On the nature of the Møller-Plesset critical point

It has been suggested [F. H. Stillinger, J. Chem. Phys. 112, 9711 (2000)] that the convergence or divergence of M?ller-Plesset perturbation theory is determined by a critical point at a negative value of the perturbation parameter z at which an electron cluster dissociates from the nuclei. This conjecture is examined using configuration-interaction computations as a function of z and using a quadratic approximant analysis of the high-order perturbation series. Results are presented for the He, Ne, and Ar atoms and the hydrogen fluoride molecule. The original theoretical analysis used the true Hamiltonian without the approximation of a finite basis set. In practice, the singularity structure depends strongly on the choice of basis set. Standard basis sets cannot model dissociation to an electron cluster, but if the basis includes diffuse functions then it can model another critical point corresponding to complete dissociation of all the valence electrons. This point is farther from the origin of the z plane than is the critical point for the electron cluster, but it is still close enough to cause divergence of the perturbation series. For the hydrogen fluoride molecule a critical point is present even without diffuse functions. The basis functions centered on the H atom are far enough from the F atom to model the escape of electrons away from the fluorine end of the molecule. For the Ar atom a critical point for a one-electron ionization, which was not previously predicted, seems to be present at a positive value of the perturbation parameter. Implications of the existence of critical points for quantum-chemical applications are discussed.     

Distinction between supercoiled and linear DNA in transverse agarose pore gradient gel electrophoresis.

Four species of linear DNA and the first four members of a linking series, generated by treatment of plasmid DNA (PUC19, 2.7 kb) with mitochondrial topoisomerase I, were differentiated by transverse agarose pore gradient gel electrophoresis. The experimental curves of migration distance vs. agarose concentration (Ferguson curves) of supercoiled DNA exhibit a steeper trajectory than those of linear DNA of the same size range. As a consequence, the four supercoiled species exhibit an increase in apparent size (relative to linear DNA standards) with increasing agarose concentration. Both the crossing of the Ferguson curves with those of linear standards as well as the apparent size increase with agarose concentration can serve to detect supercoiled plasmid-sized DNA in mixtures with linear DNA.     

Molecular sieving of lambda phage DNA in polyacrylamide solutions as a function of the molecular weight of the polymer.

Electrophoresis of lambda phage DNA was carried out in solutions at various concentrations of uncrosslinked polyacrylamide of 0.6, 1, 5 and 9 x 10(6) molecular weight (Mw) with narrow Mw distribution. By inspection of mobilities in the various concentration ranges, it appears that mobilities decrease, and retardation increases, with increasing Mw. The relation between electrophoretic retardation and the Mw of the polymer was also interpreted (i) in the manner previously applied to nonlinear Ferguson plots and compatible with the Ogston model; and (ii) empirically, on the basis of the first derivatives of the functions describing the Ferguson plots at the polymer concentrations used. Interpretation (i) shows that the retardation increases linearly in the order of 0.6, 1, 5 and 9 x 10(6) Mw of polyacrylamide. Interpretation (ii) shows a nonlinear increase of retardation in the Mw range 5 to 9 x 10(6), and a decrease in retardation as Mw is raised from 0.6 to 5.0 x 10(6). Hypothetically, interpretation (ii) can be explained mechanistically by a progressive change, as the polymer size is increased, from a collision with the surface of the polymer fiber to one occurring after permeation in the interior of a random-coiled fiber. Interpretation (i) may fail to detect that change due to the large difference between DNA mobility in solutions of the smallest polymer and the free mobility. DNA peak detection in all of the four size classes of polyacrylamide in solution is limited to relatively narrow ranges of polymer concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

DNA-templated three-branched nanostructures for nanoelectronic devices

Three-branched DNA molecules have been designed and assembled from oligonucleotide components. These nucleic acid constructs contain double- and single-stranded regions that control the hybridization behavior of the assembly. Specific localization of a single streptavidin molecule at the center of the DNA complex has been investigated as a model system for the directed placement of nanostructures. Highly selective silver and copper metallization of the DNA template has also been characterized. Specific hybridization of these DNA complexes to oligonucleotide-coupled nanostructures followed by metallization should provide a bottom-up self-assembly route for the fabrication and characterization of discrete three-terminal nanodevices.     

