Ultrafast dynamics in DNA: "fraying" at the end of the helix

The dynamics of the electric fields in the interior of DNA are measured by using oligonucleotides in which a native base pair is replaced by a dye molecule (coumarin 102) whose emission spectrum is sensitive to the local electric field. Time-resolved measurements of the emission spectrum have been extended to a six decade time range (40 fs to 40 ns) by combining results from time-correlated photon counting, fluorescence up-conversion, and transient absorption. Recent results showed that when the reporter is placed in the center of the oligonucleotide, the dynamics are very broadly distributed over this entire time range and do not show specific time constants associated with individual processes (Andreatta, D.; et al. J. Am. Chem. Soc. 2005, 127, 7270). This paper examines an oligonucleotide with the reporter near its end. The broadly distributed relaxation seen before remains with little attenuation. In addition, a new relaxation with a well-defined relaxation time of 5 ps appears. This process is assigned to the rapid component of "fraying" at the end of the helix.     

Some recent developments in the historiography of alchemy

The number of researchers and publications devoted to the history of alchemy has seen exponential growth and diversification in recent decades, to such an extent that some scholars speak of a "New Historiography of Alchemy". On the occasion of the seventy-fifth anniversary of the Society for the History of Alchemy and Chemistry, this paper outlines some highlights of the literature since 1990, with a view to identify current trends but also challenges for the future. Some of the most important changes identified are a marked awareness of the risks of presentism, a shift from ambitious histories to contextualised microhistories, a heightened recognition of the internal diversity of historical alchemy, and a greater emphasis on its practical dimensions and its role in the Scientific Revolution. Among the challenges, the paper underscores the potential risks of an excessive historiographical fragmentation, the need for further interdisciplinary training and cooperation, and the responsibilities of alchemy historians towards students and the general public alike.     

Ulivo alchemy: the bio- and tecno-molecular approach to MAC-Mediterranean aliment culture

Sccoiridoid biophenols, contained in olive drupes, are enzymatically activated to provide a complex mechanism for the biological defence against pathogen attack. Their identification, quantitation and metabolic behaviour are investigated, by HPLC and NMR experiments, for the improvement of olivegrowing and of the production of oil and table olives.     

Electron transfer in supercritical carbon dioxide: ultraexothermic charge recombination at the end of the "inverted region"

Charge-recombination rates in contact radical-ion pairs, formed between aromatic hydrocarbons and nitriles in supercritical CO(2) and heptane, decrease with the exothermicity of the reactions until they reach -70 kcal mol(-1), but from there on an increase is observed. The first decrease in rate is typical of the "inverted region" of electron-transfer reactions. The change to an increase in the rate for ultra-exothermic electron transfer indicates a new free-energy relationship. We show that the resulting "double-inverted region" is not due to a change in mechanism. It is an intrinsic property of electron-transfer reactions, and it is due to the increase of the reorganisation energy with the reaction exothermicity.     

Allocolchicines via intramolecular Nicholas reactions: the synthesis of NSC 51046

Biaryl propargyl acetate hexacarbonyldicobalt complexes (4) undergo Lewis acid mediated Nicholas reactions with a remote arene function to afford dibenzocycloheptyne complexes (9). Reductive decomplexation based on a hydrosilylation-protodesilylation protocol is facile, and the 1,2,3,9-tetramethoxy case can be converted to NSC 51046 ((S)-N-acetylcolchicinol methyl ether, 3).     

Novel formation and use of a Nicholas carbocation in the synthesis of highly substituted tetrahydrofurans

The first formation of a Nicholas carbocation through cleavage of a carbon-carbon sigma bond has allowed the preparation of highly substituted tetrahydrofurans in a formal dipolar cycloaddition reaction.     

Tandem intramolecular Nicholas and Pauson-Khand reactions for the synthesis of tricyclic oxygen-containing heterocycles

[reaction: see text] Simple acyclic enynes can be easily converted into tricyclic ethers upon treatment with Co2(CO)8 followed by Nicholas and Pauson-Khand reactions. Tricyclic [5,8,5]- and [5,7,5]-systems can be prepared in high overall yields in only seven synthetic steps.