Position dependent heavy atom effect in physical triplet quenching by electron donors

The radical yields and rate constants in the quenching reaction of thionine triplet with the complete series of monohalogen substituted anilines as electron donors were determined by flash spectroscopy. Whereas the quenching rate constants show little and unsystematic variation, the radical yields decrease with increasing spin—orbit coupling constant of the halogen substituent. This effect is very sensitive to the position of the halogen in the donor. The results are explained in terms of a heavy atom effect on the intersystem crossing rate constant in a triplet exciplex.     

Kinetics of lamellar-to-cubic and intercubic phase transitions of pure and cytochrome c containing monoolein dispersions monitored by time-resolved small-angle X-ray diffraction

We investigated the effect of incorporation of a small aqueous peripheral membrane protein (cyt c) into the three-dimensional periodic nanochannel structures formed by the lipid monoolein (MO) on its rich phase behavior as a function of temperature, pressure, and protein concentration using synchrotron X-ray small-angle diffraction. By simultaneous use of the pressure-jump relaxation technique and time-resolved synchrotron X-ray diffraction, we also studied the kinetics of various lipid mesophase transformations of the system for understanding the mechanistic pathways of their formation influenced by the protein-lipid interactions. Cyt c incorporated into the bicontinuous cubic phase Ia3d of MO has a significant effect on the lipid structure and the pressure stability of the system already at low protein concentrations. Concentrations higher than 0.2 wt % of cyt c led to an increase in interfacial curvature due to interaction of the protein with the lipid headgroups. This promotes the formation of a new, probably partially micellar cubic phase of crystallographic space group P4(3)32. Upon pressurization, the P4(3)32 phase undergoes a phase transition to a cubic Pn3m phase with smaller partial specific volume. Increase in protein concentration increases the pressure stability of the P4(3)32 phase. The formation of this phase from the cubic phase Pn3m is a slow process taking many seconds and having a time lag in the beginning. It seems to occur as a two-state process without ordered intermediate states. At temperatures above 60 degrees C, the P4(3)32 phase is unable to accommodate the unfolded protein and transforms to a bicontinuous cubic Ia3d phase. Time-resolved small-angle X-ray scattering studies show that the L(alpha) --> Ia3d transition in pure MO dispersions under limited hydration conditions occurs within a time interval of 1 s at 35 degrees C preceded by a lag phase of 1.5 s. The Ia3d cubic phase initially forms with a much larger lattice constant due to hydration and experiences an initially lower curvature that relaxes within about 1 s. Interestingly, no other cubic phases are involved as intermediates in the transition, i.e., the gyroid cubic phase is able to form directly from the L(alpha) phase. The mechanism behind the L(alpha) --> Ia3d transition in pure MO dispersions has been discussed within the framework of recent stalk models for membrane fusion. In the presence of cyt c, the L(alpha) --> Ia3d transition is much slower. The rather long relaxation times of the order of seconds are probably due to a kinetic trapping of the system and limitation by the transport and redistribution of water and lipid in the evolving new lipid phases. We also studied the transition from the pure lamellar L(alpha) phase to the Ia3d-P4(3)32 two phase region and observed a rather complex transition behavior with transient lamellar and cubic intermediate states.     

Reduction Potentials of Tetraaminecopper(II/I) Couples

The difference in steric strain between the oxidized and the reduced forms of tetraaminecopper complexes is correlated with the corresponding reduction potentials. The experimentally determined data considered range from ?0.54 to ?0.04 V (vs. NHE) in aqueous solution and from ?0.35 to ?0.08 V (vs. NHE) in MeCN. The observed and/or computed geometries of the tetraaminecopper(II) complexes are distorted octahedral or square-pyramidal (4 + 2 or 4+1) with (distorted) square-planar CuN₄ chromophores (Cu^II? N = 1.99–2.06 Å; Cu? O ≈ 2.5 Å; Cu? O ≈ 2.3 Å), those of the tetraaminecopper(I) complexes are (distorted) tetrahedral (four-coordinate; Cu^I? N = 2.12–2.26 Å; tetrahedral twist angle ?? = 30–90°). The reduction potentials of Cu^II/I couples with primary-amine ligands and those with macrocyclic secondary-amine ligands were correlated separately with the corresponding strain energies, leading to slopes of 70 and 61 kJ mol^?1 V^?1, with correlation coefficients of 0.89 and 0.91, respectively. The approximations of the model (entropy, solvation, electronic factors) and the limits of applicability are discussed in detail and in relation to other approaches to compute reduction potentials of transition-metal compounds.     

