Fall In Absorbance Research Articles

Interfacial catalytic influence of sodium lauryl sulphate in the oxidation of cyclic alcohols by Cr(VI)

In the present work effort has been made to investigate the electron transfer mechanism for the oxidation of cyclopentanol of importance be Cr(VI) in presence and absence of surfactant (Sodium lauryl sulphate, SDS) and develop a kinetic model. The progress of the reaction was monitored spectrophotometrically by measuring the fall in absorbance of Cr(VI) at its λmax (wave-length of maximum absorbance). Under the conditions of [cyclopentanol] >≫ [Cr(VI)], the ln(Absorbance) vs time plots are obtained having constant slope value indicating the reaction exhibited a first order dependence with respect to Cr(VI). Unit order dependence of rate on [cyclopentanol], complex dependence on acid strength, nil effect of added salt (K2SO4) is what we observed while monitoring the progress of the reaction. The thermodynamic parameters like activation energy, enthalpy, entropy is computed by executing the reaction at 25, 30 and 35 0C. In presence of varying [SDS] it is observed that the rate marginally increases with increase in [SDS] until an optimum [SDS] which is around cmc and there after the rate decreases exponentially. In SDS medium partitioning of both the cationic Cr(IV) species and the substrate between water and micellar pseudo-phase is considered and accordingly Berezin et al. model is extended to explain the kinetic data. Based on all experimental findings a suitable kinetic model has been proposed for both aqueous and micellar phase reaction.

Surfactant catalyzed electron transfer mechanism in the oxidation of racemic tartaric acid by Ce(IV)

Oxidation of α-hydroxy acid, racemic tartaric acid and its kinetic studies have been carried out to establish the mechanism of oxidation. The electron transfer reaction was performed in presence of surface active agents, surfactants, a clean and green catalytic material, using Ce(IV) as the oxidant. The progress of the reaction was monitored using a spectrophotometer by recording the fall in absorbance of Ce(IV) at its λ max (wave length of maximum absorbance) of 380 nm. Disappearance of Ce(IV) is observed to be first order at constant [H + ]. Plot of log(Absorbance) against time is found to be linear in the initial stages of the reaction. The results of oxidation rates show a complex dependence of rate on [H + ]. The pseudo first order rate constants ( k ) obtained with varying substrate concentration and the plot of k obs /[substrate] vs [substrate] are observed to be excellently linear with positive slope. Plots of pseudo first order rate constants in presence of surfactant (sodium dodecyl sulphate, SDS) against the [SDS] abruptly rises in the concentration range below cmc (critical micelle concentration) after which the rate exponentially falls. Based on all the findings, Berezin model has been advanced to explain the observed kinetic phenomenon in the surfactant medium.

Biological and chemical evaluation of some African plants belonging to Kalanchoe species: Antitrypanosomal, cytotoxic, antitopoisomerase I activities and chemical profiling using ultra-performance liquid chromatography/quadrupole-time-of-flight mass spectrometer

Background: Human African trypanosomiasis is one of the most serious neglected tropical diseases causing fatal symptoms and death. Natural products are a main source for anti-infective metabolites. Objectives: The objective of the study is to evaluate eight different plants belonging to the Kalanchoe species growing in Egypt for antitrypanosomal, antimalarial, antileishmanial, cytotoxic, and antimicrobial activities. Materials and Methods: The antitrypanosomal activity against Trypanosoma brucei; cytotoxic activities against human colon carcinoma, human hepatocyte carcinoma, and human breast adenocarcinoma cell lines; antileishmanial activity against Leishmania donovani; antimalarial activity against Plasmodium falciparum; and antimicrobial activities of all plant extracts have been examined. As well as the identification of the secondary metabolites for the most active extract was performed via ultra performance liquid chromatography coupled to high resolution quadrupole time of flight mass spectrometer operated in negative and positive ionization modes. Results: Among the examined plant extracts, Kalanchoe longiflora leaves extract showed promising activity against T. brucei with an inhibition concentration of sample at 50% fall in absorbance (IC50) value of 17.6 μg/mL. K. longiflora with other extracts exhibited promising cytotoxic activities. Profiling of the polar secondary metabolites of K. longiflora revealed the presence of 47 metabolites including 31 flavonoids, 9 phenolic acids, 4 anthocyanidins, 2 chalcone glucoside, and 1 coumarin. To determine the mechanism of action of K. longiflora extract as a potent antitrypanosomal and cytotoxic agent, we investigate its ability to inhibit topoisomerase I enzyme. K. longiflora extract showed an excellent activity with an IC50 value of 0.148 μg/mL. Conclusion: These interesting results open the door for further research aiming at the development of a successful treatment for Trypanosoma from K. longiflora.

