Showing papers by "Bulgarian Academy of Sciences published in 1998"

FT-IR study of NO + O2 co-adsorption on H-ZSM-5: re-assignment of the 2133 cm-1 band to NO+ species

Abstract: Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O2 to NO led to the appearance of bands at 2133 and 977 cm-1. Concomitantly, the number of acidic zeolite OH groups decreased while H2O hydrogen-bonded to zeolite OH groups developed. Introduction of small amounts of 18O2 did not change the 2133 cm-1 band wavenumber, nor the use of a partly deuteroxylated D–H-ZSM-5 sample. In such a case, HOD formation was detected. The results obtained evidence that the 2133 cm-1 band, generally considered as characterizing NO+2 species, is, in fact, due to NO+ species occupying cationic positions in the zeolite. The 977 cm-1 band is attributed to the Olattice–NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.

Abnormalities in Functional Development of the Sertoli Cells in Rats Treated Neonatally with Diethylstilbestrol: A Possible Role for Estrogens in Sertoli Cell Development

Abstract: Diethylstilbestrol (DES) was administered neonatally (Days 2-12; 10 microg on alternate days) to rats, and developmental changes in Sertoli cell function were evaluated at 18, 25, and 35 days of age and compared to those observed in rats administered a GnRH antagonist (GnRHa; Days 2 and 5; 10 mg/kg) or a vehicle (controls). DES and GnRHa treatments resulted in similar reductions in both Sertoli cell numbers (40% for DES, 48% for GnRHa) and suppression of testicular growth at 18 and 25 days, though by 35 days the suppression was more pronounced (p 60% reduction in testis weight with many Sertoli cell-only tubules and very low daily sperm production. Taken together, these data are interpreted as providing evidence for direct modulation of Sertoli cell (maturational) development by DES.

Developments in ECG acquisition, preprocessing, parameter measurement, and recording

Abstract: The subjects considered in this article follow the logical procedures of acquisition, preprocessing, wave detection, parameter measurement, and recording of the ECG. The authors present an economic procedure for simultaneous multichannel signal acquisition, digital real-time interference elimination and drift suppression, a real-time software QRS detector, and algorithms for wave onset and termination determination. The authors also discuss fast algorithms for lengthwise and crosswise lead presentation that supports the use microdot thermal printers for signal recordings.

Production of flavour compounds by yogurt starter cultures

Abstract: The present work studied the production of carbonyl compounds and saturated volatile free fatty acids by pure cultures of Streptococcus thermophilus and Lactobacillus bulgaricus, and by starter cultures for Bulgarian yogurt during cultivation and cooling. The mixed cultures formed volatile aromatic compounds more actively than the pure cultures. A guiding factor in the preparation of the starter cultures was the biochemical activity of Lactobacillus bulgaricus in synthesizing the major carbonyl compounds, acetaldehyde, diacetyl and the volatile fatty acids C2–C10. The activity of the yogurt cultures in synthesizing carbonyl compounds was at its highest during milk coagulation and cooling, up to 7 h. However, maximum concentration was reached by 22–31 h. In the cooled 22–h starter cultures, acetaldehyde predominated (1415.0–1734.2 μg per 100 g) followed by diacetyl (165.0–202.0 μg per 100 g), acetoin (170.0–221.0 μg per 100 g), acetone (66.0–75.5 μg per 100 g), ethanol (58.0 μg per 100 g), and butanone-2 (3.6–3.8 μg per 100 g). The thermophilic streptococcus and lactobacillus cultures, and the starter cultures contained predominantly acetic, butyric and caproic acids.

Persistent Interactions of Core Histone Tails with Nucleosomal DNA following Acetylation and Transcription Factor Binding

Abstract: In this study, we examined the effect of acetylation of the NH2 tails of core histones on their binding to nucleosomal DNA in the absence or presence of bound transcription factors. To do this, we used a novel UV laser-induced protein-DNA cross-linking technique, combined with immunochemical and molecular biology approaches. Nucleosomes containing one or five GAL4 binding sites were reconstituted with hypoacetylated or hyperacetylated core histones. Within these reconstituted particles, UV laser-induced histone-DNA cross-linking was found to occur only via the nonstructured histone tails and thus presented a unique tool for studying histone tail interactions with nucleosomal DNA. Importantly, these studies demonstrated that the NH2 tails were not released from nucleosomal DNA upon histone acetylation, although some weakening of their interactions was observed at elevated ionic strengths. Moreover, the binding of up to five GAL4-AH dimers to nucleosomes occupying the central 90 bp occurred without displacement of the histone NH2 tails from DNA. GAL4-AH binding perturbed the interaction of each histone tail with nucleosomal DNA to different degrees. However, in all cases, greater than 50% of the interactions between the histone tails and DNA was retained upon GAL4-AH binding, even if the tails were highly acetylated. These data illustrate an interaction of acetylated or nonacetylated histone tails with DNA that persists in the presence of simultaneously bound transcription factors.

