On the Distance‐Independent Hole Transfer over Long (A⋅T)n‐Sequences in DNA

Electron holes can travel through DNA double strands over long distances in a multistep ‘hopping’ process. But the influence of the DNA sequence on this process is still not understood in all details. We have carried out new experiments to understand the recent observation that the efficiency of the hole transport between guanines (G), which are separated from each other by long adenine?thymine (A?T) sequences, is nearly independent of the length of the (A?T)_n sequence for n ≥ 4. For this purpose, a new synthesis of the modified adenosine 16 and its incorporation into a DNA double strand was worked out. Subsequent experiments demonstrated that the hole transport between GGG units and the H₂O trapping of the guanine radical cation display similar rates. We conclude that the charge must be already partially equilibrated before being trapped by H₂O. Thus, the weak distance effect is caused not only by the rate of the hole transport, but also by its equilibration over the (A ? T)_n sequence.     

Dimeric Metal Complexes as Mediators for Radical C?C Bond-Forming Reactions

Irradiation of dicarbonyl(η⁵-cyclopentadienyl)iron dimer 1 or decacarbonyldimanganese ( 2 ) in the presence of alkyl halides leads to C-centered radicals which can be trappedby alkenes and yields saturated and/or unsaturated addition products. Carbon radicals are generated via halogen abstraction by the initially formed metal-centered radicals resulting from homolysis of the metal-metal bond of dimeric mediators 1 and 2 . No reaction occurs using octacarbonyldicobalt ( 3 ).     

Indirect determination of arsenic by flotation spectrophotometry

An indirect method of arsenic determination in the submicrogram range via the determination of molybdenum is presented here. High sensitivity is achieved by combination of the chemical amplification during formation of dodecamolybdoarsenic acid (arsenic: molybdenum ratio 1 12) with multiplication due to the formation of ion-association complexes during flotation-spectrophotometric molybdenum determination with crystal violet (molar ratio 1 2). Thus, the amplification factor relating to arsenic is 24.Dodecamolybdoarsenic acid is formed in a weakly acidic medium and is quantitatively extracted byn-butanol. Back extraction of the heteropoly acid to the aqueous phase and its simultaneous destruction provides the basis for the reaction of released molybdate ions with thiocyanate ions. The molybdenum-thiocyanate complex forms a sparingly soluble ion-association complex with crystal violet which can be floated with toluene on the phase boundary (film flotation). After separation of the aqueous phase the floated molybdenum compound is dissolved in acetone and the resulting free crystal violet ions are subjected to photometric determination at 590 nm as equivalent of the concentration of arsenic. The molar absorptivity of crystal violet is 3.2 · 10⁵1 · mol^–1 · cm^–1. Beer's law is obeyed in a concentration range from 0.01 to 1 g Mo · ml^–1 (0.001–0.1 g As · ml^–1). The resulting detection limit for arsenic is 1 ng · ml^–1.     

Studies on the mechanism of the photo-induced DNA damage in the presence of acridizinium salts-involvement of singlet oxygen and an unusual source for hydroxyl radicals

Mechanistic investigations of the photoinduced DNA damage by acridizinium salts (4a-azonia-anthracene derivatives) are presented. Irradiation of 9-bromoacridizinium in the presence of defined double- and single-stranded DNA oligomers under aerobic conditions leads to both frank strand breaks and alkali-labile sites as determined by polyacrylamide gel electrophoresis (PAGE). The extent of the DNA damage increases significantly in D(2)O and occurs selectively at guanosine residues. These observations reveal the formation of singlet oxygen ((1)O(2)) as reactive species, which oxidizes the DNA bases, above all the guanine bases. Further evidence for (1)O(2) formation was obtained from laser-flash spectroscopic investigations, which show intersystem crossing (S(1) to T(1)) of the excited states of the parent acridizinium and of the 9-bromo- and 9-amino-substituted derivatives. The resulting triplet state is efficiently quenched by oxygen (k(q) > 10(9) s(-)(1)M(-)(1)) to yield (1)O(2). Under anaerobic conditions, no significant alkali-labile lesions are observed, but frank strand breaks are induced; however, to lesser extent than under aerobic conditions. The DNA damage is suppressed in the presence of a radical scavenger, namely t-BuOH, and hydroxyl radicals are shown to be the reactive intermediates by trapping experiments with terephthalic acid. Moreover, the intercalated acridizinium molecules are not involved in the DNA damage reactions. The intercalated acridizinium salt leads to a primary PET reaction with the DNA bases; however, a fast BET transfer is proposed that regains the dye and the DNA, so that the excited intercalated dye does not contribute significantly to the overall DNA damage.     

Secondary inter­actions in gold(I) complexes with thione ligands. 5. Two ionic compounds of the form bis­(thione)gold(I) bis(4‐halobenzene­sulfon­yl)amide

Bis(1,3‐thia­zolidine‐2‐thione‐κS²)gold(I) bis­(4‐chloro­benzene­sulfonyl)amide, [Au(C₃H₅NS₂)₂](C₁₂H₈Cl₂NS₂O₄), has no imposed symmetry. Classical N—H⋯N and N—H⋯O hydrogen bonds link the residues to form chains parallel to the b axis. Weaker inter­actions involve C—H⋯O, C—H⋯Au and a number of X⋯Cl contacts (X = Cl, S or Au) clustered in the region y ≃ . In bis­(1‐methyl­imidazolidine‐2‐thione‐κS²)gold(I) bis­(4‐iodo­benzene­sulfonyl)amide, [Au(C₄H₈N₂S)₂](C₁₂H₈I₂NS₂O₄), the Au atom of the cation and the N atom of the anion lie on the twofold axis (0, y, ) in the space group C2/c. The formula unit forms a self‐contained ring with two symmetry‐equivalent N—H⋯O hydrogen bonds, and weak C—H⋯X (X = O, I or S), Au⋯I and I⋯I contacts are observed. In both compounds, the anions display extended conformations.     

Synthesis of 4′-C-Acylated Thymidines

Two synthetic pathways towards 4′-C-acylthymidines are presented. These modified mononucleosides are precursors of the 2′-deoxyribonucleotide 4′-C-radical. They were converted into their corresponding 3′-O-[(2-cyanoethyl) N,N-diisopropylphosphoramidites] 3a–c and incorporated in oligonucleotides by solid-phase synthesis. The structure of some modified nucleosides was revealed by X-ray crystal-structure analysis.     

Application of the isoselective relationship to diels-alder reactions

Diels-Alder reactions with “normal” and “reverse” electron demand” obey common isoselective relationships (Figure I).