An insight into the air stability of the benchmark polymer:fullerene photovoltaic films and devices: A comparative study

Abstract In this study, a comparative analysis of the instabilities and degradation routes of organic solar cell (OSCs) employing the three benchmarked donor polymers namely poly(3-hexylthiophene) (P3HT), poly[N-900- hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) and Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2 ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7) along with [6,6]-phenylC71 butyric acid methyl-ester (PC71BM) acceptor have been conducted using the extracted photovoltaic parameters in conjunction with the X-ray photoelectron spectroscopy (XPS), optical and morphological analysis. During the 14 days air stability test, the power conversion efficiency (PCE) decreased by 78.85%, 65.83% and 83.36% for P3HT:PC71BM, PCDTBT:PC71BM and PTB7:PC71BM based devices, respectively. However, the degradation study of the bulk heterojunction (BHJ) films was prolonged to 28 days in order to further elucidate the degradation factors affecting the device performance. XPS, optical and morphological studies enabled detailed information on the device degradation mechanisms and confirmed the oxidation of photoactive layer after ageing, morphological deterioration and fall in absorbance, particularly, the PTB7:PC71BM blend that showed the rapid degradation among all three. The results obtained in the current study advance the understanding of the stability/degradation mechanisms pertaining to the three most commonly used BHJ materials and hence, will help to improve the OSCs for longer lifetime.

Fat Globule Size Effect on Visible and Shortwave near Infrared Spectra of Milk

Step-wise homogenisation has been applied to raw milk samples of different composition to investigate the effect of fat globule size distribution on diffuse transmission spectra in the region 400–1100 nm. Homogenisation results in significant spectral changes with two distinct phases. Initial even growth of spectral intensity across the whole spectral range, observed at lower degrees of homogenisation, was followed by a drastic fall in absorbance at the long-wave end of spectrum as the fat globules reached some critical size. Fat and protein content in the sample significantly affected the observed dependences of spectra on the applied homogenisation time. These observations have been explained as a superposition of two effects: growing fat globule density and changes in scatter nature as the particle sizes approach the light wavelengths in a corresponding spectral range. The representative layer theory has been used to illustrate the nature of the spectral effects.

Optical and electrical transport properties of spray deposited CdS 1− xSe x thin films

CdS 1− x Se x thin films with different compositions have been deposited on amorphous glass substrates by spray pyrolysis technique. The spectrum shows a sharp fall in absorbance at wavelength corresponding to the band gap of the material. The optical band gap has been determined for all compositions and found to be direct allowed. The band gap has been observed to strongly depend on the film composition. The electrical conductivity of the deposited films has been studied as a function of temperature. All the films showed a transition from phonon-assisted hopping conduction through the impurity band to grain boundary limited conduction in the conduction/valence band at temperature around 370 K. The conductivity is found to vary with composition. The activation energies of the films of different compositions determined at low and high-temperature region have been observed to be in the range 0.113–0.323 eV. The Hall effect studies carried out on the deposited thin films reveal that films exhibit n-type of conductivity. The carrier concentration has been determined from Hall effect studies and is of the order of 10 15 cm −3.

Synergism in immunotoxicological effects due to repeated combined administration of arsenic and lead in mice

