Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Characterizing the dynamics of auditory perception

After listening to a sound that is presented repeatedly, subjects report hearing different transforms of the original sound. The frequency of reported transforms is a sensitive index of some speech disorders as well as cognitive flexibility in aging. In this paper, we propose and investigate quantitative measures that characterize the dynamics of this phenomenon, known as the verbal transformation effect. In particular, we show that the distribution of the dwell time, the time spent perceiving a string of a given phonemic form before switching to another form, obeys a power law for normal subjects with an exponent valued between 1 and 2. This result suggests that within this paradigm there is no characteristic time scale for the perceptual process. Additionally, we analyze the correlation properties of the transforms. We suggest that the complexity measures and techniques introduced here might be useful diagnostic tools for a number of speech and cognitive disorders. (c) 1995 American Institute of Physics.     

Dynamic behavior of DNA base pairs containing 8-oxoguanine

The process by which DNA repair enzymes recognize and selectively excise damaged bases in duplex DNA is fundamental to our mechanistic understanding of these critical biological reactions. 8-Oxoguanine (8-oxoG) is the most common form of oxidative DNA damage; unrepaired, this lesion generates a G:C-->T:A mutation. Central to the recognition and repair of DNA damage is base extrusion, a process in which the damaged base lesion or, in some cases, its partner disengages from the helix and is bound to the enzyme's active site where base excision takes place. The conformation adopted by 8-oxoG in duplex DNA is affected by the base positioned opposite this lesion; conformational changes may also take place when the damaged base binds to its cognate repair enzyme. We performed unrestrained molecular dynamics simulations for several 13-mer DNA duplexes. Oligomers containing G:C and 8oxoG:C pairs adopted Watson-Crick geometries in stable B-form duplexes; 8oxoG showed increased local and global flexibility and a reduced barrier to base extrusion. Duplexes containing the G:A mismatch showed much larger structural fluctuations and failed to adopt a well-defined structure. For the 8oxoG:A mismatch that is recognized by the DNA glycosylase MutY, the damaged nucleoside underwent spontaneous and reproducible anti-->syn transitions. The syn conformation is thermodynamically preferred. Steric hindrance and unfavorable electrostatics associated with the 8oxoG O8 atom in the anti conformation were the major driving forces for this transition. Transition events follow two qualitatively different pathways. The overall anti-->syn transition rate and relative probability of the two transition paths were dependent on local sequence context. These simulations indicate that both the dynamic and equilibrium behavior of the duplex change as a result of oxidation; these differences may provide valuable new insight into the selective action of enzymes on damaged DNA.     

Density variation in the strain-confined electron-hole liquid in Ge

In this paper we theoretically analyze two mechanisms which could account for the experimentally observed increase in pair density for the strain-confined electron-hole liquid (EHL) in Ge. We find that the change in drop density with uniform stress is insufficient to explain the experimental result. However, we find that the strain gradient in the well acts to compress the liquid sufficiently to explain the observed density increases. Densities of twice the equilibrium value can be easily obtained for large enough drop size, but the density should vary by < 10% if the drop radius is < 100 μm.     

Linkage isomerism in the binding of pentapeptide Ac-His(Ala)3His-NH2 to (ethylenediamine)palladium(II): effect of the binding mode on peptide conformation

