Radiation chemistry of an aqueous solution of glycine: compounds of interest to chemical evolution studies

Summary We have examined the radiolysis of an O₂-free aqueous solution of glycine at absorbed doses of⁶⁰Co gamma-radiation of up to 20 Mrad. At least 20 compounds are formed during radiolysis, among them several amino acids, an oligoamine, and the nitrogen-free polymers (M_w28,000 daltons). When dicyandiamide is present in the solution of glycine, various nitrogen-containing products, including some polymers (M_w12,000 daltons), are synthesized along with radiolytic products of glycine; polyglycines are not formed. We have determined the radiation-chemical yields of radiolytic-product formation and of decomposition of glycine, and have considered possible free-radical reactions leading to the radiation-induced changes observed.     

Extraction of chromatophores from Rhodospirillum rubrum in aqueous two-phase systems: A method for large-scale isolation of membrane particles

Chromatophores, membrane vesicles with the capacity of cyclic photophosphorylation, have been isolated from Rhodospirillum rubrum cells on a pilot plant scale. Results of disintegration in a glass bead mill and in a high pressure homogenizer were compared. The chromatophores were isolated from the crude extract by extraction in aqueous two-phase systems. In systems of polyethylene glycol (PEG) and dextran the chromatophores were partitioned to the upper PEG phase by the addition of PEG-palmitate. Most of the proteins and nucleic acids were forced to the bottom phase by addition of sodium chloride. Methods to prevent precipitation of the chromatophores were studied.     

The relationship between the levels of DNA-hydrocarbon adducts and the formation of sister-chromatid exchanges in Chinese hamster ovary cells treated with derivatives of polycyclic aromatic hydrocarbons

The frequencies of the induction of sister-chromatid exchanges and the levels of deoxyribonucleoside-hydrocarbon adducts formed in Chinese hamster ovary cells that had been treated with either dihydrodiols or a diol-epoxide derived from polycyclic aromatic hydrocarbons were determined. Up to 6-fold increases in the incidence of these exchanges were observed when the cells were treated either with the dihydrodiols, trans-3,4-dihydro-3,4-dihydroxy-7-methylbenz[a]anthracene,trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene or the diol-epoxide, (±)-r-7, t-8dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo[a] pyrene but when the cells were transferred to media free of these compounds, there were rapid reductions in the frequency of these exchanges. When the exchanges were induced by the diol-epoxide, the decreases in frequency were paralleled by decreases in the levels of deoxyribonucleoside-diol-epoxide adducts that were present in hydrolysates of DNA isolated from the cells. There thus appears to be a close relationship between the frequency of sister-chromatid exchanges and the levels of deoxyribonucleoside-diol-epoxide adduct formation.     

Isolation and fractionation of CHO chromosomes in aqueous two phase systems using charged polymers and base specific macroligands

Summary Chromosomes were isolated in a preparative scale by synchronisation of CHO cells with a double Thymidine block followed by an arrest in the metaphase by addition of Colcemid. Under proper cultivation conditions a mitotic index of 77% total cells could be routinely achieved. Bulk chromosome preparations free of nuclei and other subcellular particles have been obtained by low speed centrifugation followed by a 60 transfer countercurrent distribution using aqueous two phase systems composed of polyethylenglycol and dextran. The partition of CHO chromosomes previously purified in aqueous two phase systems were studied further to develop a protocol for the separation and isolation of individual chromosomes. Partition experiments with chromosomes changing the electrostatic phase potential by addition of charged PEG-derivatives suggest the existence of relatively highly charged chromosome groups. Most promising results with regard to separation were obtained using two PEG-derivatives, which interact specifically with the bases in DNA. For this affinity partitioning a GC- and AT-specific macroligand were employed. Comparing CCD's using each of these ligands information on the GC and AT content of exposed DNA in the chromosomes groups could be derived, demonstrating that specific sequences of DNA are accessible at the surface of metaphase chromosomes.     

Determination of copper concentration in blood plasma and in ocular and cerebrospinal fluids using graphite furnace atomic absorption spectroscopy

Published values for the concentration of Cu in cerebrospinal and intraocular fluids cover a very wide range (0.016 to 1.0 microgram/ml) and include values which are several times higher than those which would be consistent with normal physiology. An atomic absorption spectrophotometer equipped with a graphite furnace was used to measure the Cu concentration in these fluids and in blood plasma of toads, rabbits, and cats. Under standard conditions, these fluids yielded high background absorbance and only fractional recovery of added Cu. Parameters were therefore established which eliminated both the high background and the matrix interference and allowed the determination of Cu in 10-microliters aliquots of diluted blood plasma and undiluted cerebrospinal and ocular fluid samples. Under these conditions the Cu measured in the ocular (0.011 to 0.032 microgram/ml) and cerebrospinal fluids (0.033 to 0.050 microgram/ml) of these three species was lower than most previously reported values and only a small fraction (1-3%) of the concentration of Cu in the plasma of the same animals (0.85 to 1.22 micrograms/ml).     

