Ultraviolet photodissociation dynamics of the SH radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled SH radical (in X 2pi(3/2), nu"=0-2) is studied in the photolysis wavelength region of 216-232 nm using high-n Rydberg atom time-of-flight technique. In this wavelength region, anisotropy beta parameter of the H-atom product is approximately -1, and spin-orbit branching fractions of the S(3P(J)) product are close to S(3P2):S(3P1):S(3P0)=0.51:0.36:0.13. The UV photolysis of SH is via a direct dissociation and is initiated on the repulsive 2sigma- potential-energy curve in the Franck-Condon region after the perpendicular transition 2sigma(-)-X 2pi. The S(3P(J)) product fine-structure state distribution approaches that in the sudden limit dissociation on the single repulsive 2sigma- state, but it is also affected by the nonadiabatic couplings among the repulsive 4sigma-, 2sigma-, and 4pi states, which redistribute the photodissociation flux from the initially excited 2sigma- state to the 4sigma- and 4pi states. The bond dissociation energy D0(S-H)=29,245+/-25 cm(-1) is obtained.     

Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

The SH + CH(3) product channel for the photodissociation of CH(3)SH at 204 nm was investigated using the sliced velocity map ion imaging technique with the detection of CH(3) products using state selective (2+1) resonance enhanced multiphoton ionization (REMPI). Images were measured for CH(3) formed in the ground and excited vibrational states (v(2) = 0, 1, and 2) of the umbrella mode from which the correlated SH vibrational state distributions were determined. The vibrational distribution of the SH fragment in the SH + CH(3) channel at 204 nm is clearly inverted and peaks at v = 1. The highly negative anisotropy parameter of the CH(3) (v(2) = 0, 1, and 2) products is indicative of a fast dissociation process for C-S bond cleavage. Two kinds of slower CH(3) products were also observed (one of which was partly vibrationally resolved) that are assigned to a two-step photodissociation processes, in which the first step is the production of the CH(3)S (X(2)E) radical via cleavage of the S-H bond in CH(3)SH, followed by probe laser photodissociation of nascent CH(3)S radicals yielding CH(3)(X(2)A(1), v(2) = 0-2) + S((3)P(j)/(1)D) products.     

Kinetics and mechanism for the reactions of H atoms with CH3SH and C2H5SH

A kinetic study of the reactions of H atoms with CH₃SH and C₂H₅SH has been carried out at 298 K by the discharge flow technique with EPR and mass spectrometric analysis of the species. The pressure was 1 torr. It was found: k₁ = (2.20 ± 0.20) × 10^?12 for the reaction H + CH₃SH (1) and k₂ = (2.40 ± 0.16) × 10^?12 for the reaction H + C₂H₅SH (2). Units are cm³ molecule^?1 s^?1. A mass spectrometric analysis of the reaction products and a computer simulation of the reacting systems have shown that reaction (1) proceeds through two mechanisms leading to the formation of CH₃S + H₂ (1a) and CH₃ + H₂S (1b).     

Infrared laser spectroscopy of SH+

Many components of the fundamental band of ³²SH⁺ have been detected and measured in absorption using infrared diode laser spectroscopy. The transitions are characterized by the triplet fine-structure splitting expected for the X ³∑⁻ state of the cation and their positions are in good agreement with predictions obtained from an analysis of the A ³Π-X ³∑⁻ system. Some lines with opposite phase to the cation signals are tentatively assigned to SH⁻.     

Kinetics of the reaction of SH and SD with NO2

The rate constants for the reactions of NO2 with SH and SD were measured between 250 and 360 K to be 2.8 x 10(-11) exp{(270+/-40)/T(K)} and 2.6x10(-11) exp{(285+/-20)/T(K)} cm3 molecule-1 s-1, respectively. SH(SD) radicals were generated by pulsed laser photolysis of H2S(D2S) or CH3SH and detected via pulsed laser-induced fluorescence. The laser-induced fluorescence excitation spectrum of SH was found to be contaminated by the presence of the SO radical. This contamination is suggested as a possible reason for differences among some of the reported values of k1 in the literature. The title reaction influences the atmospheric lifetime of the SH radical when NO2 is greater than 100 pptv, but the revised value of k1 does not significantly alter our current understanding of SH oxidation in the atmosphere.     

