Complex Langmuir-Blodgett films of SiO2 and ZnO nanoparticles with advantageous optical and photocatalytical properties

Multifunctional Langmuir-Blodgett (LB) films were fabricated on the surface of glass substrates using sol-gel derived ZnO and SiO2 particles. ZnO particles of 6 and 110 nm diameter were synthesized according to the methods of Meulenkamp and Seelig et al. (Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566; Seelig, E. W.; Tang, B.; Yamilov, A.; Cao, H.; Chang, R. P. H. Mater. Chem. Phys. 2003, 80, 257). Silica particles of 37 and 96 nm were prepared by the Stober method (Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62). Alternate deposition of monoparticulate Langmuir films of SiO2 and ZnO nanoparticles provided complex (six- and nine-layered) LB films with both antireflective and photocatalytic properties. The LB films were investigated with scanning electron microscopy (morphology and structure) and UV-vis spectroscopy (optical properties and stability). The photocatalytic activity was measured by immersing the UV-irradiated films into an aqueous solution of Methyl Orange and following the photodegradation of the dye by optical spectroscopy. Adding ZnO particles to the silica films slightly lowered the antireflection property but ensured strong photocatalytic activity. Both the photocatalytic activity and antireflection properties were proved to be sensitive to the sequence of the silica and ZnO layers, with optimum properties in the case of nine-layered films with a repeated (SiO2-ZnO-ZnO) structure.     

What is so special about aerosol-OT? Part IV. Phenyl-tipped surfactants

Properties are reported for new phenyl-tipped anionic surfactants, which are aromatic chain relatives of the normal aliphatic aerosol-OT (AOT, sodium bis(2-ethyl-1-hexyl)sulfosuccinate). Variations in chain length and branching with these aromatic surfactants have important effects on aqueous and water-in-oil (w/o) microemulsion phase properties. In dilute aqueous systems, chain structure affects the cmc and surface tension behavior: compared to linear chain analogues, the branched-chain surfactants display lower surface tensions but also reduced packing as measured by molecular area at the cmc a(cmc). Owing to the phenyl-tipped structure, water-in-oil microemulsions were stabilized with aromatic toluene as an oil but not with aliphatic heptane; the latter is commonly used with normal AOT. Contrast variation small-angle neutron scattering (SANS) was used to characterize the microemulsion aggregates and adsorbed films. These SANS data show that water-in-toluene microemulsions stabilized by aromatic-AOTs contain mildly polydisperse spherical nanodroplets of similar structure to those found in systems containing normal AOT. Molecular areas at the air-water and toluene-water interfaces are found to be of similar magnitude and follow a trend that correlates with variations in surfactant chain structure. The new results with aromatic surfactants build on extensive studies of aliphatic AOT analogues (Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2002, 16, 8741. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. 2002, 18, 1505), suggesting that the versatility of normal AOT originates from an optimized head and chain spacer group rather than from any specific effects of the 2-ethyhexyl chain structure.     

Influence of surface area on charge transport and recombination in dye-sensitized TiO2 solar cells

The dependence of the electron transport and recombination dynamics on the internal surface area of mesoporous nanocrystalline TiO2 films in dye-sensitized solar cells was investigated. The internal surface area was varied by altering the average particle size in the films. The scaling of the photoelectron density and the electron diffusion coefficient at short circuit with internal surface area confirms the results of a recent study (Kopidakis, N.; Neale, N. R.; Zhu, K.; van de Lagemaat, J.; Frank, A. J. Appl. Phys. Lett. 2005, 87, 202106) that transport-limiting traps are located predominately on the surfaces of the particles. The recombination current density was found to increase superlinearly (with an exponent of 1.40 +/- 0.12) with the internal surface area. This result is at odds with the expected linear dependence of the recombination current density on the surface area when only the film thickness is increased. The observed scaling of the recombination current density with surface area is consistent with recombination being transport-limited. Evidence is also presented confirming that photoinjected electrons recombine with redox species in the electrolyte via surface states rather than from the TiO2 conduction band.     

