Eye tissue structure and refraction alterations upon nondestructive laser action

A new approach to alterations in eye refraction upon nondestructive laser action on the sclera and cornea is studied. It is demonstrated in in vivo experiments on rabbit eyes that sequential laser irradiation of the sclera and cornea yields a significant alteration in the eye refraction. The collagen structure of the sclera and cornea is studied after the nondestructive laser action with noninvasive polarization-sensitive optical coherence tomography. It is demonstrated that collagen fibers that provide for the cornea tension and applanation partially survive in the zone of the local denaturation of sclera. An irradiation mode that corresponds to an increase in the cornea’s plasticity and does not cause visible structural changes is chosen. The simplest theoretical model for alterations in the eye refraction upon the nonablative laser action on sclera is analyzed. The alteration in the cornea curvature upon stretching resulting from the local sclera coagulation and the corresponding decrease in its volume is calculated. The model makes it possible to approximately estimate the laser irradiation modes that provide the desired alterations in eye refraction.     

Structural features of glasses in the GaF3-SnF2 system

The structure of 65SnF₂-35GaF₃, 75SnF₂-25GaF₃, and 85SnF₂-15GaF₃ glasses is studied using Raman and IR spectroscopy. The glass structure in the SnF₂-GaF₃ system is formed by a mixed network of fluorogallate and fluorostannate groups. A boson peak is observed in the low-frequency region (～29 cm^?1) of the Raman spectra of the glasses. The shape of the boson peak signifies that the glass does not separate into components on the nanometer scale.     

α-Glucosidase synthesis in batch and continuous culture ofSaccharomyces cerevisiae

Glucose prevents maltose utilization bySaccharomyces cerevisiae in batch culture, whereas in a mixed carbohydrate-limited chemostat, maltose and glucose were consumed simultaneously. The specific activity of α-glucosidase was dependent on the dilution rate as well as the proportion of maltose in the mixture. Maximum specific activities in the batch and chemostat cultures on mixtures of maltose and glucose were lower than corresponding values observed on maltose alone.     

A mechanism from quantum chemical studies for methane formation in methanogenesis

The mechanism for methane formation in methyl-coenzyme M reductase (MCR) has been investigated using the B3LYP hybrid density functional method and chemical models consisting of 107 atoms. The experimental X-ray crystal structure of the enzyme in the inactive MCR(ox1)(-)(silent) state was used to set up the initial model structure. The calculations suggest a mechanism not previously proposed, in which the most remarkable feature is the formation of an essentially free methyl radical at the transition state. The reaction cycle suggested starts from a Michaelis complex with CoB and methyl-CoM coenzymes bound and with a squareplanar coordination of the Ni(I) center in the tetrapyrrole F(430) prosthetic group. In the rate-limiting step the methyl radical is released from methyl-CoM, induced by the attack of Ni(I) on the methyl-CoM thioether sulfur. In this step, the metal center is oxidized from Ni(I) to Ni(II). The resulting methyl radical is rapidly quenched by hydrogen-atom transfer from the CoB thiol group, yielding the methane molecule and the CoB radical. The estimated activation energy is around 20 kcal/mol, which includes a significant contribution from entropy due to the formation of the free methyl. The mechanism implies an inversion of configuration at the reactive carbon. The size of the inversion barrier is used to explain the fact that CF(3)-S-CoM is an inactive substrate. Heterodisulfide CoB-S-S-CoM formation is proposed in the final step in which nickel is reduced back to Ni(I). The suggested mechanism agrees well with experimental observations.     

A density functional study on a biomimetic non-heme iron catalyst: insights into alkane hydroxylation by a formally HO-FeV=O oxidant

The reactivity of [HO-(tpa)Fe(V)=O] (TPA=tris(2-pyridylmethyl)amine), derived from O-O bond heterolysis of its [H(2)O-(tpa)Fe(III)-OOH] precursor, was explored by means of hybrid density functional theory. The mechanism for alkane hydroxylation by the high-valent iron-oxo species invoked as an intermediate in Fe(tpa)/H(2)O(2) catalysis was investigated. Hydroxylation of methane and propane by HO-Fe(V)=O was studied by following the rebound mechanism associated with the heme center of cytochrome P450, and it is demonstrated that this species is capable of stereospecific alkane hydroxylation. The mechanism proposed for alkane hydroxylation by HO-Fe(V)=O accounts for the experimentally observed incorporation of solvent water into the products. An investigation of the possible hydroxylation of acetonitrile (i.e., the solvent used in the experiments) shows that the activation energy for hydrogen-atom abstraction by HO-Fe(V)=O is rather high and, in fact, rather similar to that of methane, despite the similarity of the H-CH(2)CN bond strength to that of the secondary C-H bond in propane. This result indicates that the kinetics of hydrogen-atom abstraction are strongly affected by the cyano group and rationalizes the lack of experimental evidence for solvent hydroxylation in competition with that of substrates such as cyclohexane.     