Arsenic and lead are considered potent human hazards because of their neoplastic outcomes; increasing epidemiologic evidence indicates a link between heavy metal exposure and health risk. Since health risks of singly administered metals are well-established, in the present study we determined whether simultaneous repeated multimetal (arsenic+lead) exposure influences the development of immunotoxicity in mice exposed (in vivo) to lead acetate (10 mg/kg b.w.) and sodium arsenite (0.5 mg/kg b.w.) simultaneously. We report that in vivo multimetal exposure alters cell morphology, inhibits cell adhesion, nitric oxide release, intracellular killing ability, chemotactic migration, myeloperoxidase release, bacterial clearance from blood and spleen and increases DNA fragmentation. On measuring bacterial density in blood and spleen after 0, 24, 48 and 72 h post infection (with Staphylococcus aureus MC524) in control and multimetal treated groups, bacterial load showed delayed clearance from blood and spleen in the multimetal exposed group. We also found that in vivo exposure to the multimetal caused a decrease in cell adhesion, indicated by a fall in absorbance at 570 nm with respect to control. Exposure to multimetal led to morphological changes in macrophages, since more deformed cells were obtained in repeated combined exposure to arsenic and lead compared to control. Nitric oxide, which has a potent microbicidal activity in macrophages, was found to be released in fewer amounts in the multimetal exposed group from that of control group. It was observed that the viability of bacteria gradually decreased in control macrophage with time, whereas, in macrophages of multimetal exposed mice, the viability of S. aureus gradually increased. Chemotactic migration of splenic macrophages significantly decreased in the multimetal exposed group from that of control. Lysosomal enzyme release from splenic macrophages decreased upon simultaneous exposure to arsenic and lead, as is evident from the decrease in myeloperoxidase release in multimetal group from that in control. That the structural integrity of splenic macrophages is decreased in the multimetal exposed group is also evident from the enhanced percentage of DNA fragmentation after multimetal exposure, suggesting apoptotic death of splenic macrophage. Intracellular viable bacteria in the splenic macrophage from multimetal exposed group was 89.16±3.54% while that from control group was 49.19±1.16%, whereas single metal exposed groups showed a bacterial viability of 69.6±2.45% and 71.71±1.89% in arsenic and lead treated groups respectively. What is essentially noteworthy from the observed results is that lead and arsenic causes a greater immunotoxic effect when administered together as multimetal than when singly administered.Simultaneous exposure to lead and arsenic appears to be additive as is further established from the isobologram constructed by plotting the concentration of arsenic against the concentration of lead at which effect (in this case myeloperoxidase release) remained constant, a convex line showing synergism was demonstrated. The present study reports a definite synergistic trend of immunotoxicity during simultaneous exposure to arsenic and lead, that is, a multimetal challenge, as compared to the effects of independent exposure to them.

Catalase and superoxide dismutase secreted from Helicobacter pylori.

Previously, we have reported that toxic oxidants produced by activated neutrophils play a pivotal role in the development of Helicobacter pylori-induced gastric mucosal damage. Microorganisms, however, are characterized by their ability to produce a variety of antioxidant enzymes. This study is designed to measure the oxygen radical scavenging enzymes, such as catalase and superoxide dismutase (SOD), and urease in the supernatant of H. pylori (NCTC11637) suspension. H. pylori was inoculated on a sheep blood agar plate and harvested. Bacterial suspensions (10(9) cfu/ml phosphate buffer) were washed twice and incubated at 37 degrees C for 1, 2, 12, and 24 hours or were sonicated in ice. Their supernatants were obtained by centrifugation and filtration. SOD activity was measured spectrophotometrically by the cytochrome c method. Catalase activity was assayed by the fall in absorbance at 240 nm as H2O2 is degraded. Urease activity was determined by measuring the release of ammonia using Berthelot reaction. Activities of both SOD and catalase were detected in the supernatants of 1-hour microaerophilic incubation. Their activities were almost constant in 4-, 12-, and 24-hour microaerobic incubation or sonication. Urease activity was increased dramatically in proportion to the period of microaerobic incubation. Although H. pylori possesses antioxidant as a constitutive compartment, the magnitude of its secretion was below the detectable level. It is not likely that SOD and catalase play a significant role for scavenging the oxidants from injured gastric mucosa, such as infiltrated leukocytes.

Determination of manganese superoxide dismutase activity by direct spectrophotometry.

A method to determine Mn-superoxide dismutase activity by measuring directly the rate of decay of O2- in a spectrophotometer, is described. Decay of O2- generated by KO2 at pH 9.5, was monitored as the fall in absorbance (A250nm-A360nm). Mn-superoxide dismutase was determined as the activity of cyanide-resistant superoxide dismutase, calculated from the rate of O2- dismutation. Mn-superoxide dismutase could be determined in the presence of a 700 times higher Cu,Zn-superoxide dismutase activity. The alkaline pH did not cause analytical problems. The assay was used to measure both Mn- and Cu,Zn-superoxide dismutase activity in mitochondrial preparations. The assay had a detection limit of 2.8 ng/ml when Mn-superoxide dismutase from E. coli was used, and the between-day CV was 5.8%. The assay is an alternative to indirect methods for detecting superoxide dismutase activity.

