DNA excited-state dynamics: ultrafast internal conversion and vibrational cooling in a series of nucleosides

To better understand DNA photodamage, several nucleosides were studied by femtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum pulse at wavelengths between 270 and 700 nm. A transient absorption band with maximum amplitude near 600 nm was detected in protonated guanosine at pH 2. This band decayed in 191 +/- 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the lowest excited singlet state. Excited state absorption for guanosine and the other nucleosides at pH 7 was observed in the same spectral region, but decayed on a subpicosecond time scale by internal conversion to the electronic ground state. The cross section for excited state absorption is very weak for all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state lifetimes of Ado, Guo, Cyd, and Thd were determined to be 290, 460, 720, and 540 fs, respectively (uncertainties are +/-40 fs). The decay times are shorter for the purines than for the pyrimidine bases, consistent with their lower propensity for photochemical damage. Following internal conversion, vibrationally highly excited ground state molecules were detected in experiments on Ado and Cyd by hot ground state absorption at ultraviolet wavelengths. The decays are assigned to intermolecular vibrational energy transfer to the solvent. The longest time constant observed for Ado is approximately 2 ps, and we propose that solute-solvent H-bonds are responsible for this fast rate of vibrational cooling. The results show for the first time that excited singlet state dynamics of the DNA bases can be directly studied at room temperature. Like sunscreens that function by light absorption, the bases rapidly convert dangerous electronic energy into heat, and this property is likely to have played a critical role in life's early evolution on earth.     

Analysis of blackbody-like radiation from laser-heated gas-phase tungsten nanoparticles

Thermal (blackbody-like) radiation that originated from laser-heated tungsten nanoparticles was measured using optical emission spectroscopy. The nanoparticles were generated via ArF excimer laser-assisted photolytic decomposition of WF6/H2/Ar gas mixtures, and the laser heating was applied parallel to the deposition. The temperature of the nanoparticles was determined, and its dependence on time, with respect to the 15-ns laser pulse (full width at half-maximum, fwhm) and laser fluence (phi), has been presented. At phi > 90 mJ/cm2, the particles reached the melting point (shortly after the laser pulse). Dominant cooling mechanisms, such as evaporation (above approximately 3000 K) and a combination of heat transfer by the ambient gas and radiative cooling (below approximately 3000 K), were observed for the nanoparticles, which were approximately 10 nm in diameter. The degree of inelasticity for the (predominantly) argon-gas collisions and the total emissivity of the particles (in the 2500-3000 K temperature region) could also be derived. The measured cooling rate and temperature data indicate that, depending on experimental parameters, evaporation and surface reactions can have a definite effect on the growth of particles.     

Direct optical detection of singlet oxygen from a single cell

Singlet oxygen has been detected in single nerve cells by its weak 1270 nm phosphorescence (a1deltag --> X3sigmag-) upon irradiation of a photosensitizer incorporated in the cell. Thus, one can now consider the application of direct optical imaging techniques to mechanistic studies of singlet oxygen at the single-cell level.     

Electrospray ionization mass spectrometric studies of nucleophilic carbenes derived from pyrazolium and indazolium carboxylates

Pyrazolium-3-carboxylate and indazolium-3-carboxylate, which belong to the class of pseudo-cross-conjugated mesomeric betaines and which represent the electronically relevant partial structures of the betaine alkaloid Nigellicin, were examined by electrospray ionization mass spectrometry. These compounds decarboxylate to pyrazol-3-ylidene and indazol-3-ylidene. The formation of adducts of these new nucleophilic carbenes under the measurement conditions was examined.     

Kryoskopische Bestimmung der Aktivitätskoeffizienten von Nichtelektrolytmischungen

Zusammenfassung Es wurde eine Methode entwickelt, welche die Aufnahme von Schmelzkurven von binären Nichtelektrolytmischungen mit einer Genauigkeit von 0,001° (und dementsprechend von etwa 0,00001 im Molenbruch) gestattet. Die Bestimmung der Schmelzpunkte erfolgte an Hand von Erwärmungskurven, die bei konstanter Wärmezufuhr mit einem Thermistor aufgenommen wurden.Die Methode wurde auf die Systeme 1,2,4-Trichlorbenzol-(1)-n-Hexan und Anilin-(1)-Cyclohexan angewendet und sollte hier der Überprüfung und Ergänzung der Daten dienen, die früher aus Dampfdruckmessungen¹ und Messungen der Löslichkeitskurve² abgeleitet wurden. Die Übereinstimmung mit den früheren Resultaten ist bei dem System Trichlorbenzol-Hexan ausgezeichnet, bei dem System Anilin-Cyclohexan annehmbar. Die nun genauer zu bestimmenden lnf ₁/x ₂ ² -Kurven (f _i Aktivitätskoeffizient,x _i Molenbruch der Komponentei) zeigen bei dem Übergang von verdünnter zu konzentrierterer Lösung eine plötzliche Änderung, als ob mit Erhöhung der Konzentration der Komponente 2 eine plötzliche Änderung der molekularen Struktur der Lösung einherginge. Dieser Befund steht zwar mit früheren Beobachtungen^1,2 in qualitativer Übereinstimmung, konnte aber bisher nicht so scharf formuliert werden.Mit 11 AbbildungenAuszugsweise vorgetragen auf der Chemiedozententagung der Chemischen Gesellschaft der Deutschen Demokratischen Republik in Halle (Saale) 24.–26. Juni 1959.     

