PHOTO-CIDNP DETECTION OF PYRIMIDINE DIMER RADICAL CATIONS IN ANTHRAQUINONESULFONATE- SENSITIZED SPLITTING

Anthraquinone-2-sulfonate (AQS) photosensitizes pyrimidine dimer splitting. Electron abstraction from the dimer is thought to induce dimer splitting, but direct evidence for the existence and intermediacy of dimer radical cations has been lacking. By employing photochemically induced dynamic nuclear polarization, we have found emission signals in the NMR spectra of dimers upon photolysis of dimers in the presence of anthraquinone-2-sulfonate. The two dimers employed were cis, syn-thymine dimer in which the N(1)-positions were linked by a three-carbon bridge and the N(3), N(3')-dimethyl derivative of that compound. The anthraquinone-2-sulfonate sensitized photochemically induced dynamic nuclear polarization spectrum of the methylated derivative exhibited an emission signal from the dimer-C(6) hydrogens. This result implied the existence of a dimer radical cation (mD+.) formed by electron abstraction by excited anthraquinone-2-sulfonate and nuclear spin sorting within a solvent caged radical ion pair [mD+. AQS-.]. Product pyrimidine photochemically induced dynamic nuclear polarization signals were also seen [enhanced absorption by C(6)-hydrogens and emission by C(5)-methyl groups]. Nuclear spin polarization in the product resulted from spin sorting in one or more of its precursors, including mD+. The results support the conclusion that dimer radical cations not only exist but are intermediates in the photosensitized splitting of pyrimidine dimers by anthraquinonesulfonate.     

A practical, fast, and high-yielding aziridination procedure using simple Cu(II) complexes containing N-donor pyridine-based ligands

Four-coordinate dichlorocopper(II) complexes derived from di(2-pyridyl)methanes or pyridine itself exhibit high catalytic activity in aziridination of regular olefins with PhINTs in weakly coordinating chloroform in the presence of 1-2 equiv of NaBArF4 (BArF4- = tetra[3,5-di(trifluoromethyl)phenyl]borate). High yields of aziridines exceeding 90% can be obtained with a 1:1 olefin/PhINTs ratio and 1-5 mol % catalyst loading for such reactive olefins as styrene, tri- and tetramethylethylene. For cis-cyclooctene, indene, methyl acrylate, methyl methacrylate, vinyl methyl ketone, tert-butylethylene, and neopentylethylene, as well as for 1-hexene and cyclopentene, yields of corresponding aziridines vary from 44% to 83%. The catalytic activity and efficiency of the reported copper complexes decrease moderately in the absence of NaBArF4.     

Formation of stable Meisenheimer adduct ion pairs in apolar solvents: implications for stereoselective reactions

[reaction: see text] A detailed study concerning the formation of Meisenheimer adducts in biphasic solvent systems is described. The process relies on utilizing a significantly lipophilic quaternary ammonium salt to transfer a nucleophile (e.g., hydroxide ion) between an aqueous and organic layer containing the electron-deficient aromatic substrate. Provided that the organic layer is sufficiently apolar, the resultant Meisenheimer adduct is considerably stable, likely the result of a strong ion-pairing interaction between the large polarizable anionic complex and the diffusively charged tetraalkylammonium cation. Using the diethylamide of 3,5-dinitrobenzoic acid as a model compound, the influence of ion-pairing reagents and solvents on adduct formation was investigated. Dramatically increased equilibrium formation of the Meisenheimer adduct is observed in the biphasic medium (e.g., benzene/2 M NaOH) relative to the same adduct generated in single-phase systems. Spectroscopic studies on this adduct are consistent with those conducted in single-phase polar or dipolar aprotic solvents. The methodology is extended to performing highly enantioselective biphasic kinetic resolutions of several racemic electron-deficient amides.     

Microfabricated ISEs: critical comparison of inherently conducting polymer and hydrogel based inner contacts

A rigorous side by side comparison of miniature planar potassium-selective electrodes with hydrogel and potassium hexacyanoferrate(II)/(III) doped polypyrrole (PPy/FeCN) based inner contacts is presented. The planar electrodes were manufactured by screen printing as four- and five-site arrays on ceramic substrates. These electrode arrays were incorporated into a flow-through cell, which could accommodate nine electrode sites. Two identical flow cells were connected in series and the effect of the inner contacts on the analytical performance of the respective electrodes has been critically evaluated. The time necessary to reach steady state conditions has been determined and the effect of experimental parameters (temperature, ambient light intensity, CO₂, and O₂ concentration of the sample) on the potential stability of the electrodes was analyzed. At controlled temperature, the drift of the planar potassium electrodes with hydrogel and PPy/FeCN solid contact were 0.11±0.02 mV h⁻¹ and 0.03±0.007 mV h⁻¹, respectively. The experimental data proved that there is no aqueous film formation between the PPy/FeCN film and the potassium-selective solvent polymeric membrane.     