Engineering temperature-sensitive SH3 domains.

BACKGROUND: The ability to control specific protein-protein interactions conditionally in vivo would be extremely helpful for analyzing protein-protein interaction networks. SH3 (Src homology 3) modular protein binding domains are found in many signaling proteins and they play a crucial role in signal transduction by binding to proline-rich sequences. RESULTS: Random in vitro mutagenesis coupled with yeast two-hybrid screening was used to identify mutations in the second SH3 domain of Nck that render interaction with its ligand temperature sensitive. Four of the mutants were functionally temperature sensitive in mammalian cells, where temperature sensitivity was correlated with a pronounced instability of the mutant domains at the nonpermissive temperature. Two of the mutations affect conserved residues in the hydrophobic core (Val133 and Val160), suggesting a general strategy for engineering temperature-sensitive SH3-containing proteins. Indeed mutagenesis of the corresponding positions in another SH3 domain, that of Crk-1, rendered the full-length Crk-1 protein temperature sensitive for function and stability in mammalian cells. CONCLUSIONS: Construction of temperature-sensitive SH3 domains is a novel approach to regulating the function of SH3 domains in vivo. Such mutants will be valuable in dissecting SH3-mediated signaling pathways. Furthermore, the methodology described here to isolate temperature-sensitive domains should be widely applicable to any domain involved in protein-protein interactions.     

Heat of formation for the SH radical by photoionization mass spectrometry

Photoionization mass spectrometry has been used to measure the appearance energies for [C₂H₅]⁺ from ethanethiol, [C₃H₇]⁺ from 2-propanethiol and [C₃H₅]⁺ from 2-methylthiirane. From the known thermochemistry of these cations and their precursor molecules, a 298 K heat of formation of 138.6±0.4 kJ mol^?1 for the SH radical has been derived.     

Searching for specificity in SH domains

Elucidating protein-protein interactions has been a central feature to understanding intracellular signal transduction. Many of the binding sites of the interacting proteins in these pathways are within highly sequentially homologous and structurally conserved domains. We challenge the dogma that mutual exclusivity in signalling is derived from a high level of specificity in these domains.     

Protein microarray assay for the screening of SH3 domain interactions

Analysis of cellular signal transduction processes increasingly focuses on the systematic characterization of complete protein interaction networks. Understanding the interplay of signaling components enables insight into the molecular basis of diverse diseases such as cancer. This paves the way for the rational design of specific therapeutics. Protein interactions are often mediated by conserved modular domains, e.g., SH3-domains, which recognize proline-rich sequences in their cognate ligands. In the course of this study, different microarray formats (reactive silane monolayers and nitrocellulose on glass slides) and assay work flows were evaluated to develop a microarray based screening assay that permits the reliable identification of interactions between certain target proteins with a set of SH3 domains. Nine representative SH3 domains which were produced and purified as GST-fusion proteins were spotted on the microarray substrates and probed with two well-characterized ligands, the Nef protein from HIV-1 and the human protein Sam68. The best results from these low-density model arrays were obtained with nitrocellulose slides. We show that a straightforward and highly robust detection of ligand binding is achieved by staining with a fluorescently labeled antibody directed against the N-terminal His-tag attached to these proteins. The optimized assay protocol reported here allows for the identification of SH3-interactions with high reproducibility and adequate signal-to-background and signal-to-noise ratios, as well as the quantitative determination of relative binding affinities.     

CH_3SH+CN微观反应机理的理论研究

采用BMC-CCSD//B3LYP/6-311G(d,p)方法对CH3SH+CN反应机理进行了详细的理论研究.反应中涉及的各稳定点的构型、振动频率和零点能在B3LYP/6-311G(d,p)水平下计算得到,计算结果表明,该反应存在两种反应机理,5条可能的反应通道.SN2机理由于能垒太高,与直接氢抽提机理相比可以忽略.该反应的最可行通道为CN中的C原子进攻SH中的H原子经由一个前期和一个后期分子络合物生成产物CH3S和HCN.计算得到的反应焓变与已有实验值非常吻合.     