Self-assembled organic monolayers terminated in perfluoroalkyl pentafluoro-lambda(6)-sulfanyl (-SF5) chemistry on gold

Recently synthesized (Winter, R.; Nixon, P. G.; Gard, G. L.; Radford, D. H.; Holcomb, N. R.; Grainger, D. W. J. Fluorine Chem. 2001, 107, 23-30) SF5-terminated perfluoroalkyl thiols (SF5(CF2)nCH2CH2SH, where n = 2, 4, and 6) and a symmetric SF5-terminated dialkyl disulfide ([SF5-CH=CH-(CH2)8-S-]2) were assembled as thin films chemisorbed onto gold surfaces. The adsorbed monolayer films of these SF5-containing molecules on polycrystalline gold were compared using ellipsometry, contact angle, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and infrared spectroscopy (FTIR) surface analytical methods. The resulting SF5-dialkyl disulfide monolayer film shows moderate angle dependence in depth-dependent XPS analysis, suggesting a preferentially oriented film. The SF5-terminated perfluoroalkyl thiols exhibit angular-dependent XPS compositional variance depending on perfluoroalkyl chain length, consistent with improved film assembly (increasingly hydrophobic, fewer defects, and more vertical chain orientation increasing film thickness) with increasing chain length. Tof-SIMS measurements indicate that both full parent ions for these film-forming molecules and the unique SF5 terminal group are readily detectable from the thin films without substantial contamination from other adsorbates.     

Optimizing the performance of the multiconfiguration molecular mechanics method

Multiconfiguration molecular mechanics (MCMM) is a general algorithm for constructing potential energy surfaces for reactive systems (Kim, Y.; Corchado, J. C.; Villà, J.; Xing, J.; Truhlar, D. G. J. Chem. Phys. 2000, 112, 2718). This paper illustrates how the performance of the MCMM method can be improved by adopting improved molecular mechanics parameters. We carry out calculations of reaction rate constants using variational transition state theory with optimized multidimensional tunneling on the MCMM PESs for three hydrogen transfer reactions, and we compare the results to direct dynamics. We find that the MCMM method with as little as one electronic structure Hessian can describe the dynamically important regions of the ground-electronic state PES, including the corner-cutting-tunneling region of the reaction swath, with practical accuracy.     

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles obtained by spraying: integrity of the vesicles

In a previous paper (Michel, M.; Vautier, D.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835), we showed that phospholipid vesicles can be incorporated into poly(glutamic-acid)/poly(allylamine) (PGA/PAH) multilayered polyelectrolyte films built by the alternated dipping of a surface in polyanion and polycation solutions. AFM imaging, quartz crystal microbalance, and ellipsometry suggested that the vesicles remain intact when adhering on the surface. In the present paper, we show that such films can also be realized by spraying both the polyelectrolyte solutions and the vesicles onto the surface. Using such vesicles filled with ferrocyanide ions, we prove by cyclic voltammetry that the sprayed vesicles remain intact when embedded in the multilayers. We show that multilayers containing two distinct layers of intact vesicles separated by several polyanion/polycation bilayers can also be constructed. Polyelectrolyte multilayers containing layers of phospholipid vesicles could act as reservoirs for drug or other biologically active molecules in controlled release bioactive coatings.     

Do spin state changes matter in organometallic chemistry? A computational study

Spin changes occur often in organometallic chemistry, and their effect on kinetics is not well understood. We report computations on the singlet and triplet potential energy surfaces of several processes of this type and show that the topology of the individual surfaces, as well as of the crossing regions between them, can be used to rationalize the observed reactivity in all cases. In particular, the slow addition of dihydrogen to W[N(CH(2)CH(2)NSiMe(3))(3)]H (Schrock, R. R.; Shih, K. Y.; Dobbs, D. A.; Davis, W. M. J. Am. Chem. Soc. 1995, 117, 6609) is shown to be a "spin-blocked" reaction with a high barrier due to the crossing between reactant triplet and product singlet surfaces. In contrast, addition of CO to TpCo(CO) (Detrich, J. L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1995, 117, 11745) is fast because the triplet and singlet surfaces cross at low energy. Particular care is taken to use DFT methods which are in adequate agreement with experimental and high-level computational energetics for these systems.     

