Studies of intermolecular interactions by matrix isolation vibrational spectroscopy and normal coordinate analysis: Self-association of hydrogen cyanide

A detailed study of the infrared spectra of hydrogen and deuterium cyanide in noble gas, nitrogen and carbon monoxide matrices at 4 K and 20 K has enabled the bands observed to be assigned to monomer, dimer, trimer, tetramer and higher multimer species. Force constants calculated for the linear dimer are used to predict the spectrum of the trimer. The nature of the trapping sites occupied by the monomer and linear dimer, and the structure and trapping site of a second type of dimer observed only in argon matrices, are discussed.     

The photochemistry of methyl cyclobutyl ketone. Part 2. Temperature dependence and the acetyl radical decomposition

Following earlier room-temperature studies, gaseous mixtures of methyl cyclobutyl ketone (MCK) diluted in argon have been photolyzed at temperatures up to 205°C. Experiments have been carried out at a variety of pressures (up to ca. 2 atm) at wavelengths of 313 nm (steady state conditions) and 308 nm (pulsed photolysis). The results are consistent with a mechanism dominated by radical-radical reactions involving acetyl, methyl, and cyclobutyl radicals. Acetyl radical processes predominate at lower temperatures while methyl radical reactions are more important at high temperatures. The results are interpreted via kinetic modelling of a mechanism in which a key role is played by the acetyl radical decomposition reaction Values for k₃ have been obtained and its temperature and pressure dependence are fitted by RRKM theory and a weak-collisional activation model to yield This high-pressure limiting Arrhenius equation is consistent with other studies in the same temperature range, but is difficult to reconcile with higher temperature investigations.     

Measurement of Er:YAG laser ablation plume dynamics

The dynamics of tissue ablation using an Er:YAG laser were studied using flash photography and optical pump-probe techniques. Both normal-spiking-mode and Q-switched Er:YAG laser radiation were used to study the ablation of skin and bone. Time-resolved photographs of the ablation plume were obtained using a microscope-mounted camera together with pulsed illumination from an excimer-pumped dye laser. The velocity of the plume front, obtained from the photographs, was approximately 1400 m/s. The same velocity was also measured using an optical pump-probe technique. Both techniques indicate that material removal occurred after the end of the 90-ns-long Q-switched laser pulse and that each micropulse in the normal-spiking-mode pulse train was capable of ablating and rapidly ejecting tissue.This work was supported in part by the SDIO-MFEL Program under contract # N00014-86-K-0117 and by the Arthur O. and Gullan M. Wellman Foundation     

Rapid high-yield purification of neuropeptides from canine intestinal muscle

A simplified method for purification of gastrointestinal neuropeptides from relatively small amounts of tissue is described. Sequential adsorption, gel filtration, ion-exchange chromatography, and high-performance liquid chromatography without lyophilization produced good yields of somatostatin, bombesin, and vasoactive intestinal polypeptide immunoreactivity. The method is suitable for simultaneous purification of several small, basic peptides from limited amounts of starting material.     

Microarrays of tagged combinatorial triazine libraries in the discovery of small-molecule ligands of human IgG

A novel and highly diverse tagged triazine library incorporating a triethylene glycol-based linker was synthesized using an orthogonal combinatorial approach on the solid phase and covalently immobilized on a glass substrate as a small molecule microarray (SMM). The SMM was screened with a fluorophore-conjugated human IgG, and 4 novel binders from a library of 2688 compounds were identified from the fully spatially addressable array without the need for compound decoding. Using surface plasmon resonance (SPR) analysis, binding seen on the array was confirmed, and a binding constant as low as Kd = 2.02 x10(-6) M was measured.     

Incorporation of glucose analogs by GtfE and GtfD from the vancomycin biosynthetic pathway to generate variant glycopeptides

Analogs of the glycopeptide antibiotics vancomycin and teicoplanin with alterations in one or both sugar moieties of the disaccharide have been prepared by tandem action of the vancomycin pathway glycosyltransferases GtfE and GtfD. All four regioisomers (2-, 3-, 4-, 6-) of TDP-deoxyglucoses and UDP/TDP-aminoglucoses were prepared, predominantly by action of D-glucopyranosyl-1-phosphate thymidylyltransferase, E(p). GtfE transferred the deoxyglucoses or aminoglucoses onto the 4-OH of 4-hydroxyphenylglycine of both the vancomycin and teicoplanin aglycone scaffolds. Kinetic analysis indicated the 2-, 3-, 4-, and 6-amino-glucoses were transferred by GtfE with only a 4- to 30-fold drop in k(cat) and no effect on K(m) compared to the native substrate, UDP/TDP-glucose, suggesting preparative utility. The next enzyme, GtfD, could utilize the variant glucosyl-peptides as substrates for transfer of L-4-epi-vancosamine. The aminosugar moieties in these variant glycopeptides introduce sites for acylation or reductive alkylation.     

Ionisation characteristics and elimination rates of some aminoindanones determined by capillary electrophoresis

Capillary electrophoresis (CE) has emerged as an important tool for evaluating the ionisation characteristics of compounds and their corresponding pKa values. A particular strength of CE in this context is that its relative selectivity allows one to measure the extent of ionisation of materials that are impure. In this study, using CE, we have measured the pKa values of a series of anti-inflammatory aminoindanones, which underwent degradation to indenone during the course of the determination. We subsequently monitored the elimination reactions measuring remaining indanone over the pH range 2.6-10 at constant ionic strength and temperature. The decomposition of the tertiary amino derivatives was especially fast with first-order half-lives of less than 10 min observed at pH 7.4. The resulting sigmoidal pH rate profiles can be accounted for by assuming unimolecular elimination of the protonated (slow) and neutral (fast) forms of the amines. This study provides further support for the use of CE in evaluating amino ionisation especially in cases where degradation might be expected.     

Comparison of reaction pathways calculated by different algorithms for disilane and water trimer

There still exists some confusion in the literature concerning the definition of a minimum energy pathway and the coordinate system in which it is calculated. Here we compare steepest-descent and eigenvector-following pathways, both with and without a mass-weighted metric. The systems studied are disilane and the water trimer, and we employ various basis sets at the SCF level of theory. We find that paths calculated using eigenvector-following and steepestdescent are practically the same, at least in terms of the reaction mechanism.We find that for the mass-weighted metric the pathways are similar, although in principle they do not have to be identical. Finally, we verify that the geometrical symmetry selection rules hold for a pathway mediated by a recently discovered transition state of the disilane system.