A novel oxygen and/or carbon dioxide-sensitive optical transducer

A novel oxygen (O 2) and/or carbon dioxide (CO 2)-sensitive transducer for the measurement of both gaseous O 2 and CO 2 over the concentration ranges of O 2, 0–100% and CO 2, 0–10% has been described employing a solution of 10.6 μ M fluorescein (FL) and 190 μ M potassium hydroxide in a solvent mixtures of 1:1 (v/v) N, N′-diethylaniline (DEA) and N, N-dimethylformamide. Increasing O 2 concentrations cause the absorbance of the solution at a wavelength of 400 nm to increase owing to a contact charge transfer reaction existing between O 2 and DEA molecules, and increasing CO 2 concentrations produce a non-linear fall in absorbance at 520 nm as the colour of FL changes from its orange dianion form to the colourless neutral, lactonic form. Both processes are independent of each other and reversible. The response to changes in O 2 concentrations is in good agreement with Beer-Lambert's law and the response to changes in CO 2 concentrations in non-linear. A fibre optic sensing system based on this solvent-dye solution has been set up for continuous and reversible determination of both gaseous O 2 and CO 2. Possible applications include environmental and physiological monitoring of O 2 over the ranges of 0–100% and CO 2, 0–10%.

Copper chelation‐induced reduction of the biological activity of S‐nitrosothiols

1. The effect of copper on the activity of the S-nitrosothiol compounds S-nitrosocysteine (cysNO) and S-nitrosoglutathione (GSNO) was investigated, using the specific copper chelator bathocuproine sulphonate (BCS), and human washed platelets as target cells. 2. Chelation of trace copper with BCS (10 microM) in washed platelet suspensions reduced the inhibition of thrombin-induced platelet aggregation by GSNO; however, BCS had no significant effect on the anti-aggregatory action of cysNO. BCS inhibited cyclic GMP generation in response to both cysNO and GSNO. 3. The effect of BCS was rapid (within 30 s), and could be abolished by increasing the platelet concentration to 500 x 10(9) l-1. 4. In BCS-treated platelet suspensions, the addition of Cu2+ ions (0.37-2.37 microM) led to a restoration of both guanylate cyclase activation and platelet aggregation inhibition by GSNO. 5. The anti-aggregatory activity of GSNO was reduced in a concentration-dependent manner by the copper (I)-specific chelators BCS and neocuproine, and to a smaller extent by desferal. No effect was observed with the copper (II) specific chelator, cuprizone, the iron-specific chelator, bathophenanthroline sulphonate, or the broader-specificity copper chelator, D-penicillamine. 6. In both BCS-treated and -untreated platelet suspensions, cys NO was more potent than GSNO as a stimulator of guanylate cyclase. In BCS-treated platelet suspensions there was no significant difference between the anti-aggregatory potency of cysNO and GSNO; however, in untreated suspensions, GSNO was significantly more potent than cysNO. Thus, when copper was available, GSNO produced a greater inhibition of aggregation than cysNO, despite being a less potent activator of guanylate cyclase. 7. The breakdown of cysNO and GSNO was measured spectrophotometrically by decrease in absorbance at 334 nm. In Tyrode buffer, cysNO (10 microM) broke down at a rate of 3.3 microM min-1. BCS (10 microM)reduced this to 0.5 microM min-1. GSNO, however, was stable, showing no fall in absorbance over a period of 7 min even in the absence of BCS.8. We conclude that copper is required for the activity of both cysNO and GSNO, although its influence on anti-aggregatory activity is only evident with GSNO. The stimulatory effect of copper is unlikely to be explained solely by catalysis of S-nitrosothiol breakdown. The enhancement by copper of the anti-aggregatory activity of GSNO, relative to cysNO, suggests that copper may be required for biological activity of GSNO which is independent of guanylate cyclase stimulation.

Oxidation of microbial iron-sulfur centers by the myeloperoxidase-H2O2-halide antimicrobial system.