Molecular transfer of surfactant bilayers: widening the range of substrates

Handling nanometer-thick films and nano-objects remains a challenge. Applying self-assembly properties of surfactants to nanomaterials manipulation may be the key to the fast, easy, cost-effective growth of 2D and 3D nanostructures. Newton black films (NBFs) are self-assembled bilayers of surfactant, well-organized, but fragile objects. To render such films amenable to practical applications, it is necessary to find ways to transfer them onto solid substrates. A method developed recently to transfer NBFs onto a solid substrate while preserving their molecular organization (Benattar, J.-J.; Nedyalkov, M.; Lee, F. K.; Tsui, O. K. C. Angew. Chem., Int. Ed. 2006, 45, 4186) is broadened here to different surfaces. The method requires hydrophobic, planar, atomically smooth surfaces. This study presents the adhesion of a fluorinated NBF surfactant onto hydrophobically treated silica and silicon surfaces (with etching or silanization). The structures of the free-standing film, bare substrates, and transferred films are investigated using X-ray reflectivity. The homogeneity of the surfaces before and after bilayer deposition is examined by atomic force microscopy (AFM). Multiple transfers are tested and described for the future development of more complex architectures involving many surfactant layers and inserted nanosized objects.     

Control of wettability of molecularly thin liquid films by nanostructures

The patterning of liquid thin films on solid surfaces is very important in various fields of science and engineering related to surfaces and interfaces. A method of nanometer-scale patterning of a molecularly thin liquid film on a silicon substrate using the lyophobicity of the oxide nanostructures has recently been reported (Fukuzawa, K.; Deguchi, T.; Kawamura, J.; Mitsuya, Y.; Muramatsu, T.; Zhang, H. Appl. Phys. Lett. 2005, 87, 203108). However, the origin of the lyophobicity of the nanostructure with a height of around 1 nm, which was fabricated by probe oxidation, has not yet been clarified. In the present study, the change in thickness of the liquid film on mesa-shaped nanostructures and the wettability for the various combinations of the thickness of the liquid films and the height of ridge-shaped nanostructures were investigated. These revealed that lyophobicity is caused by a lowering of the intermolecular interaction between the liquid and silicon surfaces by the nanostructure and enables the patterning of a liquid film along it. The tendency of the wettability for a given liquid film and nanostructure size can be predicted by estimating the contributions of the intermolecular interaction and capillary pressure. In this method, the height of the nanostructure can control the wettability. These results can provide a novel method of nanoscale patterning of liquid thin films, which will be very useful in creating new functional surfaces.     

From exponential to linear growth in polyelectrolyte multilayers

There exist two types of polyelectrolyte multilayers: those whose thickness increases linearly with the number of deposition steps, which are nicely structured, and those whose thickness increases exponentially, which resembles hydrogels. This simple picture has recently slightly evolved with the finding that some exponentially growing films enter into a linear growth phase after a certain number of deposition steps. In this study, we investigate the buildup process of hyaluronic acid/poly(L-lysine) (HA/PLL) multilayers that constitute one of the best known exponentially growing systems. The films are built by using two deposition methods: the well-known dipping method and the more recent spraying method where the polyelectrolyte solutions are sprayed alternately onto a vertical substrate. The goal of this study is twofold. First, we investigate the influence of the main parameters (i.e., spraying rate and spraying time) of the spraying method on the film growth process. We find that, as for the dipping method, the film thickness first evolves exponentially with the number of deposition steps, and after a given number of deposition steps, it follows a linear evolution. We find that similar behavior is observed with the dipping method. Second, because the spraying method allows the very fine variation of the different parameters of the buildup, we use this method to investigate the exponential-to-linear transition. We find that this transition always takes place after about 12 deposition steps whatever the values of the parameters controlling the deposition process. We discuss our results in light of a model proposed by Hübsch et al. (Hübsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980-1985) and later by Salom?ki et al. (Salom?ki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232-11240) in which it is assumed that the exponential-to-linear transition is due to a film restructuring that progressively forbids the diffusion of one of the polyelectrolytes constituting the film over part of the film. This "forbidden" zone then grows with the number of deposition steps so that the outer zone of the film that is still concerned with diffusion keeps a constant thickness and moves upward as the total film thickness increases.     

