Specific assay for endotoxin using immobilized histidine, Limulus amoebocyte lysate and a chromogenic substrate.

The Limulus amoebocyte lysate (LAL) test is inhibited or enhanced by many substances. In order to overcome this problem, a specific endotoxin assay method using a membrane filter unit, a chromogenic LAL reagent, and immobilized histidine (which is a specific adsorbent for endotoxins) was developed. Endotoxins are quantitatively adsorbed on immobilized histidine. The adsorbed endotoxins are separated from LAL-inhibiting or -enhancing substances by the membrane filter unit, and their activities are directly assayed with the LAL reagent in a filter cup without any inhibition or enhancement. The reproducibility and the accuracy of this method are high. This new endotoxin assay method using immobilized histidine can be used for the determination of endotoxins in a solution containing LAL-inhibiting or -enhancing substances such as amino acids and antibiotics, as an alternative to the more common gel-clot technique.     

A note on the importance of including monoatomic overlap densities in the calculation of CNDO/2 charge distributions

It is demonstrated that in the calculation of CNDO/2 charge density distributions the monoatomic overlap densities necessarily must be taken into account. Otherwise electron densities are obtained which are not invariant to molecular rotations and generally of wrong symmetry.     

Strikingly Different Effects of Hydrogen Bonding on the Photodynamics of Individual Nucleobases in DNA: Comparison of Guanine and Cytosine

Ab initio surface hopping dynamics calculations were performed to study the photophysical behavior of cytosine and guanine embedded in DNA using a quantum mechanical/molecular mechanics (QM/MM) approach. It was found that the decay rates of photo excited cytosine and guanine were affected in a completely different way by the hydrogen bonding to the DNA environment. In case of cytosine, the geometrical restrictions exerted by the hydrogen bonds did not influence the relaxation time of cytosine significantly due to the generally small cytosine ring puckering required to access the crossing region between excited and ground state. On the contrary, the presence of hydrogen bonds significantly altered the photodynamics of guanine. The analysis of the dynamics indicates that the major contribution to the lifetime changes comes from the interstrand hydrogen bonds. These bonds considerably restricted the out-of-plane motions of the NH(2) group of guanine which are necessary for the ultrafast decay to the ground state. As a result, only a negligible amount of trajectories decayed into the ground state for guanine embedded in DNA within the simulation time of 0.5 ps, while for comparison, the isolated guanine relaxed to the ground state with a lifetime of about 0.22 ps. These examples show that, in addition to phenomena related to electronic interactions between nucleobases, there also exist relatively simple mechanisms in DNA by which the lifetime of a nucleobase is significantly enhanced as compared to the gas phase.     

Precursor Phenomenon of Laser Ablation in Bismuth

We have investigated the precursor phenomenon of laser ablation in Bi through the observation of the coherent phonons using a reflection-type pump-probe technique with amplified femtosecond pulses. The time-resolved reflectivity changes in Bi under high-density photo-excitation indicate the coherent A1 g phonon oscillation with a time-dependent oscillation period. Analysis of the coherent oscillation with the time-dependent period using a time-partitioning Fourier transform (FT) reveals that the frequency of the A1 g mode depends on the squared amplitude of the oscillation. We discuss these results in terms of an anharmonicity of the lattice potential.     

Kinetics of chemically activated ethane

Chemically activated ethane, with an excitation energy of 114.9 ± 2 kcal/mole, was formed by reaction with methane of excited singlet methylene radicals produced by the 4358 Å photolysis of diazomethane. A decomposition rate constant of (4.6 ± 1.2) × 10⁹ sec^?1 was measured for the chemically activated ethane. This result agrees, via RRKM theory, with most other chemically activated ethane data, and the result predicts, via RRKM and absolute rate theory for E₀ = 85.8 kcal/mole, E* = 114.9 kcal/mole, and k_E = 4.6 × 101 sec^?1, a thermal A-factor at 600°K of 10^16.6±0.2 sec^?1, in approximate agreement with the more recent experimental values. Combining 2 kcal/mole uncertainties in E₀ and E* with the uncertainty in our rate constant yields an A-factor range of 10^16.6±0.7 sec^?1. It is emphasized that this large uncertainty in the A-factor results from an improbable combination of uncertainty limits for the various parameters. These decomposition results predict, via absolute rate theory (with E₀(recombination) = 0) and statistical thermodynamic equilibrium constants, methyl radical recombination rates at 25°C of between 4.4 × 10⁸ to 3.1 × 10⁹ l.-mole^?1-sec^?1, which are 60 to 8 times lower, respectively, than the apparently quite reliable experimental value. A value of E₀(recombination) greater than zero offers no improvement, and a value less than zero would be quite unusual. Activated complexes consistent with the experimental recombination rate and E₀(recombination) = 0 greatly overestimate the experimental chemical activation and high pressure thermal decomposition rate data. Absolute rate theory as it is applied here in a straightforward way has failed in this case, or a significant amount of internally consistent data are in serious error. Some corrections to our previous calculations for higher alkanes are discussed in Appendix II.     

Vibrational spectra of zinc halide complexes with trimethylphosphine oxide

The Raman and IR spectra are reported for the complexes ZnCl₂.2TMPO, ZnBr₂.2TMPO and ZnI₂.2TMPO. The observed frequencies for polycrystalline samples are assigned on the basis of a C_3v structure for the TMPO ligand and a C_2y structure for the ZnX₂O₂ skeletal fundamentals. To discuss the coordination effects of TMPO, the ligand vibrations are compared with those of free TMPO and the skeletal vibrations with those of acetonitrile complexes. Using the ratios of G matrix elements and of observed frequencies for symmetric and asymmetric Zn-X stretching modes, the ZnX₂O₂ skeletal structures are found to be tetrahedral.     

Low wavenumberRaman bands (300−50cm−1) for four isotopically substituted lithium hydrogen oxalate monohydrates LiXC2O4·X 2O

The solid stateRaman spectra (300?50 cm^?1) of lithium hydrogen oxalate monohydrate have been investigated for four isotopically substituted compounds,⁶LiHC₂O₄·H₂O,⁷LiHC₂O₄·H₂O,⁶LiDC₂O₄·D₂O and⁷LiDC₂O₄·D₂O. The observed spectral data have been discussed on the basis of the factor group analysis by considering the H/D and⁶Li/⁷Li isotope effects on the fundamental lattice vibrations.