The reaction of the pentapeptide Ac-His1-Ala2-Ala3-Ala4-His5-NH2 (AcHAAAHNH2) (1) with [Pd(en)(ONO2)2] (en = NH2CH2CH2NH2) in either DMF-d(7) or H2O:D2O (90%:10%) gave three linkage isomers of [Pd(en)(AcHAAAHNH2)](2+) (2), 2a, 2b, and 2c, which differ only in which pair of imidazole nitrogen atoms bind to Pd. In the most abundant isomer, 2a, Pd is bound by N1 from each of the two imidazole rings. In the minor isomers 2b and 2c, Pd is bound by N1(His1) and N3(His5) and by N3(His1) and N1(His5), respectively. The reactions of [Pd(en)(ONO2)2] with the N-methylated peptides Ac-(N3-MeHis)-Ala-Ala-Ala-(N3-MeHis)-NH2 (AcH*AAAH*NH2) (3), Ac-(N3-MeHis)-Ala-Ala-Ala-(N1-MeHis)-NH2 (AcH(*)AAAH(#)NH2) (4), and Ac-(N1-MeHis)-Ala-Ala-Ala-(N3-Me-His)-NH2 (AcH(#)AAAH(*)NH2) (5) each gave a single species [Pd(en)(peptide)](2+) in N,N-dimethylformamide (DMF) or aqueous solution, 7, 8, and 9, respectively, with Pd bound by the two nonmethylated imidazole nitrogen atoms in each case. These complexes were analogous to 2a, 2b, and 2c, respectively. Ac-(N1-MeHis)-Ala-Ala-Ala-(N1-MeHis)-NH2 (AcH(#)AAAH(#)NH2) (6) with [Pd(en)(ONO2)2] in DMF slowly gave a single product, [Pd(en)(AcH(#)AAAH(#)NH2)](2+) (10), in which Pd was bound by the N3 of each imidazole ring. The corresponding linkage isomer of 2 was not observed. Complex 10 was also the major product in aqueous solution, but other species were also present. All compounds were exhaustively characterized in solution by multinuclear 1D ((1)H , (13)C, and, with (15)N-labeled ethylenediamine, (15)N) and 2D (correlation spectroscopy, total correlation spectroscopy, transverse rotating-frame Overhauser effect spectroscopy (T-ROESY), heteronuclear multiple-bond correlation, and heteronuclear single quantum coherence) NMR spectra, circular dichroism (CD) spectra, electrospray mass spectroscopy, and reversed-phase high-performance liquid chromatography. ROESY spectra were used to calculate the structure of 2a, which contained a single turn of a peptide alpha helix in both DMF and water, the helix being better defined in DMF. The Pd(en)(2+) moiety was not used in structure calculations, but its location and coordination by one imidazole N1 from each histidine to form a 22-membered metallocycle were unambiguously established. Convergence of the structures was greatest when calculated with two hydrogen-bond constraints (Ala4 peptide NH...OC acetyl and His5 peptide NH...OC-His1) that were indicated by the low temperature dependence of these NH chemical shifts. Vicinal HN-CHalpha coupling constants and chemical shifts of alpha-H atoms were also consistent with a helical conformation. Similar long-range ROE correlations were observed for [Pd(en)(AcH(*)AAAH(*)NH2)](2+) (7), which displayed a CD spectrum in aqueous solution that suggested the presence of some helicity. Long-range ROE correlations were not observed for 8, 9, or 10, but a combination of NMR data and CD spectroscopy was interpreted in terms of the conformational behavior of the coordinated pentapeptide. Only for the linkage isomer [Pd(en)(AcH(*)AAAH(#)NH2)](2+) (8) was there evidence of a contribution from a helical conformation. The data for 8 were interpreted as interconversion between the helix and random coil conformations. Zn(2+) with peptides gave broad NMR peaks attributed to lability of this metal ion, while reactions of cis-[Pt(NH3)2(ONO2)2] were slow, giving a complex mixture of products rather than the macrochelate ring observed with Pd(en)(2+). In summary, these studies indicate that Pd(en)(2+) coordinates to histidine with similar preference for each of the two imidazole nitrogens, enabling the formation of up to four linkage isomers in its complexes with pentapeptides His-xxx-His. Only the N1-N1 linkage isomer that forms a 22-membered macrochelate ring is able to induce an alpha-helical peptide conformation, whereas the 20- and 21-membered rings of linkage isomers do not. This suggests that linkage isomeric mixtures may compromise histidine coordination to metal ions and reduce alpha-helicity.     

A dynamic study of earthworm feeding ecology -using stable isotopes

Changes in the specific diet of earthworms with time in relation to landuse changes and two different climates were studied by analysing (13)C and (15)N natural abundance in soils and animals. Soil samples from three depths (0-10, 10-20 and 20-30 cm) and earthworms were collected from two sites: Santiago (Northwest Spain) and North Wyke (Southwest England) both consisting of replicated long-term grasslands and recently converted to maize plots. Earthworms were hand-sorted in the field at the peak of the maize growth and after harvesting at both sites. In the Spanish plots, nine and eight earthworm species, all belonging to the three ecological categories (epigeic, anecic and endogeic), were found under maize and permanent pasture, whereas at the English site five and seven different species were, respectively, identified. At both sites (13)C isotopic values of the earthworm tissues reflected changes in diet from C(3) to C(4) with epigeic and epi/anecic worms in the maize plots showing one delta unit difference in relation to the ones found in the grassland plots. Anecic worms seemed to be less responsive to landuse changes. The higher (13)C values of the Spanish soils were also reflected in the earthworm tissues when compared with the English samples. (15)N values showed no clear relationship with the cropping treatments but were clearly related to the ecological grouping, with endogeic worms reaching the highest values whereas for the epigeic and epi/anecic species the lowest values were obtained. This finding was also previously recorded by other authors1 and suggests that, in the future, stable isotope techniques could also be a useful tool in taxonomic studies. Copyright 1999 John Wiley & Sons, Ltd.