Reaction of ethylene with hydroxyl radicals: a theoretical study

Ab initio calculations of portions of the C2H5O potential energy surface critical to the title reaction are presented. These calculations are based on QCISD geometries and frequencies and RQCISD(T) energies extrapolated to the complete-basis-set limit. Rate coefficients for the reaction of C2H4 with OH are calculated using this surface and the two transition-state model of Greenwald and co-workers [J. Phys. Chem. A 2005, 109, 6031] for the association of OH with C2H4. The present calculations reproduce most of the experimental data, including the temperature and pressure dependence of the rate coefficients, with only a small (0.4 kcal/mol) adjustment to the energy barrier for direct hydrogen abstraction. We confirm the importance of this channel above 800 K and find that a significant fraction of the total rate coefficient (approximately 10%) is due to the formation of vinyl alcohol above this temperature. Calculations of the vinyl alcohol channel are consistent with the recent observation of this molecule in low-pressure flames [Taatjes, C. A.; Hansen, N.; McIlroy, A.; Miller, J. A.; Senosiain, J. P.; Klippenstein, S. J.; Qi, F.; Sheng, L.; Zhang, Y.; Cool, T. A.; Wang, J.; Westmoreland, P. R.; Law, M. E.; Kasper, T.; Kohse-H?inghaus, K. Science 2005, 308, 1887] and suggest that this reaction should be included in hydrocarbon oxidation mechanisms.     

Determinants of cyanuric acid and melamine assembly in water

While the recognition of cyanuric acid (CA) by melamine (M) and their derivatives has been known to occur in both water and organic solvents for some time, analysis of CA/M assembly in water has not been reported (Ranganathan, A.; Pedireddi, V. R.; Rao, C. N. R. J. Am. Chem. Soc.1999, 121, 1752-1753; Mathias, J. P.; Simanek, E. E.; Seto, C. T.; Whitesides, G. M. Macromol. Symp.1994, 77, 157-166; Zerkowski, J. A.; MacDonald, J. C.; Seto, C. T.; Wierda, D. A.; Whitesides, G. M. J. Am. Chem. Soc.1994, 116, 2382-2391; Mathias, J. P.; Seto, C. T.; Whitesides, G. M. Polym. Prepr.1993, 34, 92-93; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1993, 115, 905-916; Zerkowski, J. A.; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1992, 114, 5473-5475; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc.1990, 112, 6409-6411; Wang, Y.; Wei, B.; Wang, Q. J. Chem. Cryst.1990, 20, 79-84; ten Cate, M. G. J.; Huskens, J.; Crego-Calama, M.; Reinhoudt, D. N. Chem.-Eur. J.2004, 10, 3632-3639). We have examined assembly of CA/M, as well as assembly of soluble trivalent CA and M derivatives (TCA/TM), in aqueous solvent, using a combination of solution phase NMR, isothermal titration and differential scanning calorimetry (ITC/DSC), cryo-transmission electron microscopy (cryo-TEM), and synthetic chemistry. While the parent heterocycles coprecipitate in water, the trivalent system displays more controlled and cooperative assembly that occurs at lower concentrations than the parent and yields a stable nanoparticle suspension. The assembly of both parent and trivalent systems is rigorously 1:1 and proceeds as an exothermic, proton-transfer coupled process in neutral pH water. Though CA and M are considered canonical hydrogen-bonding motifs in organic solvents, we find that their assembly in water is driven in large part by enthalpically favorable surface-area burial, similar to what is observed with nucleic acid recognition. There are currently few synthetic systems capable of robust molecular recognition in water that do not rely on native recognition motifs, possibly due to an incomplete understanding of recognition processes in water. This study establishes a detailed conceptual framework for considering CA/M heterocycle recognition in water which enables the future design of molecular recognition systems that function in water.     

Energy transfer in molecular layer-by-layer films of water-soluble perylene diimides

Multilayer films of water-soluble anionic and cationic perylene diimide (PDI) moieties have been prepared using the molecular layer-by-layer method described in an earlier publication (Tang, T. J.; Qu, J. Q.; Müllen, K.; Webber, S. E. Langmuir 2006, 22, 26-28) and the fluorescence intensity compared with and without a base layer prepared using an anionic terrylene diimide dye (n-TDI), which serves as an energy-trapping layer for the PDI exciton. The fluorescence quenching data could be fit equally well to a modification of a model used by Kuhn to describe energy transfer from a J aggregate or a model developed by Kenkre and Wong to describe excitonic transfer. For both models, we obtain a characteristic energy-transfer distance on the order of 5.4 nm. Fluorescence quenching of the PDI via a single F?rster energy-transfer step to the n-TDI layer is ruled out on the basis of the observed power-law dependence. We also consider a model in which the excitation is trapped at the outermost surface. This model provides a reasonable fit to the data only if the Kuhn relationship is used.     

