A STUDY OF THE PHOTODYNAMIC EFFICIENCIES OF SOME EYE LENS CONSTITUENTS

We have studied the photochemical quantum yields of singlet oxygen production (using the RNO bleaching method) and superoxide production (using the EPR-spin trapping method and the SOD-inhibitable ferricytochrome c reduction spectral assay) of kynurenine (Ky), N-formylkynurenine (NFK), 3-hydroxykynurenine (3HK), kynurenic acid (KUA), and the flavins, riboflavin (RF) and flavin mononucleotide (FMN). Such a study of the photodynamic efficiencies is important since these compounds appear endogenously in the eye. The singlet oxygen quantum yields of the flavins and KUA are high, while Ky and 3HK generate no detectable amounts of singlet oxygen. The superoxide quantum yields of the sensitizers are low compared to their singlet oxygen, and Ky and 3HK produce no detectable amounts of superoxide. The production of the superoxide radical is enhanced in the presence of electron donor molecules such as EDTA and NADH. These results suggest that the production of oxyradicals in the lens may be modulated by the presence of endogenous electron donor molecules such as the coenzymes NADH and NADPH, which are present in significant amounts in some lenses. They also suggest that Ky and 3HK, which are known to be present in aged lenses, might play a protective rather than a deleterious role in the eye.     

Crystal structure prediction of aminols: advantages of a supramolecular synthon approach with experimental structures

The supramolecular synthon approach to crystal structure prediction (CSP) takes into account the complexities inherent in crystallization. The synthon is a kinetically favored unit, and through analysis of commonly occurring synthons in a group of related compounds, kinetic factors are implicitly invoked. The working assumption is that while the experimental structure need not be at the global minimum, it will appear somewhere in a list of computationally generated structures so that it can be suitably identified and ranked upward using synthon information. These ideas are illustrated with a set of aminophenols, or aminols. In the first stage, a training database is created of the 10 isomeric methylaminophenols. The crystal structures of these compounds were determined. The prototypes 2-, 3-, and 4-aminophenols were also included in the training database. Small and large synthons in these 13 crystal structures were then identified. Small synthons are of high topological but low geometrical value and are used in negative screens to eliminate computationally derived structures that are chemically unreasonable. Large synthons are more restrictive geometrically and are used in positive screens ranking upward predicted structures that contain these more well-defined patterns. In the second stage, these screens are applied to CSP of nine new aminols carried out in 14 space groups. In each space group, up to 10 lowest energy structures were analyzed with respect to their synthon content. The results are encouraging, and the predictions were classified as good, unclear, or bad. Two predictions were verified with actual crystal structure determinations.     

Laser Mass Spectrometry of Organophosphorus Pesticides and Related Compounds

Abstract

Laser mass spectra obtained for 20 organophosphorus (OP) compounds were systematically evaluated for groups containing analogous structural features. Variations in fragmentation can be understood based on simple organic reactions. While detailed mechanistic interpretations of the laser mass spectra (LMS) were not possible, the qualitative features in the LMS obtained from five compounds, not in the original set, could be predicted based on the characteristics of the other OP compounds studied. The success of the prediction lends credence to the qualitative models developed for rationalizing the LMS. A specific feature in the LMS of aromatic thionophosphates is a thiono-thiolo rearrangement. Detailed investigation into the phenomena involved comparison of LMS obtained from aromatic thionophosphates with spectra from electron impact, chemical ionization, field desorption, and secondary ion mass spectrometry. These results led to the conclusion that the rearrangement in laser mass spectrometry must occur during volatilization while the molecule/ion is in the “cloud” present immediately above the laser impact area.     

Polymerization (Polycondensation) of Alkanes over Fluosulfonic Acid and Antimony Pentafluoride

Several low molecular weight alkanes have been polymerized (polycondensed) over HSO₃ F-SbF₅ to yield a highly branched oily oligomer with a molecular weight range from the molecular weight of the monomer to around 700. The order of reactivity for butane and lower molecular alkanes is isobutane > n-butane > propane > ethane > methane. The reactivity is explained by the ease of attack of an acid proton derived from the acid on the alkane sigma bonds as well as the relative stability of the resulting cation formed. A cationic mechanism for this reaction is proposed.     

Short-Chain Branching in Polyethylene and Poly(vinyl Chloride) Using Pyrolysis Hydrogenation Gas Chromatography and 13C Nuclear Magnetic Resonance Analysis

This study compares the use of pyrolysis hydrogenation gas chromatography (PHGC) and ¹³C Fourier transform nuclear magnetic resonance (FTNMR) methods for the analysis of reference polyethylene (PE) samples, ethylene-α-olefin copolymers, and specially prepared poly(vinyl chloride) (PVC) samples which were reduced to their PE skeletal structures. The nature and relative quantities of the short branches along the polymer chains were determined using both techniques. Improved high-resolution PHGC data, obtained with a fused silica capillary separation column, gave results which were in satisfactory agreement with the ¹³C FTNMR data. This approach confirms that detailed microstructural information can be obtained with these methods by using carefully controlled experimental conditions and appropriate reference systems.     

Tentative Identification of the Reactive Species in the Reaction of Cp2 TiCl2 with Salts of Diacids

The tentative identification of the reactive species in the condensation of Cp₂ TiCl₂ with salts of diacids to form titanium polysters is made. The reactive species are believed to be the same for both aqueous solution and interfacial systems, i.e., R -CO₂ ^? and Cp₂ Ti²⁺ with reaction occurring in the aqueous phase. The condensation of Cp₂ TiCl₂ with disodium terephthalate in interfacial systems occurs via a pseudo-first-order reaction:

Rate = K[Cp₂ TiCl₂] The rate-determining step (s) is believed to be diffusion of CP₂ TiCl₂ into the aqueous layer and/or hydrolysis of Cp₂ TiCl₂     

Funneling light through a subwavelength aperture with epsilon-near-zero materials

We present a comprehensive study of enhanced light funneling through a subwavelength aperture with realistic (lossy) epsilon-near-zero (ENZ) materials. We realize experimentally an inclusion-free ENZ material layer operating at optical frequencies and characterize its performance. An analytical expression describing light funneling through several structures involving ENZ coupling layers is developed, validated with numerical solutions of Maxwell equations, and utilized to relate the performance of the ENZ coupling systems to their main limiting factor, material losses.     

Stereoselective total synthesis of (+)-β-conhydrine from d-mannitol

An asymmetric synthesis of the piperidine alkaloid (+)-β-conhydrine has been developed starting from (R)-protected glyceraldehyde as the chiral precursor using Barbier allylation, azide nucleophilic substitution, and ring-closing metathesis as the key steps.