K-stabilized high-oxygen-coverage states on Rh(110): a low-pressure pathway to formation of surface oxide

Using scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy, we studied the evolution of the structure and chemical state of a Rh(110) surface, modified by K adlayers and exposed to high O2 doses at elevated temperatures. We find that oxygen coadsorption on the K-covered Rh(110) leads to massive reconstruction of the Rh(110) surface. Stable reconstructed (10 x 2) and (8 x 2) segmented phases with a local coverage of more than two oxygen atoms per surface Rh atom were observed. Formation of surface oxide, which coexists with the (10 x 2) and (8 x 2) segmented adsorption phases, is evidenced at the highest O2 doses. The development of strongly reconstructed adsorption phases with oxide-like stoichiometry and surface oxide under UHV conditions is explained in terms of the stabilization of the (1 x 2) reconstruction and promotion of O2 dissociation by the K adatoms.     

Rapid method for quantifying the extent of methionine oxidation in intact calmodulin

We have developed a method for rapidly quantifying the extent to which the functionally important Met¹⁴⁴ and Met¹⁴⁵ residues near the C-terminus of calmodulin (CaM) are converted to the corresponding sulfoxides, Met(O). The method utilizes a whole protein collision-induced dissociation (CID) approach on an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer. Using standards of CaM oxidized by hydrogen peroxide (H₂O₂) or peroxynitrite (ONOO⁻), we demonstrated that CID fragmentation of the protein ions resulted in a series of C-terminal singly charged y₁–y₁₅ ions. Fragments larger than y₄ exhibited mass shifts of +16 or +32 Da, corresponding to oxidation of one or two methionines, respectively. To assess the extent of oxidative modification for Met¹⁴⁴ and Met¹⁴⁵ to Met(O), we averaged the ratio of intensities for y _n , y _n + 16, and y _n + 32 ions, where n = 6–9. By alternating MS and CID scans at low and high collision energies, this technique allowed us to rapidly determine both the distribution of intact CaM oxiforms and the extent of oxidative modification in the C-terminal region of the protein in a single run. We have applied the method to studies of the repair of fully oxidized CaM by methionine sulfoxide reductases (MsrA and MsrB), which normally function in concert to reduce the S and R stereoisomers of methionine sulfoxide. We found that repair of Met(O)¹⁴⁴ and Met(O)¹⁴⁵ did not go to completion, but was more efficient than average Met repair. Absence of complete repair is consistent with previous studies showing that accumulation of methionine sulfoxide in CaM can occur during aging (Gao, J.; Yin, D.; Yao, Y.; Williams, T. D.; Squier, T. C. Biochemistry 1998, 37, 9536–9548).     

Peroxide‐mediated crosslinking of poly(ethylene oxide)

The crosslinking of poly(ethylene oxide) (PEO) with a difunctional peroxide such as 2,5‐bis(tert‐butylperoxy)‐2,5‐dimethylhexane (Luperox 101) may proceed through at least two different pathways. Thus, the thermally formed alkoxy radicals abstract hydrogen from PEO, and the resulting PEO radicals may combine or react with the peroxide radicals to give ether or acetal crosslinks. However, these reactions also produce noncrosslinking acetals and vinyl ethers. The acetals may react further with peroxy or other radicals and yield orthoesters and ketene acetals. These and similar groups, in the presence of dilute acids, are susceptible to acid hydrolysis, leading to the disintegration of the gels. The gels, upon swelling in dilute acids, undergo a slow degradation process that is not observed for unmodified PEO. The acid hydrolysis is confirmed by a combination of size exclusion chromatography and dynamic mechanical analysis. IR analysis shows the presence of hydroxyl end groups and the presence of carboxylates rather than aldehyde groups, this is consistent with orthoesters and ketene acetal intermediates. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3021–3026, 2002     

One-way optoelectronic switching of photochromic molecules on gold

We investigate photochromic molecular switches that are self-assembled on gold. We use two experimental techniques; namely, the mechanically controllable break-junction technique to measure electronic transport, and UV/Vis spectroscopy to measure absorption. We observe switching of the molecules from the conducting to the insulating state when illuminated with visible light (lambda=546 nm), in spite of the gold surface plasmon absorption present around this wavelength. However, we fail to observe the reverse process which should occur upon illumination with UV light (lambda=313 nm). We attribute this to quenching of the excited state of the molecule in the open form by the presence of gold.     

Two-step reaction on a strained, nanoscale segmented surface

By means of scanning tunneling microscopy and density functional theory calculations we demonstrate that on the Rh(110)-(10 x 2)-O surface, a prototypical multiphase surface of an oxidized transition metal model catalyst, water formation upon H2 exposure is a two-step reaction, with each step requiring special active sites. The 1st step initiates at (2 x 1)p2mg-O defect islands in the (10 x 2) structure and propagates across the surface as a reaction front, removing half of the adsorbed oxygen. The oxygen decorated Rh ridges of the (10 x 2) structure lose their tensile strain upon this reduction step, whereby nanoscale patches of clean Rh become exposed and act as special reaction sites in the 2nd reaction step, which therefore initiates homogeneously over the entire surface.     

Living tert‐butyllithium initiated anionic polymerization of 2‐vinylnaphthalene in toluene‐tetrahydrofuran mixtures

The tert‐butyllithium (t‐BuLi) initiated polymerization of carefully purified 2‐vinylnaphthalene in toluene containing small amounts of tetrahydrofuran with respect to t‐BuLi proceeds on a timescale of several hours without significant deactivation and allows the synthesis of very narrow molecular weight distribution poly‐(2‐vinylnaphthalene) (P2VN) (polydispersities as low as 1.04) and molecular weights between 1000 and 20,000. The absence of P2VN‐Li deactivation at these conditions is also indicated by high degrees of trimethylsilyl end functionalization (>95%) and coupling with dibromoxylene. The respective polymerizations of conventionally purified monomer reveal a complex polymerization profile consistent with deactivation by 2‐acetylnaphthalene during the early stages of the reaction. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3034–3041, 2001     

The screensaver map: Dynamics on elliptic curves arising from polygonal folding

We study the repeated folding of a two-parameter family of quadrilaterals about their successively transformed diagonals by examining the evolution of the diagonal lengths. Successively mapped pairs of squared lengths lie on an elliptic curve on which folding acts as translation under the group law. We prove the rotation number attains all possible values and any value determines a unique curve in parameter space. For rational parameters we give an algorithm to determine if the folding map is periodic. This gives a partial explanation for the diversity and intricacy of the curves traced out by the paths of the vertices of the transformed quadrilaterals. This research was supported by a grant from NSERC.     

Polymeric Fluorosurfactant‐Mediated Perfluorocarbon Emulsions

Polymeric fluorosurfactants, using poly(N,N‐dimethylacrylamide) (PDMA) or PDMA copolymers containing acrylic acid (AA) or 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid (AMPS) comonomers were synthesized by copolymerization of DMA, DMA/AA, or DMA/AMPS and methacrylates composed of 1,1‐dihydroperfluorodecanoyl groups separated from the methacrylate by polyethylene glycol (PEG) groups with molecular weights ranging from 1 to 6 kD. The stability of aqueous perfluorocarbon (PFC) emulsions mediated by these polymeric fluorosurfactants was found to be affected by polymer concentration, ionic comonomer content, perfluoroalkyl (R_F) comonomer content, and PEG spacer length. Thus, emulsion stability characterized by average particle size and morphology was improved by increased AA or AMPS content (up to 30 weight percent), increased PEG chain length and R_F comonomer content, and greater polymer concentration.