Einwirkung von p-Nitroso-dimethylanilin auf Brenztraubensäure und ihre Derivate, 6. Mitt.: Kondensation der Phenylbrenztraubensäure mit primären aromatischen Aminen

Phenylpyruvic acid reacts with p-nitroaniline to yield the 3-hydroxypyrrolinone2a, and with anilines bearing+M substituents in the p-position to yield the 3-enaminopyrrolinones1b-1e.

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5. Mitt.:M. Ruse, E. Hamburg undM. Petri, Chem. Ber.103, 3727 (1970).     

Improved quantification limits in chiral capillary electrochromatography by peak compression effects

The peak compression effect has been applied to improve quantification limits in chiral capillary electrochromatography (CEC). A stationary phase based on the chiral selector vancomycin (Chirobiotic V) was used for separations of the enantiomers of mianserin. By adding solvents with a low dielectric constant, e.g. 2-propanol or tetrahydrofuran, to the sample solution, peak compression could be induced. The plate numbers for the minor enantiomer increased from approximately 100,000 to 1.4-1.6 million plates/m, when the composition of the mobile phase was adjusted so that the analyte eluted within either one of two system zones originating from the sample solution. A 10-fold improvement in the quantification limit for the minor enantiomer was obtained compared to elution under non-focused conditions.     

Peak compression effects in capillary electrochromatography of basic drug substances using a strong cation-exchanger

Peak compression effects in capillary electrochromatography of basic drug substances using a strong cation-exchanger have been studied. Extremely narrow peaks with apparent efficiencies of several million plates per meter could be obtained when the composition of the sample zone differed from that of the mobile phase. The increased efficiencies were predominately observed when the analyte had an elution time similar to that of the electroosmotic flow marker. Peak compression was found to be reproducible and could be obtained for all investigated basic drug substances by altering the composition of the mobile phase in such a way that the analyte co-eluted with the sample zone. An explanation of the observed phenomena is proposed. A sample zone differing in composition from the mobile phase will disturb the equilibrium between the stationary and mobile phase. The elution rate of an analyte will consequently be different when residing inside the sample zone. If the analyte migrates through the sample zone at a higher speed than the rest of the mobile phase and is strongly retained after passing through a boundary in the sample zone, a continuous stacking can be obtained trapping the analyte as a very narrow band.     

Titration of sulphides and thiols in natural waters

The concentrations of the species present during the procedure of Boulègue for determination of sulphide, thiols, thiosulphate and sulphite in mineral waters by titration with mercury(II) chloride have been calculated and functions suitable for the evaluation of the equivalence points have been derived. It is shown that the halide ions in seawater interfere only in the titration of sulphite.     

1-Lithio-2,2-diphenyl-1-(phenylsulfonyl)ethen: Synthese und Kristallstruktur

Syntheses and Crystal Structure of 1-Lithio-2,2-diphenyl-1-(phenylsulfonyl)ethene Crystals of [1-lithio-2,2-diphenyl-1-(phenylsulfonyl)ethene]– N,N,N′,N′ -tetramethylethylenediamine (2/2) ( 2 ) were prepared by addition of BuLi to 1,1-diphenyl-2-(phenylsulfonyl)ethene ( 1 ) in the presence of N,N,N′,N′ -tetramethylethylenediamine (TMEDA) at low temperature. The X-ray structure analysis shows a centrosymmetric dimer bridged over an eight-membered (Li? O? S? O)₂ ring. There are no Li–C contacts to the C(α) atoms. Both Li cations are tetracoordinated via the sulfonyl O-atoms and the N-atoms of the TMEDA ligand. The X-ray structure analysis of 1,1-diphenyl-2-(phenylsulfonyl)ethene ( 1 ) also was determined to compare interatomic distances and angles.