Perturbations of quadratic centers

Abstract: We study the bifurcation of limit cycles in general quadratic perturbations of plane quadratic vector fields having a center at the origin. For any of the cases, we determine the essential perturbation and compute the corresponding bifurcation function. As an application, we find the precise location of the subset of centers in Q 3 R surrounded by period annuli of cyclicity at least three. Two specific cases are considered in more detail: the isochronous center S 1 and one of the intersection points ( Q 4 + ) of Q 4 and Q 3 R . We prove that the period annuli around S 1 and Q 4 + have cyclicity two and three respectively. The proof is based on the possibility to derive appropriate Picard-Fuchs equations satisfied by the independent integrals included in the related bifurcation function.

The influence of water on the structure of hydrous sodium tetrasilicate glasses

Abstract: The structure of sodium tetrasilicate (Na 2Si4O9) glasses containing 0 to 10 wt% water was investigated by a combination of Raman, IR, and NMR methods. Both the 29 Si magic angle spinning NMR data and Raman spectra in the Si-O stretching region clearly show that water depolymerizes the silicate network of the glasses. Q-species distributions calculated from Raman spectra, assuming equal scattering cross sections of all bands in the Si-O stretching region, closely agree with results obtained from NMR data. At low total water contents, the silicate network is depolymerized mainly by breaking of Q 4 -Q 4 bonds, whereas breaking of Q 3 -Q 3 bonds dominates at high water contents. Near IR spectra show the presence of both OH groups and molecular H2O in the glasses. The number of nonbridging O atoms per silicon atom, calculated from the near IR data, closely agrees with the results obtained from Raman and NMR, and confirms the assignment of the 4500 cm 21 band in the near IR to a combination mode of Si-OH groups. Moreover, the intensity of the fundamental Si-OH stretching band at 910 cm 21 in the Raman spectra varies proportionally to the intensity of the 4500 cm 21 near IR band. Both IR and Raman spectra show three main bands in the OH-stretching region, centered at 3580, 3000, and 2350 cm 21 , due to hydrous species with different hydrogen bond strengths. The relative intensities of these three bands are insensitive to total water content and OH/H2O ratio, suggesting that both OH and H2O contribute to each of these bands. This is consistent with the fine structure of the H2O bending vibration in the IR spectra around 1640 cm 21 and with the polarization dependence of the OH-stretching bands in the Raman spectra. Near IR spectra of hydrous sodium tetrasilicate glasses and hydrous aluminosilicate glasses are very similar and show a similar dependence of band intensity on total water content, suggesting that there is no fundamental difference in the dissolution mechanism of water in these systems.

Au/α-Fe2O3 catalyst for water–gas shift reaction prepared by deposition–precipitation

Abstract: A modified version of the preparation methods for the synthesis of highly-active Au/α-Fe2O3 catalysts for WGS reaction has been proposed. The results obtained show that gold dispersion is highly preserved and prevents cluster formation. On the other hand, a major part of gold remains on the surface, thus being accessible to catalysis. As a result of the preparation technique, it seems that the interaction between gold and support is weaker and, likely, more favourable for catalysis. This study demonstrates that a specific approach to preparation of catalysts of optimum structure and texture is crucial for high-catalytic activity.

Diffraction from Polarization Holographic Gratings with Surface Relief in Side-chain Azobenzene Polyesters

Abstract: We investigate the polarization properties of holographic gratings in side-chain azobenzene polyesters in which an anisotropic grating that is due to photoinduced linear and circular birefringence is recorded in the volume of the material and a relief grating appears on the surface. A theoretical model is proposed to explain the experimental results, making it possible to understand the influence of the different photoinduced effects. It is shown that at low intensity the polarization properties of the diffraction at these gratings are determined by the interaction of the linear and circular photobirefringences, and at larger intensity the influence of the surface relief dominates the effect of the circular anisotropy. Owing to the high recording efficiency of the polyesters, the ±1-order diffracted waves change the polarization interference pattern during the holographic recording, resulting in the appearance of a surface relief with doubled frequency.