Chiral ditopic receptors. Application to palladium-catalyzed allylic alkylation

Chiral pyridinooxazoline, quinolinooxazoline, bis(oxazolino)pyridine (pybox), and bisoxazoline (box) derivatives containing crown ether residues were prepared. Some of the ligands were assessed in substrate binding studies and in palladium catalyzed allylic alkylations.     

Illustration of a simple and versatile scheme for reversing enantiomeric elution order and facilitating enantiomeric impurity determination in capillary electrophoresis

Determination of enantiomeric purity is most often done under overload conditions, which leads to deformed peaks. In general, the best resolutions are obtained when the small peak appears before the large peak in the electropherogram. To be able to determine the R(+)-impurity in the S(-)-form as well as the S(-)-impurity in the R(+)-form the elution orders have to be reversed. The present paper describes reversal of enantiomeric elution order for the basic analyte propranolol and the acidic analyte ibuprofen. For propranolol, a charged heptakis-(6-sulfo)-beta-cyclodextrin (CD) is used in the background electrolyte. For ibuprofen, a mix of the charged heptakis-(6-sulfo)-beta-CD and the uncharged heptakis-(2,3,6-tri-O-methyl)-beta-CD is used in the background electrolyte. The use of a coated capillary and reversal of the polarity shift the elution order, buffer composition is unchanged in both cases. The enantiomers of propranolol and ibuprofen are well separated on both the coated and uncoated capillaries. Detection limits of enantiomer impurities are investigated using spiked samples of both propranolol and ibuprofen.     

Shaping of gelling biopolymer drops in an elongation flow

Shaping, defined as deformation in combination with gel formation of gelatine and kappa-carrageenan drops in an elongation flow, was studied. The focus was to investigate the possibility of shaping and fixating small drops in the diameter range 20 to 229 mum. In the shaping progress and the influence of experimental properties, the viscosity, temperature, and flow of the deforming fluid were examined on the final drop shape. In the experiments a hot emulsion of an aqueous biopolymer solution in silicone oil was injected into cold silicone oil where a deforming elongation flow field existed. After injection, a temperature decrease in the drops resulted in a gel formation of the biopolymer and a fixation of the deformed drop in the flow. The shape was measured and the effect on the drop aspect ratio was determined by image analysis. Over the total drop diameter range, kappa-carrageenan was more ellipsoid-shaped than gelatine, with a maximum aspect ratio of 6 compared to 4 for gelatine. For small drops, around 22 mum, it is possible to shape kappa-carrageenan, but for gelatine small drops tend to be unaffected. An increase in viscosity, temperature, and flow resulted in an increase in the final fixated shape of the drops. The differences in drop deformation between the biopolymers were explained by drop-viscosity/oil differences and differences in the kinetics of gel formation. The different gel formation kinetics resulted in a short, well-defined, shaping process for kappa-carrageenan, while for gelatine the process was more complex, with both deformation and relaxation present at different stages.     

Bis(dimethylmetall(III)glyoximato)metallate(II) (metall(III) = Al,Ga, In,metall(II) = Ni,Pd, Pt,Cu)

When the trimethyl derivatives of aluminium, gallium and indium react with glyoximato metallates, (R₂C₂N₂(O)OH)₂Met^II (R = H, CH₃; Met^II = Ni, Pd, Pt, Cu), in a $\text{2}\text{1}$ molar ratio, 2 mol of methane are evolved and monomeric bis(dimethylmetal(III)glyoximato)metallates(II) (metal(III) = Al, Ga, In) are formed in high yields. The vibrational and NMR spectra of the new complexes were measured and were partly resolved. The X-ray structure determinations of two of these compounds show non-planar structures of approximate C_2h and C₂ symmetry, respectively, with weak metal(III)?metal(II) π-interactions.