Seams near seams: the Jahn-Teller effect in the 1E" state of N3+

The (1)E(") electronic state of cyclic N(3) (+) arising from the singly excited electron configuration e('3)e(") is studied using multireference configuration interaction wave functions and a quadratic Jahn-Teller Hamiltonian determined from those calculations. It is shown that these two states have both a symmetry-required seam of conical intersections at D(3h) geometries and three proximal symmetry equivalent C(2v) seams, located on a circle with radius rho(0) from the D(3h) intersection. rho(0), a function of Q(s), the breathing mode, is quite small but only attains a value of zero at Q(s) (crit)=1.252 A, resulting in a confluence or intersection node of the three C(2v) seams with the D(3h) seam. At this point only, g= parallelg(Q(s)) parallel, the norm of half the energy difference gradient, the linear Jahn-Teller term, vanishes and the intersection is of the Renner-Teller type. The close proximity of the previously unreported C(2v) seams to the D(3h) seam over the range of Q(s) considered is a consequence of the small values of g, compared to the quadratic Jahn-Teller term. The present analysis has important implications in the study of Jahn-Teller effects in ring systems and provides insight into a recent report that characterized this D(3h) seam as a Renner-Teller or glancing intersection.     

Compositional studies of human RPE lipofuscin: mechanisms of molecular modifications

The accumulation of lipofuscin has previously been implicated in several retinal diseases including Best's macular dystrophy, Stargardt's disease and age-related macular degeneration (AMD). Previously one of the major fluorophores of lipofuscin was identified as a bis-retinoid pyridinium salt called A2E, which is known to photochemically cause damage. In addition to A2E, there are numerous components in RPE lipofuscin that are unidentified. These compounds were determined to be structurally related to A2E by their fragmentation pattern with losses of 106, 190, 174 and/or 150 amu from the parent ion and the formation of fragments of ca 592 amu. The vast majority consists of relatively hydrophobic components corresponding to derivatized A2E with molecular weights in discrete groups of 800-900, 970-1080 and > 1200 m/z regions. In order to determine the mechanism of these modifications, A2E was chemically modified by; (1) the formation of specific esters, (2) reaction with specific aldehydes and (3) spontaneous auto-oxidation. The contribution of ester formation to the naturally occurring components of lipofuscin was discounted since their fragmentation patterns were different to those found in vivo. Alternatively, reactions with specific aldehydes result in nearly identical products as those found in vivo. Artificial aging of RPE lipofuscin gives a complex mixture of structurally related components. This results from the auto- and/or photooxidation of A2E to form aldehydes, which then back react with A2E giving a series of higher molecular weight products. The majority of these modifications result in compounds that are much more hydrophobic than A2E. These higher molecular weight materials have increased values of log P compared to A2E. This increase in hydrophobicity most likely aids in the sequestering of A2E into granules with the concomitant diminution of its reactivity. Therefore, these processes may serve as protective mechanisms for the RPE.     

Catalytic and Stoichiometric Transformations with Triphenyl(P,P,P-Triphenylphosphine Imidato-N) Phosphorus Cyanide, [Pnp Cyanide]

Abstract

Reactions involving anhydrous [PNP]⁺CN^?, 1, are described for a series of nucleophilic displacements. The reagent may be used stoichiometrically with moisture sensitive substrates, and also acts as an efficient phase transfer catalyst.     

In situ transmission electron microscopy observation of silver oxidation in ionized/atomic gas

The interaction between silver and ionized and atomic gas was observed directly by in situ transmission electron microscopy with an environmental cell for the first time. The electron beam provides dual functions as the source of both gas ionization and imaging. The concentration of ionized gas was tuned via adjusting the current density of the electron beam. Oxidation of the silver is observed in situ, indicating the presence of ionized and/or atomic oxygen. The evolution of microstructure and phase constituents was characterized. Then the oxidation rate was measured, and the relationships among grain size, mass transport rate, and electron flux were characterized. The role of the electron beam is discussed, and the results are rationalized with respect to ex situ results from the literature.     

