice that of benzene, toluene, or ethylbenzene. The lower formation rate constants and higher chemiluminescence/[NO(A)] 0 yields for the xylenes are explained by differences in the details of NO(A) production. The triplet-state argument is bolstered by the observation of NO( A 2 Σ + ) chemiluminescence following the focused 532-nm irradiated mixture of NO 2 and acrylaldehyde.