Myeloperoxidase, H2O2, and a halide (chloride, bromide, or iodide) form a potent microbicidal system that contributes to the antimicrobial activity of neutrophils. The mechanism of toxicity is not completely understood. Powerful oxidants are formed that presumably attack the microbe at a variety of sites. Among the consequences of this attack is the release of a large proportion of 59Fe of prelabeled organisms. We report here that the myeloperoxidase-H2O2-halide system oxidizes the iron-sulfur centers of model compounds (spinach ferredoxin) and intact microorganisms (Escherichia coli) with the loss of labile sulfide. The oxidation of the iron-sulfur centers of ferredoxin was measured by the fall in absorbance at 420 nm (bleaching) and by the loss of 5,5'-dithiobis-(2-nitrobenzoic acid) reducing activity. The latter compound is a sulfhydryl reagent that is reduced by ferredoxin labile sulfide during denaturation. The oxidation of E. coli iron-sulfur centers by the peroxidase system was determined by the loss of labile sulfide content, as measured by the release of H2S by acid and its reaction with zinc acetate to form ZnS. The halides were effective as components of the peroxidase system in the order I greater than Br greater than Cl. The oxidation of E. coli iron-sulfur centers by the peroxidase system was rapid and preceded the loss of viability. Gentamicin, at a concentration which produced a loss of viability comparable to that of the peroxidase system, did not cause a loss of labile sulfide from E. coli, suggesting that labile sulfide loss is not a nonspecific reflection of the loss of viability, but a direct consequence of the action of the myeloperoxidase system. The oxidation of iron-sulfur centers in microorganisms by the myeloperoxidase-H2O2-halide system may contribute to the death of the organism.

Oxidation of Escherichia coli iron centers by the myeloperoxidase-mediated microbicidal system.

Myeloperoxidase, HeO2, and a halide (chloride, bromide, iodide) constitute a powerful microbicidal system which is active against a wide variety of microorganisms and is believed to contribute to the antimicrobial activity of neutrophils. The precise mechanism by which this system exerts its toxicity is unknown. We report here that the microbicidal activity of the myeloperoxidase-H2O-chloride system on Escherichia coli is associated with the loss of iron into the medium as measured by the release of 59Fe from prelabeled organisms. Iron loss (but not bactericidal activity) was considerably increased by the addition of EDTA or other iron chelators; it was not associated with a corresponding release of protein with 14C-amino-acids. Iron loss was observed with chloride or bromide as the halide, but not when iodide was employed in microbicidal concentrations. Microbicidal activity was detected at an earlier time period and at a lower halide concentration than was iron loss. Analogous changes were observed when cytochrome c was oxidized by the myeloperoxidase H2O2-halide system. The initial response was a shift in the Soret maximum, followed by a fall in absorbance accompanied by the loss of iron. As with the intact organism, iron loss was evident with chloride and bromide, but not with iodide as the halide. These findings suggest that microbial iron centers are a target for the myeloperoxidase-mediated antimicrobial system and that their oxidation may contribute to microbicidal activity.

Germination properties of a spore coat-defective mutant of Bacillus subtilis.

The presence of the gerE36 mutation in strains of Bacillus subtilis 168 resulted in poor germination of their spores in a range of germinants, as measured by the fall in absorbance of spore suspensions. Although resistant to heat and organic solvents, spores were sensitive to lysozyme; electron microscopy revealed that their coat structure was incomplete. These spores responded to germinants by losing heat resistance and changing from phase bright to phase gray. The release of dipicolinic acid and the fall in absorbance of spore suspensions reached only 75 and 50% of wild-type levels, respectively, but followed the same time course as the loss of heat resistance. Although the germination response was incomplete, the concentration of L-alanine required to elicit it was the same for the mutant as for the wild type. The properties of mutant spores suggest that an intact spore coat is not required for the initial interaction between germinant and spore, but that the coat layers may contain molecules important in later stages of germination. In transduction with phage SPP1, the gerE36 mutation mapped between citF and ilvB and was 90% cotransduced with citF2. The gerE mutation identifies the location of a gene important for the progress of late stages of spore formation.

Properties of a Mutant of Bacillus subtilis 168 in which Spore Germination is Blocked at a Late Stage

Summary: A mutation, gerj50, causing defective spore germination has been located between lys-1 and trpC2 on the Bacillus subtilis map and is 92% cotransduced with trpC2 by phage PBS1. Spores of strains containing gerJ50 will respond to germinants but the fall in absorbance is only 60% of that of the wild-type. During germination, mutant spores reach only an intermediate phase-grey stage, in contrast to those of the wild-type which become dark. The germination response, measured by the release of dipicolinic acid and the loss of resistance to heat, chloroform, octanol or toluene, is normal. The release of hexosamine-containing fragments from mutant spores is incomplete but has similar kinetics to that of wild-type spores. The mutant spores are more sensitive to heating at 90 °C which may point to a structural abnormality, but there is no evidence of a spore coat deficiency from electron microscopy of thin sections or freeze fractures or from the chemical resistances of the spores. Thus, the germination of mutant spores is initiated normally, but blocked at a late stage.