Fiber-to-waveguide coupler based on the parabolic reflector

We present a fiber-to-waveguide coupling structure, the so-called vertical J coupler, based on the parabolic reflector. The device addresses the multiple objectives of high coupling efficiency, large bandwidth operation, polarization insensitivity, and compact footprint. The optical mode emanating from a fiber arranged normal to the plane of the substrate is incident underneath the parabolic reflector, turned through 90 degrees and focused into a dielectric waveguide. The viability of the coupler is demonstrated by finite-difference time-domain electromagnetic simulation as well as preliminary fabrication and optical testing of the device.     

Evaluation of intracranial stenoses and aneurysms with accelerated 4D flow

The aim of this study was to evaluate intracranial arterial stenoses and aneurysms with accelerated time-resolved three-dimensional (3D) phase-contrast MRI or 4D flow. The 4D flow technique was utilized to image four normal volunteers, two patients with intracranial stenoses and two patients with intracranial aneurysms. In order to reduce scan time, parallel imaging was combined with an acquisition strategy that eliminates the corners of k-space. In the two patients with intracranial stenoses, 4D flow velocity measurements showed that one patient had normal velocity profiles in agreement with a previous magnetic resonance angiogram (MRA), while the second showed increased velocities that indicated a less significant narrowing than suspected on a previous MRA, as confirmed by catheter angiography. This result may have prevented an invasive angiogram. In the two patients with 4-mm intracranial aneurysm, one had a stable helical flow pattern with a large jet, while the other had a temporally unstable flow pattern with a more focal jet possibly indicating that the second aneurysm may have a higher likelihood of rupture. Accelerated 4D flow provides time-resolved 3D velocity data in an 8- to 10-min scan. In the stenosis patients, the addition of 4D flow to a traditional MRA adds the velocity data provided from transcranial Doppler ultrasound (TCD) possibly allowing for more accurate grading of stenoses. In the aneurysm patients, visualization of flow patterns may help to provide prognostic information about future risk of rupture.     

Pauling's rules for oxide surfaces

Determination of surface structures currently requires careful measurement and computationally expensive methods since, unlike bulk crystals, guiding principles for generating surface structural hypotheses are frequently lacking. Herein, we discuss the applicability of Pauling's rules as a set of guidelines for surface structures. The wealth of solved reconstructions on SrTiO₃ (100), (110), and (111) are considered, as well as nanostructures on these surfaces and a few other ABO₃ oxide materials. These rules are found to explain atomic arrangements for reconstructions and thin films just as they apply to bulk oxide materials. Using this data and Pauling's rules, the fundamental structural units of reconstructions and their arrangement are discussed.     

Methylation of some cyclic triphosphenium ions

Six cyclic triphosphenium ions, including examples of five-, six-, and seven-membered ring systems, have been successfully methylated by excess methyl triflate to form the corresponding dications. This has been unequivocally established by means of ³¹P NMR spectroscopy; results are in good agreement with literature data for this type of compound. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:150–154, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10228     

Side-Chain Chemistry Governs Hierarchical Order of Charge-Complementary β-sheet Peptide Coassemblies

Self-assembly of proteinaceous biomolecules into functional materials with ordered structures that span length scales is common in nature yet remains a challenge with designer peptides under ambient conditions. This report demonstrates how charged side-chain chemistry affects the hierarchical co-assembly of a family of charge-complementary β-sheet-forming peptide pairs known as CATCH(X+/Y−) at physiologic pH and ionic strength in water. In a concentration-dependent manner, the CATCH(6K+) (Ac-KQKFKFKFKQK-Am) and CATCH(6D−) (Ac-DQDFDFDFDQD-Am) pair formed either β-sheet-rich microspheres or β-sheet-rich gels with a micron-scale plate-like morphology, which were not observed with other CATCH(X+/Y−) pairs. This hierarchical order was disrupted by replacing D with E, which increased fibril twisting. Replacing K with R, or mutating the N- and C-terminal amino acids in CATCH(6K+) and CATCH(6D−) to Qs, increased observed co-assembly kinetics, which also disrupted hierarchical order. Due to the ambient assembly conditions, active CATCH(6K+)-green fluorescent protein fusions could be incorporated into the β-sheet plates and microspheres formed by the CATCH(6K+/6D−) pair, demonstrating the potential to endow functionality.