Eine empfindliche Farbenreaktion des Histidins

Ohne Zusammenfassung     

Enhancing Fluorescence Brightness: Effect of Reverse Intersystem Crossing Studied by Fluorescence Fluctuation Spectroscopy

Experiments based on fluorescence detection are limited by the population of the fluorescence marker’s long‐lived dark triplet state, leading to pronounced photobleaching reactions and blinking which reduces the average fluorescence signal obtained per time interval. By irradiation with a second, red‐shifted laser line, we initiate reverse intersystem crossing (ReISC) which enhances the fluorescence signal of common fluorophores up to a factor of 14. The reverse intersystem crossing from the triplet state back to the singlet system is achieved by photoexcitation to higher‐excited triplet states, which are, however, prone to photobleaching. We gain insights into the competing pathways of ReISC and photobleaching. The relative efficiencies of these two pathways and the triplet lifetime determine the achievable fluorescence enhancement, which varies strongly with the choice of dye, excitation irradiance and wavelength, and with environmental conditions. The study of ReISC not only results in a better understanding of a fluorescent label’s photophysics, but the method is a possible approach to optimize fluorescence emission in experiments, where signal strength is a critical parameter.     

Standardization of methods for the determination of traces of mercury : Part I. Determination of total inorganic mercury in inorganic samples

A method is described for the routine determination of mercury in the process and product streams of chlor alkali electrolysis plants (with the exception of chlorine) and also in the plant waste waters. The procedure is based on flameless atomic absorption spectrophotometry. Results for interlaboratory trials involving up to 37 participating laboratories are presented. Both synthetic and technical samples were analysed with mercury contents varying between 20 μg kg^-1 and 20 mg kg^-1. Statistical evaluation of the results gave a value of from 4 to 8% for the repeatability and from 6 to 22% for the reproducibility.     

Boron‐Doped Tri(9,10‐anthrylene)s: Synthesis,Structural Characterization,and Optoelectronic Properties

The reaction of 9,10‐dibromo‐9,10‐dihydro‐9,10‐diboraanthracene (9,10‐dibromo‐DBA, 3 ) with two equivalents of 9‐lithio‐2,6‐ or 9‐lithio‐2,7‐di‐tert‐butylanthracene gave the corresponding 9,10‐dianthryl‐DBAs featuring two ( 4 ) or four ( 5 ) inward‐pointing tert‐butyl groups. Compound 4 exists as two atropisomers, 4 and 4′ , due to hindered rotation about the exocyclic B? C bonds. X‐ray crystallography of 5 suggests that the overall interactions between facing tert‐butyl groups are attractive rather than repulsive. Even in solution, 4 / 4′ and 5 are stable toward air and moisture for several hours. Treatment of 3 with 10‐lithio‐9‐R‐2,7‐di‐tert‐butylanthracenes carrying phenyl (R=Ph), dimesitylboryl (R=Mes₂B), or N,N‐di(p‐tolyl)amino (R=Tol₂N) groups gave the corresponding 9,10‐dianthryl‐DBA derivatives 9 – 11 in moderate to good yields. In these molecules, all four solubilizing tert‐butyl groups are outward pointing. The solid‐state structures of 4 , 5 , 9 , and 10 reveal twisted conformations about the exocyclic B? C bonds with dihedral angles of 70–90°. A significant electron‐withdrawing character was proven for the Mes₂B moiety, but no appreciable +M effect was evident for Tol₂N. Compounds 5 , 9 , and 11 show two reversible DBA‐centered reduction waves in the cyclic voltammogram. In the case of 10 , a third reversible redox transition can be assigned to the Mes₂B–anthryl substituents. The UV/Vis absorption spectrum of 5 is characterized by a very broad band at λ_max=510 nm, attributable to a twisted intramolecular charge‐transfer interaction from the anthryl donors to the DBA acceptor. The corresponding emission band shows pronounced positive solvatochromism (λ_em=567 nm, C₆H₁₂; 680 nm, CH₂Cl₂) in line with a highly polar excited state. The charge‐transfer bands of 10 and 11 , as well as the emission bands of 9 and 10 , are redshifted relative to those of 5 . The Tol₂N derivative 11 is essentially nonfluorescent in solution, but emits bright wine‐red light in the solid state.     

On Real-Time Word Problems

It is proved that the word problem of the direct product oftwo free groups of rank 2 can be recognised by a 2-tape real-timebut not by a 1-tape real-time Turing machine. It is also provedthat the Baumslag–Solitar groups B(1,r) have the 5-tapereal-time word problem for all r 0.     

Liquid column-chromatographic separation and determination of long-chain primary n-alkylamines (n-C10H21NH2 to n-C22H45NH2) after their fluorescence derivatization with salicylaldehyde diphenylboron chelate

Summary Long-chain primary n-alkylamines (fatty amines) were separated by reversed-phase high-performance liquid chromatography (RP HPLC) after derivatization with salicylaldehyde diphenylboron chelate to fluorescent chelate-like azomethine compounds. Thin-layer chromatography was used as a pilot procedure. A RP-18 HPLC column and methanol/water mixtures are suitable for the separation of the derivatized fatty amines up to n-C₂₂H₄₅NH₂, they can be detected fluorimetrically. Only n-alkylamines of specific chain length ranges are separable under isocratic conditions with water concentrations of 0 to 25% by volume. On the other hand gradient elution makes it possible to separate all the n-alkylamines investigated within 45 min, thus allowing quantitative determination. The peak areas are proportional to concentrations in the range 2 × 10^–6 to 8 × 10^–5 mol/l. The determination of derivatized n-dodecylamine (concentration 5 × 10^–5 mol/l) can be performed with a relative standard deviationS _rel = 0.023 ( 2.3%). The smallest determinable concentration varies from 1.5 × 10^–6 for n-C₁₂H₂₅NH₂ to 4.5 × 10^–6 mol/l for n-C₁₈H₃₇NH₂ (sample volume 50 l).