The influence of pH and chloride on the retention of cadmium,lead, mercury,and zinc by soils

The extent of contamination of soils by toxic heavy metals not only depends on the rate of loading of the metal but also on the nature of the adsorbing surfaces, the degree of alkalinity or acidity of the soil and the presence of aqueous complexant ligands. This work reports on the role of pH on the retention of Cd, Hg, Pb and Zn by two soils and on the influence of the chloride, Cl‐, ion on the chemical speciation and retention of the four metals. Batch adsorption experiments were conducted from pH 3 to 7 in the presence of either 0.1 M LiCl or LiClO₄. The results of the study showed that high concentrations of Cl^‐ ions can greatly decrease the retention of Hg and have an increasingly lesser effect on Cd, Pb and Zn retention. The effect of the Cl^‐ons was directly related to the metal‐Cl formation constants. The results of computer modeling of Cd and Hg retention by goethite and humic acid fractions indicated the relative importance of aqueous vs. surface complexation on metal retention. For organic surfaces, which do not form ternary surface complexes, the presence of aqueous complexant ligands should always decrease the adsorption of the metal. For mineral surfaces, which do form ternary surface complexes, there may be increased or decreased metal retention depending on the formation constant of the aqueous metal‐ligand species, the intrinsic complexation constants for the various binary and ternary complexes of the metal and the concentration of the complexant ligand. Thus for Hg, which forms very strong aqueous species with Cl^‐ ions, reduced adsorption on goethite was predicted in the presence of 0.1 M LiCl, while enhanced adsorption was predicted for Cd and Pb. The results suggest caution in the disposal of Cl‐containing wastes onto metal‐contaminated soils. The deleterious effects of Cl^‐ ion addition would be greatest for soils with relatively high organic matter contents and low contents of hydrous ferric oxides.     

Existence and uniqueness of a sharp travelling wave in degenerate non-linear diffusion Fisher-KPP equations

In this paper we use a dynamical systems approach to prove the existence of a unique critical value c ^* of the speed c for which the degenerate density-dependent diffusion equation u _ct = [D(u)u _x ] _x + g(u) has: 1. no travelling wave solutions for 0 < c < c ^*, 2. a travelling wave solution u(x, t) = (x - c ^* t) of sharp type satisfying (– ) = 1, () = 0 ^*; '(^*–) = – c ^*/D'(0), '(^*+) = 0 and 3. a continuum of travelling wave solutions of monotone decreasing front type for each c > c ^*. These fronts satisfy the boundary conditions (– ) = 1, '(– ) = (+ ) = '(+ ) = 0. We illustrate our analytical results with some numerical solutions.     

Bifurcation analysis of periodic SEIR and SIR epidemic models

The bifurcations of the periodic solutions of SEIR and SIR epidemic models with sinusoidally varying contact rate are investigated. The analysis is carried out with respect to two parameters: the mean value and the degree of seasonality of the contact rate. The corresponding portraits in the two-parameter space are obtained by means of a numerical continuation method. Codimension two bifurcations (degenerate flips and cusps) are detected, and multiple stable modes of behavior are identified in various regions of the parameter space. Finally, it is shown how the parametric portrait of the SEIR model tends to that of the SIR model when the latent period tends to zero.     

Modelling the effect of treatment and behavioral change in HIV transmission dynamics

In this paper we analyze a model for the HIV-infection transmission in a male homosexual population. In the model we consider two types of infected individuals. Those that are infected but do not know their serological status and/or are not under any sort of clinical /therapeutical treatment, and those who are. The two groups of infectives differ in their incubation time, contact rate with susceptible individuals, and probability of disease transmission. The aim of this article is to study the roles played by detection and changes in sexual behavior in the incidence and prevalence of HIV. The analytical results show that there exists a unique endemic equilibrium which is globally asymptotically stable under a range of parameter values whenever a detection /treatment rate and an indirect measure of the level of infection risk are sufficiently large. However, any level of detection/ treatment rate coupled with a decrease of the transmission probability lowers the incidence rate and prevalence level in the population. In general, only significant reductions in the transmission probability (achieved through, for example, the adoption of safe sexual practices) can contain effectively the spread of the disease.     

Characterization of a non-detergent PS II-cytochrome b/f preparation (BS)

A non-detergent photosystem II preparation, named BS, has been characterized by countercurrent distribution, light saturation curves, absorption spectra and fluorescence at room and at low temperature (–196°C). The BS fraction is prepared by a sonication-phase partitioning procedure (Svensson P and Albertsson P-Å, Photosynth Res 20: 249–259, 1989) which removes the stroma lamellae and the margins from the grana and leaves the appressed partition region intact in the form of vesicles. These are closed structures of inside-out conformation. They have a chlorophyll a/b ratio of 1.8–2.0, have a high oxygen evolving capacity (295 mol O₂ per mg chl h), are depleted in P700 and enriched in the cytochrome b/f complex. They have about 2 Photosystem II reaction centers per 1 cytochrome b/f complex.The plastoquinone pool available for PS II in the BS vesicles is 6–7 quinones per reaction center, about the same as for the whole thylakoid. It is concluded, therefore, that the plastoquinone of the stroma lamellae is not available to the PS II in the grana and that plastoquinone does not act as a long range electron transport shuttler between the grana and stroma lamellae.Compared with Photosystem II particles prepared by detergent (Triton X-100) treatment, the BS vesicles retain more cytochrome b/f complex and are more homogenous in their surface properties, as revealed by countercurrent distribution, and they have a more efficient energy transfer from the antenna pigments to the reaction center.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_v variable fluorescence - LHC light-harvesting complex - PpBQ phenyl-p-benzoquinone - PQ plastoquinone pool - P700 reaction center of PS I - PS I, PS II Photosystem I, II - Q_A first bound plastoquinone accepter - RC reaction centre