Multiple forms of starch branching enzyme of maize: Evidence for independent genetic control

Purification of starch branching enzymes from kernels of two nonlinked mutants of maize, sugary and amylose-extender , showed the basis of the two mutations to be associated with branching enzymes I and IIb, respectively. Branching enzyme I from sugary kernels purified as nonmutant branching enzyme I, but had an altered pattern of activity when amylose was used as a substrate. In addition to the typical fall in absorbance at high wavelengths (550–700 nm) of the amylose-iodine complex, branching of amylose by sugary branching enzyme I caused an increase in absorbance at low wavelengths (400–550 nm). Branching enzyme IIb was undetected in extracts of amylose-extender kernels, while branching enzymes I and IIa appeared unaltered. Low umprimed starch synthase activity was also observed in DEAE-cellulose fractions of amylose-extender maize, but this activity was regenerated by the addition of any branching enzyme.

Hypoxanthine-guanine phosphoribosyltransferase: A simple spectrophotometric assay

A simple spectrophotometric assay is described based on the conversion of hypoxanthine to inosine monophosphate and precipitation of both the reaction product and protein with lanthanum phosphate. The extent of conversion is determined by the fall in absorbance of hypoxanthine at 249 nm. The assay is suitable for screening red cell lysates for hypoxanthine-guanine phosphoribosyl-transferase deficiency.

Ligand-induced structural changes in amylose partially complexed with iodine.

The influence of complexing agents such as methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, cyclohexanol and 2-octanol on the formation of a blue coloured amylose - iodine complex (pH 4.8), under suboptimum concentrations of iodine and in the absence of potassium iodide, is studied by recording the absorbance at 640 nm. A drop in absorbance at 640 nm accompanied by a blue shift in the spectrum (640-580 nm) was observed at higher concentrations of the complexing agents. This behaviour of amylose partially complexed with iodine appears to be due to ligand-induced structural changes in the amylose chain. The fall in absorbance at 640 nm observed when the temperature of amylose - iodine complex in the presence of complexing agents is raised, and the subsequent regeneration of the absorbance on cooling, indicates the possible helix to random coil transition of the amylose chain in an aqueous system.

Hydrolysis of adenosine monophosphates by acid phosphatases as measured by a continuous spectrophotometric assay

A method has been devised for measuring the initial rate of hydrolysis of adenonsine monophosphates by acid phosphatase. This utilizes the enzymatic deamination of adenosine by exogenous adenonsine deaminase and permits the fall in absorbance at 265 nm to be continuously monitored. The reaction is extremely sensitive and activity was proportional to the amount of enzyme taken in the case of human prostatic and potato acid phosphatases, but not in the case of rat liver and human kidney preparations. The velocities with different monophosphates of adenosine were studied for the above enzymes and for calf intestinal alkaline phosphate. The latter showed no preference with respect to position of the phosphate ester, but the prostate, kidney, and potato enzymes showed a preference for 3′AMP while the liver enzyme hydrolyzed 3′- and 5′AMP at similar rates. Analysis of kinetic constants suggested that the velocities were more dependent upon rate of dissociation of enzyme-substrate complex than changes in the substrate-enzyme affinity. Preliminary studies suggested activation by adenine and inhibition by guanine these effects possibly being mediated allosterically at a site other than the catalytic center. Glycerophosphates inhibited almost completely AMP hydrolysis by potato and prostatic enzymes, but liver retained considerable activity towards 3′-and 5′AMP.

Conformation of U.V.-irradiated DNA.

SummaryThe effects of u.v. on DNA have been found to be manifold. The initial effect characterized by a fall in absorbance, rise in sedimentation coefficient and appearance of kinks in many places of the molecule has been attributed to dimerization of the pyrimidines and formation of single-strand breaks. Together with the occurrence of kinks, the molecule becomes folded up and assumes a ring-like appearance at several places; this is partly due to intramolecular crosslinks. With an increase in dose a third effect, i.e. double-strand break which results in back-bone scission, comes into play and predominates. This effect is manifested in a further decrease in absorbance, a fall in sedimentation constant, and the appearance of shorter molecules in the electron microscope. At still higher doses, an equilibrium state is reached when both absorbance and sedimentation coefficient remain steady. The resulting molecules were so short that the effect of dimer formation with the increase in dose was negligible.