Comparison of P···D (D = P,N) with other noncovalent bonds in molecular aggregates

All the minima on the potential energy surfaces of homotrimers and tetramers of PH(3) are identified and analyzed as to the source of their stability. The same is done with mixed trimers in which one PH(3) molecule is replaced by either NH(3) or PFH(2). The primary noncovalent attraction in all global minima is the BP···D (D = N,P) bond which is characterized by the transfer of charge from a lone pair of the donor D to a σ? B-P antibond of the partner molecule which is turned away from D, the same force earlier identified in the pertinent dimers. Examination of secondary minima reveals the presence of other weaker forces, some of which do not occur within the dimers. Examples of the latter include PH···P, NH···P, and PH···F H-bonds, and "reverse" H-bonds in which the source of the electron density is the smaller tail lobe of the donor lone pair. The global minima are cyclic structures in all cases, and exhibit some cooperativity, albeit to a small degree. The energy spacing of the oligomers is much smaller than that in the corresponding strongly H-bonded complexes such as the water trimer.     

Photodissociation dynamics of the methyl radical at 212.5 nm: effect of parent internal excitation

Photodissociation dynamics of the CH3 radical at 212.5 nm has been investigated using the H atom Rydberg tagging time-of-flight method with a pure CH3 radical source generated by the photolysis of CH3I at 266 nm. Time-of-flight spectra of the H atom products from the photolysis of both cold and hot methyl radicals have been measured at different photolysis polarizations. Experimental results indicate that the photodissociation of the methyl radical in its ground vibrational state at 212.5 nm excitation occurs on a very fast time scale in comparison with its rotational period, indicating the CH3 dissociation at 212.5 nm occurs on the excited 3s Rydberg state surface. Experimental evidence also shows that the photodissociation of the methyl radical in the nu2 = 1 state of the umbrella mode at 212.5 nm excitation is characteristically different from that in the ground vibrational state.     

Thermodynamics of binding of 2-methoxy-3-isopropylpyrazine and 2-methoxy-3-isobutylpyrazine to the major urinary protein

In the present study we examine the thermodynamics of binding of two related pyrazine-derived ligands to the major urinary protein, MUP-I, using a combination of isothermal titration calorimetry (ITC), X-ray crystallography, and NMR backbone (15)N and methyl side-chain (2)H relaxation measurements. Global thermodynamics data derived from ITC indicate that binding is driven by favorable enthalpic contributions, rather than the classical entropy-driven hydrophobic effect. Unfavorable entropic contributions from the protein backbone and side-chain residues in the vicinity of the binding pocket are partially offset by favorable entropic contributions at adjacent positions, suggesting a "conformational relay" mechanism whereby increased rigidity of residues on ligand binding are accompanied by increased conformational freedom of side chains in adjacent positions. The principal driving force governing ligand affinity and specificity can be attributed to solvent-driven enthalpic effects from desolvation of the protein binding pocket.     

Improvement of the traceability of reference materials certified for purity: evaluation of a semi-preparative high-performance liquid chromatography approach

A semi-preparative high-performance liquid chromatography process was evaluated as a tool to quantitatively determine the purity or percentage mass fraction content (% m/m) of organic compounds. The method is simple and does not require the identification and subsequent quantitation of organic-related structure impurities. A protocol was developed and tested on four reference materials certified for purity from 95% m/m to 99.3% m/m. Comparing the purity results of each certified reference material using the new approach with their respective certified values showed no significant analytical bias. Semi-preparative high-performance liquid chromatography has proved the potential to be a primary method directly traceable to mass with an uncertainty statement written down also in terms of mass with expanding uncertainty ranging from 0.8% to 1.3% m/m compared to 0.3 to 2.0% m/m for the certified purity values at the 95% confidence interval.     

Characterization of ethyl chloroformate derivative of <Emphasis Type="Italic">β</Emphasis>-methylamino-<Emphasis Type="SmallCaps">l</Emphasis>-alanine

Beta-methylamino-L-alanine (BMAA) is a neurotoxic amino acid that can be produced by cyanobacteria in aqueous environments. To analyze this compound by gas chromatography/mass spectrometry (GC/MS), BMAA must be derivatized to a nonpolar, volatile compound. This can be accomplished by reacting BMAA with ethyl chloroformate. While carrying out electron ionization (EI) mass spectrometric analysis on the (13)C-labeled derivative, it was discovered that the formation of an ion with a peak at m/z 245.12 is the result of [CH(3)CH(2)O.] loss from the amino groups resulting from alpha-cleavage. This differs from previous reports that attributed this peak to alpha-cleavage of the carboxylic ester portion of the BMAA derivative. This finding is important for understanding BMAA derivative mass spectrometric fragmentation patterns and ultimately to properly identifying and quantifying BMAA. Fragmentation pathways for the formation of other major peaks observed in the EI mass spectra are also proposed.     

Use of ETAAS,FAAS and ICP-OES for production and post-certification monitoring of a multielemental certified reference material according to national regulation limits for elements in drinking water

Accreditation and Quality Assurance - This paper describes the development of a certified reference material (CRM), produced in the National Institute of Quality from Peru (INACAL) in order to...     

Computing inertia sets using atoms

We consider the problem of computing inertia sets for graphs. By using tools for combining the inertia sets of smaller graphs we can reduce this problem to understanding the inertia sets for three-connected graphs that are not joins. We term such graphs atoms and give the inertia sets for all atoms on at most seven vertices. This can be used to compute the inertia sets for all graphs on at most seven vertices.