Single-sweep analysis of the theta frequency band during an auditory oddball task.

Abstract: The P300 component and the oscillatory 4-7 Hz electroencephalographic activity of auditory event-related brain potentials (ERPs) were assessed to study differences between passive and oddball task conditions. Theta responses from 15 adults were analyzed for single-sweep amplitude, phase locking, and enhancement against prestimulus activity. ERPs were characterized by enhanced and strongly phase-locked theta oscillations in the early (0-300 ms) poststimulus epoch, with only the late (300-600 ms) theta responses at Fz and Pz affected by the oddball condition. P300 was strongly associated not only with the concurrent theta oscillations but also with the evoked theta activity preceding P300 (0-300 ms). It was concluded that single theta response parameters can reveal specific functional differences between passive and oddball conditions and that a strong relationship exists between the theta frequency component and the time domain P300 ERP component.

A model for geometric and mechanical adaptation of arteries to sustained hypertension

Abstract: This study aimed to model phenomenologically the dynamics of arterial wall remodeling under hypertensive conditions. Sustained hypertension was simulated by a step increase in blood pressure. The arterial wall was considered to be a thick-walled tube made of nonlinear elastic incompressible material. Remodeling rate equations were postulated for the evolution of the geometric dimensions of the hypertensive artery at the zero-stress state, as well as for one of the material constants in the constitutive equations. The driving stimuli for the geometric adaptation are the normalized deviations of wall stresses from their values under normotensive conditions. The geometric dimensions are modulated by the evolution of the deformed inner radius, which serves to restore the level of the flow-induced shear stresses at the arterial endothelium. Mechanical adaptation is driven by the difference between the area compliance under hypertensive and normotensive conditions. The predicted time course of the geometry and mechanical properties of arterial wall are in good qualitative agreement with published experimental findings. The model predicts that the geometric adaptation maintains the stress distribution in arterial wall to its control level, while the mechanical adaptation restores the normal arterial function under induced hypertension.

Seasonal variations of the chemical composition of Brazilian propolis

Abstract: Institute of Organic Chemistry Centre of Phytochemistry Bulgarian Academy of Sciences, 1113 Sofia

Bimodality and Gaps on Globular Cluster Horizontal Branches. II. The Cases of NGC 6229, NGC 1851, and NGC 2808

Abstract: The outer halo globular cluster NGC 6229 has a peculiar horizontal-branch (HB) morphology, with clear indications of a bimodal HB and a gap on the blue HB. In this paper, we present extensive synthetic HB simulations to determine whether peculiar distributions in the underlying physical parameters are needed to explain the observed HB morphology. We find that a unimodal mass distribution along the HB can satisfactorily account for the observed HB bimodality, provided the mass dispersion is substantially larger than usually inferred for the Galactic globular clusters. In this case, NGC 6229 should have a well-populated, extended blue tail. A truly bimodal distribution in HB masses can also satisfactorily account for the observed HB morphology, although in this case the existence of an extended blue tail is not necessarily implied. The other two well-known bimodal-HB clusters, NGC 1851 and NGC 2808, are briefly analyzed. While the HB morphology of NGC 1851 can also be reproduced with a unimodal mass distribution assuming a large mass dispersion, the same is not true of NGC 2808, for which a bimodal, and possibly multimodal, mass distribution seems definitely required. The problem of gaps on the blue HB is also discussed. Applying the standard Hawarden and Newell χ2 test, we find that the NGC 6229 gap is significant at the 99.7% level. However, in a set of 1000 simulations, blue-HB gaps comparable to the observed one are present in ~6%-9% of all cases. We employ a new and simple formalism, based on the binomial distribution, to explain the origin of this discrepancy, and conclude that Hawarden's method, in general, substantially overestimates the statistical significance of gaps.

Rigidity for metrics with the same lengths of geodesics

Abstract: We prove that we can recover a Riemannian metric in a bounded smooth domain in R 3 up to an isometry which is the identity on the boundary, by knowing the lengths of the geodesics joining points on the boundary. We assume that the metrics are close to the euclidian metric e.