Singularity and unanimity

Examples are adduced which lead one to ask if the following rule of unanimity makes sense: Given, a classical dynamical problem. Given, that all solutions of the equations of motion (a) run into a singularity [or (b) are free of singularity], except a set of measure zero. Then (rule of unanimity), all solutions of the corresponding quantum mechanical problem are (a) singular [or (b) free of singularity]. If valid, this rule would imply that quantization of Einstein's standard general relativity model for a closed universe gives no escape from the singularity of gravitational collapse.Dedicated to Achille Papapetrou on the occasion of his retirement.     

DETECTION OF DNA-PSORALEN PHOTOADDUCTS in situ

Abstract— An immunological method, with the use of specific immune serum, has been developed for detection of 8-methoxypsoralen (8-MOP) photoadducts to DNA, formed in situ in cell nuclei, after combined treatment with 8MOP and UV-A irradiation (Zarçbska et al. , 1978). Lymphocytes fixed on slides or in suspension, and cryostat sections of different mammalian tissues, served as antigenic substrate, after treatment with 8-MOP and UV-A in vitro. Specific fluorescence in these substrates was detected in the nuclei after treatment with 30 ˜ 140 kJ/m² UV-A in the presence of 0.1-0.3 μg/cm² 8-MOP. PHA-stimulated-lymphocytes appeared to be the most sensitive substrate.
However, hairless mice treated with high doses of UV-A in vivo , 70 ˜ 360 kJ/m² did not reveal a specific fluorescence of epidermal nuclei, unless a high local concentration of 8-MOP was attained.
The apparent discrepancy in the level of photoadduct detection between the in vitro and in vivo treated specimens was explained by the low number of DNA-8-MOP-photoadducts formed in vivo under these experimental conditions. The relevance of these findings to the role of DNA-8-MOP-photoadducts formed during PUVA photochemotherapy is discussed.     

Information on DNA conformation derived from transverse pore gradient gel electrophoresis in conjunction with an advanced data analysis applied to capillary electrophoresis in polymer media.

Abnormally slow migration of DNA is conventionally viewed as being due to an abnormal conformation relative to "linear" standards. The evidence for this rests on a few instances where nonlinear DNA structures have been established by independent methods and yield low mobilities relative to standards. Transverse pore gradient gel electrophoresis of authentically bent kinetoplast DNA and of an upstream activator sequence (UAS) of an E. coli operon promoter shows in addition that curves of migration distance vs. gel concentration ("Ferguson curves") of such abnormally conformed DNA differ from those of "linear" standards. Since Ferguson curves are interpretable with regard to molecular size in concordance with a mathematical model (Ogston model), transverse pore gradient gel electrophoresis provides a simple means of correlating abnormally slow migration of DNA with molecular size. In addition, transverse pore gradient gel electrophoresis is able to distinguish between DNA banding which exhibits a steeper dependence on gel concentration than "linear" standards from one which shows the same dependence. The former appears characteristic of circularly bent DNA and gives rise to a substantial retardation, the latter of bending across a knot or kink in the DNA chain associated with a relatively minor retardation relative to standards. Circularly bent restriction fragments formed from kinetoplast DNA retain the characteristic intersecting Ferguson curves on the transverse pore gradient gel. Another authentically "abnormal" DNA structure recognizable on transverse pore gradient gels is supercoiled DNA derived from the reaction of topoisomerase with a plasmid. Different lengths of supercoiled sequences give rise to parallel Ferguson curves clearly intersecting with those of linear standards.(ABSTRACT TRUNCATED AT 250 WORDS)     

Proteomic analysis of the endoplasmic reticulum from developing and germinating seed of castor (Ricinus communis)