Sulfur atom exchange in the reaction of SH radicals with S atoms

The structural and energetic properties of the HSS-->SSH transition state are examined using the single and double coupled-cluster method. The energy change for the isomerization reaction is estimated to be 31.7+/-1 kcal mol(-1). The results suggest that the reaction between SH radicals and S atoms should isotopically exchange because the isomerization barrier is significantly less than the S-S bond dissociation energy in the HSS radical.     

Fluorescence lifetimes and predissociation of the A3Π state of SH+

The high frequency deflection (HFD) technique has been used to obtain fluorescence lifetimes of the SH⁺ A³Π state. The A³Π?X³∑^? electronic transition moment function has been calculated with the complete active space SCF(CASSCF) method. Radiative lifetimes calculated with this transition moment function are compared with measured lifetimes and found to be in good agreement (τ_exp(v′=0)=1.4±0.3 µs; τ_calc=1.6 µs). Deviations between measured and calculated lifetimes forv′=0,J′ ≥ 15 andv′=1,J′=1?7 are interpretated as due to a predissociation caused by a repulsive⁵∑^? state. A simple model, based on a perturbation approach to the predissociation, is used to obtain quantitative estimates of predissociation rates. These calculated rates are in reasonable agreement with the predissociation rates that are inferred from the measured and calculated lifetimes.     

Theoretical study on the atmospheric reaction of CH3SH with O2

A detailed study on the reaction mechanism of CH₃SH with O₂ was carried out using quantum chemical methods. Eleven singlet pathways and four triplet pathways were found based on CCSD(T)//M06-2x calculations. The nature of chemical bonding evolution was also studied using electron localization function and atoms in molecules analysis. Moreover, reaction rate constants were calculated between 200 and 800 K at the level of the transition state theory by Wigner tunneling correction. The results suggest that the main products should be CH₂SO, H₂O, CH₃OH, SO, CH₄, and SO₂, respectively, basically coinciding with the experimental results. The corresponding feasible pathways are channels R7, R8, and R9, respectively, with an effective energy barrier of 56.21 kJ/mol. Obviously, given the low energy barrier similar to the main paths mentioned above, the products CH₂SH and HO₂ should assume a definite proportion in all possible products, although such species were not yet detected in experiment.     

An Antifungal Chitosanase from Bacillus subtilis SH21

Bacillus subtilis SH21 was observed to produce an antifungal protein that inhibited the growth of F. solani. To purify this protein, ammonium sulfate precipitation, gel filtration chromatography, and ion-exchange chromatography were used. The purity of the purified product was 91.33% according to high-performance liquid chromatography results. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis and liquid chromatography–tandem mass spectrometry (LC–MS/MS) analysis revealed that the molecular weight of the protein is 30.72 kDa. The results of the LC–MS/MS analysis and a subsequent sequence-database search indicated that this protein was a chitosanase, and thus, we named it chitosanase SH21. Scanning and transmission electron microscopy revealed that chitosanase SH21 appeared to inhibit the growth of F. solani by causing hyphal ablation, distortion, or abnormalities, and cell-wall depression. The minimum inhibitory concentration of chitosanase SH21 against F. solani was 68 µg/mL. Subsequently, the corresponding gene was cloned and sequenced, and sequence analysis indicated an open reading frame of 831 bp. The predicted secondary structure indicated that chitosanase SH21 has a typical a-helix from the glycoside hydrolase (GH) 46 family. The tertiary structure shared 40% similarity with that of Streptomyces sp. N174. This study provides a theoretical basis for a topical cream against fungal infections in agriculture and a selection marker on fungi.     

SH2 binding site comparison: a new application of the SURFCOMP method

To avoid side effects, it is often desirable to increase the specificity of a drug candidate when targeting one member of a family of related proteins, whereby one exploits small differences between the structures of the binding sites. Identification of such differences can be carried out by analyzing the distributions of physicochemical properties mapped onto molecular surfaces. Here we demonstrate that SURFCOMP, our local surface similarity detection method, is able to detect differences between the binding sites of two closely related proteins. We analyzed the SH2 domains of Sap and Eat-2, two highly similar signal transduction molecules involved in inflammatory processes and found differences between their binding sites that can possibly lead to a better understanding of the different specificities of the two proteins.     

Imaging the photodissociation of CH3SH in the first and second absorption bands: the CH3(X2A1)+SH(X2Pi) channel

The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).