Lipid bilayer disruption by polycationic polymers: the roles of size and chemical functional group

Polycationic polymers are used extensively in biology to disrupt cell membranes and thus enhance the transport of materials into the cell. The highly polydisperse nature of many of these materials makes obtaining a mechanistic understanding of the disruption processes difficult. To design an effective mechanistic study, a monodisperse class of polycationic polymers, poly(amidoamine) (PAMAM) dendrimers, has been studied in the context of supported dimyristoylphosphatidylcholine (DMPC) lipid bilayers using atomic force microscopy (AFM). Aqueous solutions of amine-terminated generation 7 (G7) PAMAM dendrimers caused the formation of 15-40-nm-diameter holes in lipid bilayers. This effect was significantly reduced for smaller G5 dendrimers. For G3, no hole formation was observed. In addition to dendrimer size, surface chemistry had a strong influence on dendrimer-lipid bilayer interactions. In particular, acetamide-terminated G5 did not cause hole formation in bilayers. In all instances, the edges of bilayer defects proved to be points of highest dendrimer activity. A proposed mechanism for the removal of lipids by dendrimers involves the formation of dendrimer-filled lipid vesicles. By considering the thermodynamics, interaction free energy, and geometry of these self-assembled vesicles, a model that explains the influence of polymer particle size and surface chemistry on the interactions with lipid membranes was developed. These results are of general significance for understanding the physical and chemical properties of polycationic polymer interactions with membranes that lead to the transport of materials across cell membranes.     

A highly emissive Cu2N2 diamond core complex supported by a [PNP]- ligand

A Cu2N2 diamond core structure, {(PNP)CuI}2 (2), supported by a [PNP]- ligand (1) ([PNP]- = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (tau > 10 mus) and an unusually high quantum yield (phi > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]- ligand. An estimate for the excited-state reduction potential of *2 (ca. -3.2 V vs Fc+/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations.     

Delocalization in platinum-alkynyl systems: a metal-bridged organic mixed-valence compound

A complex featuring two triarylamine redox centers bridged by Pt, trans-bis(triethylphosphine)-bis{4-[bis(4-methoxyphenyl)amino]phenylethynyl} platinum(II), has been synthesized as a model system for pi-conjugated Pt-containing polymers. Analysis of the intervalence charge-transfer band displayed by its mixed-valence monocation affords a quantitative assessment of electronic delocalization through the Pt bridge; this is found to be only slightly smaller than that determined for a benzene-bridged analogue. These results are supported by density functional theory calculations, which show that the active orbitals involved in the electron-transfer process in both cases have similar delocalization through the bridging unit.     

TRANSIENT INTERMEDIATES IN INTRAMOLECULARLY PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING BY INDOLE DERIVATIVES

Abstract— Intramolecularly photosensitized pyrimidine dimer splitting can serve as a model for some aspects of the monomerization of dimers in the enzyme-substrate complex composed of a photolyase and UV-damaged DNA. We studied compounds in which a pyrimidine dimer was covalently linked either to indole or to 5-methoxyindole. Laser flash photolysis studies revealed that the normally observed photoejection of electrons from the indole or the 5-methoxyindole to solvent was diminished by an order of magnitude for indoles with dimer attached (dimer-indole and dimer-methoxyindole). The fluorescence lifetime of dimer-indole in aqueous methanol was 0.85 ns, whereas that of the corresponding indole without attached dimer (tryptophol) was 9.7 ns. Similar results were obtained for the dimer-methoxyindole (0.53 ns) and 5-methoxytryptophol (4.6 ns). The quantum yield of dimer splitting for the dimer-methoxyindole (φ₂₈₇K7 = 0.08) was only slightly greater than the value found earlier for the dimer bearing the unsubstituted indole (4>_2K7= 0.04). Transient absorption spectroscopy also revealed lower yields of indole radical cations following laser flash photolysis of dimer-indole compared to the indole without attached dimer. Dimer-methoxyindole behaved similarly. These results are interpreted in terms of an enhanced rate of radiationless relaxation of the indole and methoxyindole excited singlet states in dimer-indoles. The possible quenching of the indole and methoxyindole excited states via electron abstraction by the covalently linked dimer is discussed.     

Elastic π±-d and π±-4He scattering below 70 MeV

Elastic scattering differential cross section data of ^± d at 65 MeV and ^{± 4}He at 51 MeV are presented and compared respectively to fully relativistic 3-body and optical potential calculations including true pion absorption.Presented at the symposium Mesons and Light Nuclei, Liblice, Czechoslovakia, June 1981.