Endoplasmic reticulum (ER) has been prepared and analysed from germinating and developing castor bean endosperm. A combination of one- and two-dimensional (1-D and 2-D) gel electrophoresis was used to study the complexity of sample and protein differences between the two stages. The ER of the developing oilseed is central to the synthesis, sorting and storage of protein and lipid reserves while the germinating seed is concerned with their degradation. Sample complexity has been reduced by separation of ER proteins into lumenal, peripheral membrane and integral membrane subfractions. Membrane proteins pose specific problems in aggregation and binding to passive surfaces. We have overcome this by collection of membranes at density gradient interfaces and by silanization of plastic ware. Several major components have been identified from 1-D gels by N-terminal sequencing and matrix-assisted laser desorption/ionization (MALDI) peptide mass fingerprints. These include protein disulphide isomerase (PDI), calreticulin and developing-ER-specific oleate-12-hydroxylase involved in the biosynthesis of ricinoleic acid. In excess of 300 spots are detectable in each developmental fraction by high sensitivity 2-D gels. This is the first 2-D electrophoretic analysis of plant ER. These gels reveal significant differences between germinating and developing ER. Preparative loading 2-D gels of germinating ER have been run and 14 selected spots characterized by quadrupole time of flight tandem mass spectrometry (Q-TOF MS/MS). Ten of these proteins were assigned function on the basis of identity with existing castor database entries, or by homology with other species. Two proteins, aspartate proteinase precursor and N-carbamyl-L-aminohydrolase-like protein, appear to be absent from developing profiles. Most of the proteins identified are concerned with roles in protein processing and storage, and lipid metabolism which occur in the ER. Data from three of the assigned spots included unidentified peptides indicating the presence of more than one protein in these spots following 2-D electrophoresis. More extensive analysis will have to await developments in genomics but the basic separation technologies to simplify sample identity for a plant ER preparation have been established.     

Electrostatic influence on rotational mobilities of sol-gel-encapsulated solutes by NMR and EPR spectroscopies

The rotational mobilities of small solute molecules encapsulated in tetramethyl orthosilicate (TMOS) sol-gels have been investigated by EPR spectroscopy of encapsulated nitroxide probes and by high-resolution NMR spectroscopic measurements of transferred NOE's (trNOE's), of T(1)'s, and of T(1)'s in the rotating frame (T(1)rho). The two spectroscopic methods are sensitive to motions on different time scales and hence, are nicely complementary. Suites of neutral, positively, and negatively charged nitroxide probes (EPR) and of simple diamagnetic small molecules (NMR) were selected to disclose influences of electrostatic interactions with the sol-gel walls and to probe the presence of multiple populations of molecules in distinct regions of the sol-gel pores. For neutral and negatively charged solute probes, both techniques disclose a single population with a significantly increased average rotational correlation time, which we interpret at least in part as resulting from exchange between free-volume and transiently immobilized surface populations. The electrostatic attraction between cationic probes and the negatively charged sol-gel walls causes the positively charged probes to be more effectively immobilized and/or causes a greater percentage of probes to undergo this transient immobilization. The EPR spectra directly disclose a population of cationic probes which are immobilized on the X-band EPR time scale: tau(c) greater than or approximately equal 10(-7) s. However, NMR measurements of trNOE's and of T(1)rho demonstrate that this population does exchange with the free-volume probes on the slower time scale of NMR. This approach is equally applicable to the study of solutes within other types of confined spaces, as well.     

Solid-Phase Extraction and Capillary Gas Chromatographic Determination of Triazine Herbicides in Water

Abstract

A facile and efficient method is described for the determination of trace quantities of triazine herbicides, terbutryn, prometryn and ametryn in water. The procedure involved preconcentration of water samples by sorption on chromatographic grade silica gel particles with chemically modified surface, being covalently bonded with a nonofunctional C₈H₁₇ group. This was followed by solvent desorption with 2-propanol. The determinative step was achieved by capillary gas chromatography on Supelcowax-10 fused silica column using a nitrogen-phosphorus detector. The limit of detection was 0.1 μg-10 μgL^?1.