THE SOLUTION PHOTODIMERIZATION OF URIDINE

Abstract— Quenching of the degradative photobleaching of FMN in anaerobic aqueous solution, and of the flavin-sensitized photo-oxidation of EDTA, alcohols, glycols, and glycerol, has been examined using flash techniques. The quenching can be produced by addition of KI, by successive flashes, and by increasing the flavin concentration (self-quenching). It is concluded that the lowest triplet is the photoreactive species in all systems, that the successive flash effect is due to triplet quenching by a reaction product, and that the concentration quenching may be due to either triplet-triplet annihilation or to reaction between a ground-state flavin and a flavin triplet.     

The stereochemistry of the hydroboration rearrangement.

Hydroboration of 1,2-dimethylcyclopentene with BH₃·THF affords a product in which boron migrates $\text{stereospecifically}$ at low temperature into the cyclopentane ring. At higher temperatures subsequent (non-stereospecific) isomerisation occurs via a competing reaction mechanism.     

Investigation of the ceriumIV oxidation of poly(vinyl alcohol)

Initiation of ceric ion-induced grafting from poly(vinyl alcohol) (PVOH) was studied by examining the formation and lifetime of colored complexes between cerium^IV and (1) a commercial PVOH, (2) a diol-free PVOH, and (3) several model 1,2-diols. UV/visible spectroscopy was employed using a wavelength of 520 nm. Results showed that the 1,2-diols, which are an inherent defect in PVOH, are the sites most reactive toward ceric ion. The observed rate constant for the reaction was found to increase with decreasing [Ce^IV] until the maximum rate was reached at which time reaction took place exclusively with 1,2-diols. The activation energy for oxidation of commercial PVOH at the maximum rate was 15.4 kcal/mol while that for diol-free PVOH was 22.4 kcal/mol. The 1,2-diols of PVOH displayed a high equilibrium constant for complexation with ceric ion, compared to the model 2,3-butanediol. It was suggested that the PVOH chain segments bring about crowding of the diol group, and this leads to a higher equilibrium constant for complexation. The lowest energy conformation of a PVOH chain segment containing a 1,2-diol linkage was calculated using the CHEMLAB-II molecular design program, and the 1,2-diol was predicted to be at the center of a sharp bend in the chain brought about by its presence.     

Using a non-invasive stable isotope tracer to measure the absorption of water in humans

The development of solutions that prevent dehydration or promote adequate re-hydration play a vital role in preventing fatigue during exercise, however, the methods commonly used to assess the hydration ability of such solutions are invasive and often assess the components of absorption separately. This paper describes using a non-invasive deuterium tracer technique that assesses gastric emptying and intestinal absorption simultaneously to evaluate the uptake of water during rest and exercise. The kinetics of absorption are further examined by mathematical modelling of the data generated. For the rest group, 0.05 g/kg of body weight of deuterium, contained in gelatine capsules, was ingested with ordinary tap water and saliva samples were collected every 5 min for one hour while the subject remained seated. The deuterium was administered as above for the exercise group but sample collection was during one hour of exercise on a treadmill at 55% of the subject's maximum heart rate. The enrichment data for each subject were mathematically modelled and the parameters obtained were compared across groups using an independent samples t-test. Compared with the rest condition, the exercise group showed delayed absorption of water as indicated by significant differences for the modelling parameters t2, t1/2, maximum absorption rate and solution absorption amount at t1. Labelling with a deuterium tracer is a good measure of the relative rate ingested fluids are absorbed by the body. Mathematical modelling of the data generates rates of maximum absorption and allows calculation of the percentage of the solution that is absorbed at any given time during the testing period.     

[(C6H5)3P]2Ni(Me3Si?CC?SiMe3). Darstellung,Eigenschaften und Struktur des ersten stabilen Nickel(0)-Komplexes von Bis-trimethylsilylacetylen

[(C₆H₅)₃P]₂Ni(Me₃Si? C?C? SiMe₃). Preparation, Properties, and Structure of the First Stable Nickel(0) Complex with Bis(trimethylsilyl)acetylene The title compound is the first example of a nickel(0) complex with bis(trimethylsilyl)acetylene and obtained from (ph₃P)₂Ni(C₂H₄) and bis(trimethylsilyl)acetylene in tetrahydrofuran. The complex is characterized by some reactions, the i.r. spectrum and by a structural analysis with the aid of X-ray diffraction data. (ph₃P)₂Ni(Me₃Si? C?C? SiMe₃) crystallizes monoclinic in the space group C2/c with four formula units per unit cell (2468 observed, independent reflexions, R = 0.038). The cell dimensions are a = 20.927, b = 13.812, c = 14.238 Å, α = γ = 90°, β = 91.02°. The molecules are monomer in crystalls. The molecule is planar with ligands arranged trigonally about the central atome but distorted to the tetrahedral complex.     

Magnetic and spectroscopic properties of benzylammomium tetrahalogenochromates(II) containing mixed halides (Cl and Br)

Summary The complexes [PhCH₂NH₃]₂[CrCl_xBr_4–x], x=0, 1, 1.8, 3.3 or 4, have been prepared, and the new mixed halides characterised by magnetic and spectroscopic measurements. They are further examples of two dimensional ferromagnets. The magnitude of the magnetic interaction increases slightly with replacement of chloride by bromide.     

Ab initio excited-state dynamics of the photoactive yellow protein chromophore

The photoisomerization mechanism of the neutral form of the photoactive yellow protein (PYP) chromophore is investigated using ab initio quantum chemistry and first-principles nonadiabatic molecular dynamics (ab initio multiple spawning or AIMS). We identify the nature of the two lowest-lying excited states, characterize the short-time behavior of molecules excited directly to S2, and explain the origin of the experimentally observed wavelength-dependent photoisomerization quantum yield.     

Hydrogen-bonding contributions to the lattice energy of salts for second harmonic generation

The lattice energies of a series of organic dihydrogenphosphate salts capable of second harmonic generation (SHG) have been calculated. These calculations, coupled with empirical data, indicate that a minimum of 20–25% of the lattice energy arises from hydrogen-bond interactions. Hydrogen bonding is shown to be a strong enough force to have a profound effect on the overall packing and crystal geometry of such ionic materials, and is thus an important factor to consider for crystal engineering.     

Reduction of 1,4-dichlorobut-2-yne by titanocene to a 1,2,3-butatriene. Formation of a 1-titanacyclopent-3-yne and a 2,5-dititanabicyclo[2.2.0]hex-1(4)-ene

The 2,5-dititanabicyclo[2.2.0]hex-1(4)-ene (bis-titanocene-mu-(Z)-1,2,3-butatriene complex)3 is formed starting from [Cp2Ti(eta2-Me3SiC2SiMe3)] by in situ generated titanocene and 1,4-dichlorobut-2-yne via the 1-titanacyclobut-3-yne (2).     

Chemische synthese von nonadesoxyribonucleotiden mit den abgewandelten basen uracil, 5-bromuracil und 5-methylcytosin nach dem triester-verfahren

In order to investigate the interaction of Eco RII restriction and modification enzymes with synthetic DNA fragments three nonadeoxyribonucleotides containing the modified bases uracil, 5-bromouracil and 5-methylcytosine were synthesized according to the phosphate tri-ester approach using TPS/l-methylimidazole as the condensation agent. The patterns of these modified DNA fragments obtained by Maxam/Gilbert sequence technique are presented.