DETECTION OF RADICAL ION INTERMEDIATES IN FLAVIN-PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING

Photosensitized splitting of cis-syn- and trans-syn-l,3-dimethyluracil dimers by 2′,3′,4′,5′-tetraacetylri-boflavin in acetonitrile containing a trace of perchloric acid was studied by laser flash photolysis. Protonation of the flavin prior to excitation resulted in excited singlet and triplet states that abstracted an electron from the dimers and yielded the protonated flavin radical (F1H₂′⁺), which was detected by absorption spectroscopy. Electron abstraction by the excited singlet state predominated over abstraction by the triplet state. Approximately one-third to one-half of the excited states quenched by the trans-syn dimer yielded F1H₂⁺, the balance presumably undergoing back electron transfer within the geminate radical ion pair generated by the initial electron transfer. A covalently linked dimer-flavin exhibited very inefficient flavin radical ion formation, consistent with the known low efficiency of dimer splitting in this system. These results constitute the first identification of a flavin radical ion intermediate in photosensitized pyrimidine dimer splitting.     

Ein beliebig startender SIMPLEX-Algorithmus

Zusammenfassung Die betrachtete Aufgabe der linearen Programmierung lautet: man maximiereC _T x unter den Bedingungenx0,A xd, wobeix,c R ⁿ ,d R ^m ,A=(m×n)-Matrix. Fallsd0, muß man nach herkömmlichen Verfahren zuerst einen zulässigen Ausgangspunkt finden, um den eigentlichen SIMPLEX-Algorithmus starten zu können. Die beschriebene Methode wendet den eigentlichen SIMPLEX-Algorithmus sofort solange auf jeweils noch verletzte Restriktionend _i <0 an, bis entweder alle eingehalten sind, und optimiert schließlich die gegebene Zielfunktion, oder gibt an, daß eine zulässige Lösung der Aufgabe nicht existiert.

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Consider the linear programming problem: maximizec ^T x s.t.x0,A xd, wherex,cR _n ,d R _m ,A=(m×n)-matrix. Ifd0, the ordinary SIMPLEX-algorithm can only be started after some feasible solution has been found. The above procedure instead takes advantage of the SIMPLEX-algorithm from the very beginning by using violated constraints as objective functions until either all of them hold (which allows subsequent optimization of the original objective function) or it can be stated that there exists no feasible solution at all.

     

Heteroleptic copper dipyrromethene complexes: synthesis, structure, and coordination polymers

The synthesis of neutral [Cu(dpm)2] and [Cu(dpm)(acac)] (dpm = dipyrromethene, acac = acetylacetonato) complexes is presented. The formation of the asymmetric metal complexes was monitored by electronic absorption and infrared spectroscopy. Two of the complexes investigated, containing pyrdpm ligands (pyrdpm = pyridyldipyrromethene), form 1-dimensional coordination polymers. The coordination polymers formed by these complexes have been characterized by single-crystal X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. The complexes possess square pyramidal coordination geometries with the apical position occupied by the meso-pyridyl donor of a neighboring complex in the crystal lattice. The features of these coordination complexes that facilitate formation of extended solids have been probed. Symmetric [Cu(pyrdpm)2] complexes are unable to form coordination solids due to steric hindrance at the metal center. Use of cyano donors in complexes such as [Cu(cydpm)(acac)] (cydpm = cyanodipyrromethene) in lieu of pyridyl donors also fail to form network solids. Through these systematic studies, both the basic coordination chemistry of these complexes and the fundamental design requirements for synthesizing this novel class of coordination polymers have been defined.     

SOLVENT DEPENDENCE OF PYRIMIDINE DIMER SPLITTING IN A COVALENTLY LINKED DIMER-INDOLE SYSTEM

Cyclobutadipyrimidines (pyrimidine dimers) undergo splitting that is photosensitized by indole derivatives. We have prepared a compound in which a two-carbon linker connects a dimer to an indolyl group. Indolyl fluorescence quenching indicated that the two portions of the molecule interact in the excited state. Intramolecular photosensitization of dimer splitting was remarkably solvent dependent, ranging from phi spl = 0.06 in water to a high value of phi spl = 0.41 in the least polar solvent mixture examined, 1,4-dioxane-isopentane(5 : 95). A derivative with a 5-methoxy substituent on the indolyl ring behaved similarly. These results have been interpreted in terms of electron transfer from the excited indolyl group to the dimer, which would produce a charge-separated species. The dimer anion within such a species could split or undergo back electron transfer. The possibility that back electron transfer is in the Marcus inverted region can be used to rationalize the observed solvent dependence of splitting. In the inverted region, the high driving force of a charge recombination exceeds the reorganization energy of the solvent, which is less for solvents of low polarity than those of high polarity. If this theory is applicable to the hypothetical charge-separated species, a slower back electron transfer, and consequently higher splitting efficiencies, would be expected in solvents of lower polarity. Photolyases may have evolved in which a low polarity active site retards back transfer of an electron and thereby contributes to the efficiency of the enzymatic dimer splitting.     

THERMAL REQUIREMENTS OF PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING

Abstract— A cis, syn -pyrimidine dimer (derived from thymine and orotate) covalently linked to 5-methoxyindole has been studied as a mechanistic model of photosensitized pyrimidine dimer splitting. In this dimer-indole, photoinitiated electron transfer to the dimer causes splitting in a manner that parallels the mechanism by which the DNA photolyases are thought to act. Dissolved in EPA (diethyl ether-isopentane-ethyl alcohol, 5: 5: 1, by vol) at room temperature, the dimer-indole exhibited indole fluorescence quenching and underwent splitting upon irradiation at 300 nm. In an EPA glass at 77 K, however, no splitting was detectable. To distinguish the effects of temperature and immobilization, photolysis experiments were performed on PMM [poly(methyl methacrylate)] films containing dimer-indole. In PMM at room temperature, dimer-indole underwent splitting when irradiated at 300 nm, which indicated that immobilization per se was not responsible for the failure of dimer-indole to split at low temperature. Furthermore, no splitting was observed when dimer-indole was irradiated in PMM at 77 K. These results imply that a step following photoinitiated, intramolecular electron transfer from indole to dimer has an insurmountable activation barrier at 77 K. The mechanistic implications for the photolyases are considered.     

High charge-carrier mobility in an amorphous hexaazatrinaphthylene derivative

An isomeric mixture of a tris(pentafluorobenzyl ester) derivative of hexaazatrinaphthylene forms stable amorphous films with an effective charge-carrier mobility of 0.02 cm2/Vs, while the pure 2,8,15-isomer exhibits widely differing morphologies and carrier mobilities (0.001-0.07 cm2/Vs), depending critically on the processing conditions.