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Flüssigkeits-chromafographische Trennung und Bestimmung von langkettigen primären n-Alkylaminen (n-C₁₀H₂₁NH₂ bis n-C₂₂H₄₅NH₂) nach ihrer Fluorescenzderivatisierung mit Salicylaldehyd-diphenylborchelat

Zusammenfassung Langkettige primäre n-Alkylamine (Fettamine) wurden nach der Derivatisierung mit Salicyl-aldehyd-diphenylborchelat zu fluorescierenden chelatartigen Azomethinverbindungen mit Hilfe der reversed-phase-Hochleistungsflüssigkeits-Chromatographie (RP-HPLC) getrennt. Als Pilot-Verfahren diente die Dünnschicht-Chromatographie. Für die Trennung der derivatisierten Fettamine bis n-C₂₂H₄₅NH₂ und deren fluorimetrischer Detektion sind eine RP-18-HPLC-Säule und Methanol/Wasser-Gemische geeignet. Unter isokratischen Bedingungen sind bei Wassergehalten zwischen 0 und 25 Vol.-% nur n-Alkylamine bestimmter Kettenlängenbereiche trennbar. Dagegen ermöglicht die Gradientelution eine für quantitative Bestimmungen geeignete Trennung aller angewendeten n-Alkylamine innerhalb von 45 min. Die Peakflächen sind den Konzentrationen im Bereich 2 · 10^–6 bis 8 · 10^–5mol/l proportional. Die Bestimmung von derivatisiertem n-Dodecylamin (Konzentration 5 · 10^–5 mol/l) ist mit einer relativen StandardabweichungS _rel = 0,023 ( 2,3%) durchführbar. Die kleinste bestimmbare Konzentration beträgt 1,5 · 10^–6 für n-C₁₂H₂₅NH₂ bis 4,5 · 10^–6 mol/l für n-C₁₈H₃₇NH₂ (Probenvolumen 50 l).

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Boron chelates and boron metal chelates, Part XVII. For Part XVI see ref. [1]

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Dedicated to Prof. Dr. H. Hartkamp on the occasion of his 60th birthday     

Formation and stability of high-spin alkali clusters

Helium nanodroplet isolation has been applied to agglomerate alkali clusters at temperatures of 380 mK. The very weak binding to the surface of the droplets allows a selection of only weakly bound, high-spin states. Here we show that larger clusters of alkali atoms in high-spin states can be formed. The lack of strong bonds from pairing electrons makes these systems nonmetallic, van der Waals-like complexes of metal atoms. We find that sodium and potassium readily form such clusters containing up to 25 atoms. In contrast, this process is suppressed for rubidium and cesium. Apparently, for these heavy alkalis, larger high-spin aggregates are not stable and depolarize spontaneously upon cluster formation.     

THE ACTION SPECTRA FOR UV-INDUCED SUPPRESSION OF MLR and MECLR SHOW THAT IMMUNOSUPPRESSION IS MEDIATED BY DNA DAMAGE

-Ultraviolet-B (UVB,280–320 nm) radiation can promote the induction of skin cancer by two mechanisms: damage of epidermal DNA and suppression of the immune system, allowing the developing tumor to escape immune surveillance. The mixed lymphocyte reaction (MLR) and the mixed epidermal cell lymphocyte reaction (MECLR) are commonly used methods to study the immunosuppressive effects of UVB radiation. To obtain a better understanding of the mechanism by which UVB radiation decreases the alloactivating capacity of in vitro-irradiated cells, action spectra for the MLR and MECLR were determined. Suspensions of peripheral blood mononuclear cells or epidermal cells were irradiated with monochromatic light of 254, 297, 302 or 312 nm and used as stimulator cells in the MLR or MECLR. Using dose-response curves for each wavelength, the action spectra were calculated. Both MLR and MECLR action spectra had a maximum at 254 nm and a relative sensitivity at 312 nm that was a thousand times lower than at 254 nm. Strikingly, the action spectra corresponded very closely to the action spectra that were found by Matsunaga et al. (Photochem. Photobiol. 54,403–410, 1991) for the induction of thymine dimers and (6-4)photoproducts in irradiated calf thymus DNA solutions, strongly suggesting that the UV-induced abrogation of the MLR and MECLR responses is mediated by UV-induced DNA damage. Furthermore, the action spectra for the MLR and MECLR were similar, suggesting that they share a common mechanism for UV-induced suppression.     

An electrophilic cleavage procedure for the asymmetric dihydroxylation: direct enantioselective synthesis of cyclic boronic esters from olefins

A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of olefins is described that yields cyclic boronic esters from olefins in a straight-forward manner. This process represents the first real product alteration in asymmetric dihydroxylation, since all previous protocols lead to free diols exclusively. A protocol based on the Sharpless AD conditions (for enantioselective oxidation of prochiral olefins) was developed that gives cyclic boronic esters with excellent enantiomeric excesses (ee's). Some of the ee's are higher than those reported for conventional AD. The unprecedented role of phenyl boronic acid on the course of the AD reaction was investigated in detail. PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of olefin oxidation intact. The main role of the boronic acids-apart from protecting the diol products against potential overoxidation-relies on removing the diol entity in an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols. Thus, a mechanistically new cleavage for enantioselective dihydroxylation reactions is introduced within the present work.