Interprotein electron transfer in a confined space: uncoupling protein dynamics from electron transfer by sol-gel encapsulation

In this paper, we describe the first observations of photoinitiated interprotein electron transfer (ET) within sol-gels. We have encapsulated three protein-protein complexes, specifically selected because they represent a full range of affinities, are sensitive to different types of dynamic processes, and thus are expected to respond differently to sol-gel encapsulation. The three systems are (i) the [Zn, Fe(3+)L] mixed-metal hemoglobin hybrids, where the alpha(1)-Zn and beta(2)-Fe subunits correspond to a "predocked" protein-protein complex with a crystallographically defined interface (Natan, M. J.; Baxter, W. W.; Kuila, D.; Gingrich, D. J.; Martin, G. S.; Hoffman, B. M. Adv. Chem. Ser. 1991, 228 (Electron-Transfer Inorg., Org., Biol. Syst.), 201-213), (ii) the Zn-cytochrome c peroxidase complex with cytochrome c, [ZnCcP, Fe(3+)Cc], having an intermediate affinity between its partners (Nocek, J. M.; Zhou, J. S.; De Forest, S.; Priyadarshy, S.; Beratan, D. N.; Onuchic, J. N.; Hoffman, B. M. Chem. Rev. 1996, 96, 2459-2489), and (iii) the [Zn-deuteromyoglobin, ferricytochrome b(5)] complex, [ZnDMb, Fe(3+)b(5)], which is loosely bound and highly dynamic (Liang, Z.-X.; Nocek, J.; Huang, K.; Hayes, R. T.; Kurnikov, I. V.; Beratan, D. N.; Hoffman, B. M. J. Am. Chem. Soc. 2002, 124, 6849-6859. Intersubunit ET within the hybrid does not involve second-order processes or subunit rearrangements, and thus is influenced only by perturbations of high-frequency motions coupled to ET. For the latter two complexes, sol-gel encapsulation eliminates second-order processes: protein partners encapsulated as a complex must stay together throughout a photoinitiated ET cycle, while proteins encapsulated alone cannot acquire a partner. It further modulates intracomplex motions of the two partners.     

The reaction OH + C2H4: an example of rotational channel switching

The low-temperature data for the reaction between OH and C(2)H(4) is treated canonically as either a two-well or one-well problem using the "Multiwell" suite of codes, in which a "well" refers to a minimum in the potential energy surface. The former is analogous to the two transition state model of Greenwald et al. [Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein, S. J. J. Phys. Chem. A2005, 109, 6031], while the latter reflects the dominance of the so-called "inner transition state". External rotations are treated adiabatically, causing changes in the magnitude of effective barriers as a function of temperature. Extant data are well-described with either model using only the average energy transferred in a downward direction, upon collision, ΔE(d)(T), as a fitting parameter. The best value for the parameters describing the rate coefficient as a function of temperature (200 < T/K < 400) (Data at lower temperature is too sparse to yield a recommendation.) and pressure in the form used in the NASA/JPL format [Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 17, Jet Propulsion Laboratory, 2011] are k(0) = 1.0 × 10(-28)(T/300)(-3.5) cm(6) molecule(-2) s(-1) and k(∞) to 8.0 × 10(-12)(T/300)(-2.3) cm(3) molecule(-1) s(-1).     

Direct measurement by laser flash photolysis of intraprotein electron transfer in a rat neuronal nitric oxide synthase