Efficient loading of sulfonamide safety-catch linkers by Fmoc amino acid fluorides

[reaction: see text] Fmoc-protected amino acid fluorides were found to be excellent reagents for the acylation of sulfonamide safety-catch linkers (SCL) suitable for the subsequent preparation of peptide C-terminal thioesters. High loadings were obtained on different types of resins with low levels of epimerization.     

AES investigations of plasma sprayed Al2O3 coatings on Ni

Summary The system of plasma sprayed Al₂O₃ on Ni substrates is investigated by means of AES/depth profiling. The influence of two process parameters — preoxidation procedure and spraying temperature — is examined. Rupture between substrate and ceramic layer occurs between a residual — or, in the case of excessive preoxidation, a superfluous — NiO layer on Ni, the thickness of the former depending on preoxidation conditions and the Al₂O₃ layer, the back side of which being partially covered with NiO. The thickness of this NiO layer increases up to about 1 m with the thickness of the initial NiO layer on the substrate, until this layer is about 1.3 m thick, and remains constant thereafter. The same dependence is observed for the width (0.1–1 m range) of the mixed oxide interface between the sprayed Al₂O₃ layer and the NiO layer below. These results represent the chemical contribution to adherence. Contrary to excessive preoxidation, an increase of the spraying temperature from 300°C to 500°C effects broader interfaces.This poster was awarded the First Prize in Poster Section C by the Deutscher Arbeitskreis für Spektroskopie (DASp)     

b1Σ+ Emissions from group V–VII diatomic molecules. b 0+ → X1 0+, X2 1 emissions of AsI and SbI

Chemiluminescence from the b 0⁺ → X₁ 0⁺ band system of AsI and of the b 0⁺ → X₁ 0⁺, X₂ 1 systems of SbI in the near-infrared spectral region has been observed in a discharge flow system. Analysis of the spectra led to the spectroscopic constants (in cm^?1) of AsI: ω_e(X₁, X₂) = 257 ± 2, ω_ex_e(X₁, X₂) = 0.82 ± 0.2, T_e(b 0⁺) = 11738 ± 5, ω_e(b 0⁺) = 271 ± 2, ω_ex_e(b 0⁺) = 0.66 ± 0.2, and of SbI: T_e(X₂ 1) = 965 ± 10, ω_e(X₁, X₂) = 206 ± 6, T_e(b 0⁺) = 12328 ± 10, ω_e(b 0⁺) = 211 ± 6. The intensity ratio of the two sub-systems b 0⁺ → X₂ 1 and b 0⁺→ X₁ 0⁺ was found to be ≈0.013 in the case of SbI and ? 0.01 for AsI.     

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.     

Crystal Structures of Twisted Di(2-Pyridyl)ketone and Planar Azo-di(2-Pyridine) and Their Discussion Based on PM3 Enthalpy Hypersurfaces for the Isoelectronic Molecules

Single crystal structure determinations prove the two pyridine substituents in di(2-pyridyl)ketone (H₄C₄NC)₂C=O to be twisted out of the carbonyl skeleton plane by torsion angles (OCCN) of 41° and –163°, in contrast to their planar arrangement in azo-di(2-pyridine) H₄C₄NC)-N=N-(CNC₄H₄). In order to rationalize the surprising difference between the two isoelectronic molecules, approximate PM3 enthalpy of formation hypersurfaces have been calculated for each of the two ring torsions, which are assumed to be the dominant ones among the 3N – 6 = 60 degrees of freedom. For both the ketone and the azo derivative, global minima are calculated, the torsion angles of which deviate from the crystal structure results, and, therefore, support the assumption that both the experimentally determined twisting of di(2-pyridyl)ketone as well as the flattening of azo-di(2-pyridine) might be affected by the crystal packing.     

Separation of dixanthogen and sulphur xanthates by H.P.L.C.: Disproportionation of Sulphur Dixanthates

High-performance liquid chromatography on C₁₃ reverse-phase or silica gel columns, with methanol—water (85:15) or n-hexane, respectively, as mobile phase, is used to separate dixanthogens and sulphur xanthates with different alkyl groups. Detection limits are as low as 1 ng (0.05 ppm) with u.v. detectors. When molecular emission cavity analysis is used for the detector, detection limits are reduced to ca. 0.25 ng (0.013 ppm). In aqueous methanol, alkyl sulphur dixanthates, (ROCS₂)₂S, disproportionate to yield mixtures of the corresponding dixanthogen, (ROCS₂)₂ , alkyl sulphur monoxanthates, (ROCS₂)₂S₂ , and dixanthates.