Intraprotein interdomain electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in nitric oxide (NO) synthesis by NO synthase (NOS). Our previous laser flash photolysis studies have provided a direct determination of the kinetics of IET between the FMN and heme domains in truncated oxyFMN constructs of rat neuronal NOS (nNOS) and murine inducible NOS (iNOS), in which only the oxygenase and FMN domains along with the calmodulin (CaM) binding site are present [Feng, C. J.; Tollin, G.; Holliday, M. A.; Thomas, C.; Salerno, J. C.; Enemark, J. H.; Ghosh, D. K. Biochemistry 2006, 45, 6354-6362. Feng, C. J.; Thomas, C.; Holliday, M. A.; Tollin, G.; Salerno, J. C.; Ghosh, D. K.; Enemark, J. H. J. Am. Chem. Soc. 2006, 128, 3808-3811]. Here, we report the kinetics of IET between the FMN and heme domains in a rat nNOS holoenzyme in the presence and absence of added CaM using laser flash photolysis of CO dissociation in comparative studies on partially reduced NOS and a single domain NOS oxygenase construct. The IET rate constant in the presence of CaM is 36 s-1, whereas no IET was observed in the absence of CaM. The kinetics reported here are about an order of magnitude slower than the kinetics in a rat nNOS oxyFMN construct with added CaM (262 s-1). We attribute the slower IET between FMN and heme in the holoenzyme to the additional step of dissociation of the FMN domain from the reductase complex before reassociation with the oxygenase domain to form the electron-transfer competent output state complex. This work provides the first direct measurement of CaM-controlled electron transfer between catalytically significant redox couples of FMN and heme in a nNOS holoenzyme.     

Validation studies of the site-directed docking program LibDock

The performance of the site-features docking algorithm LibDock has been evaluated across eight GlaxoSmithKline targets as a follow-up to a broad validation study of docking and scoring software (Warren, G. L.; Andrews, W. C.; Capelli, A.; Clarke, B.; Lalonde, J.; Lambert, M. H.; Lindvall, M.; Nevins, N.; Semus, S. F.; Senger, S.; Tedesco, G.; Walls, I. D.; Woolven, J. M.; Peishoff, C. E.; Head, M. S. J. Med. Chem. 2006, 49, 5912-5931). Docking experiments were performed to assess both the accuracy in reproducing the binding mode of the ligand and the retrieval of active compounds in a virtual screening protocol using both the DJD (Diller, D. J.; Merz, K. M., Jr. Proteins 2001, 43, 113-124) and LigScore2 (Krammer, A. K.; Kirchoff, P. D.; Jiang, X.; Venkatachalam, C. M.; Waldman, M. J. Mol. Graphics Modell. 2005, 23, 395-407) scoring functions. This study was conducted using DJD scoring, and poses were rescored using all available scoring functions in the Accelrys LigandFit module, including LigScore2. For six out of eight targets at least 30% of the ligands were docked within a root-mean-square difference (RMSD) of 2.0 A for the crystallographic poses when the LigScore2 scoring function was used. LibDock retrieved at least 20% of active compounds in the top 10% of screened ligands for four of the eight targets in the virtual screening protocol. In both studies the LigScore2 scoring function enhanced the retrieval of crystallographic poses or active compounds in comparison with the results obtained using the DJD scoring function. The results for LibDock accuracy and ligand retrieval in virtual screening are compared to 10 other docking and scoring programs. These studies demonstrate the utility of the LigScore2 scoring function and that LibDock as a feature directed docking method performs as well as docking programs that use genetic/growing and Monte Carlo driven algorithms.     

Two-state reactivity in alkane hydroxylation by non-heme iron-oxo complexes

Density functional theory is used to explore the mechanisms of alkane hydroxylation for four synthetic non-heme iron(IV)-oxo complexes with three target substrates (Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde; J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Münck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2004, 126, 472-473; Rohde, J.-U.; Que, L., Jr. Angew. Chem. Int. Ed. 2005, 44, 2255-2258.). The iron-oxo reagents possess triplet ground states and low-lying quintet excited states. The set of experimental and theoretical reactivity trends can be understood if the reactions proceed on the two spin states, namely two-state reactivity (TSR); an appropriate new model is presented. The TSR model makes testable predictions: (a) If crossing to the quintet state occurs, the hydroxylation will be effectively concerted; however, if the process transpires only on the triplet surface, stepwise hydroxylation will occur, and side products derived from radical intermediates would be observed (e.g., loss of stereochemistry). (b) In cases of crossing en route to the quintet transition state, one expects kinetic isotope effects (KIEs) typical of tunneling. (c) In situations where the two surfaces contribute to the rate, one expects intermediate KIEs and radical scrambling patterns that reflect the two processes. (d) Solvent effects on these reactions are expected to be very large.