Synthesis of 5-Alkyl-5-methyl-2-hydroxy-2-cyclopentenones

A hydrogen bacterium strain, N34, and its oxygen-resistant segregant strain, Y38, were subjected to a taxonomical study. Since both strains were capable of N₂-fixation, N₂-fixing facultative hydrogen autotrophs listed in “Bergey’s Manual of Systematic Bacteriology” were used for comparison. Both strains produced a water-insoluble carotenoid pigment, zeaxanthin dirhamnoside, indicating that both should be classified into the genus Xanthobacter. Then, the differential characteristics of the two species of the genus Xanthobacter, X. autotrophicus and X. flavus, were investigated as to both strains. The vitamin requirement, the sensitivity to oxygen under autotrophic conditions, the inducibility of hydrogenase, the substrate range of carbohydrates and N₂-fixing growth characteristics of both strains were almost completely opposite to those of X. flavus. Moreover, both strains coincided exactly with X. autotrophicus in morphological and other physiological characteristics. From these results both strains were identified as Xanthobacter autotrophicus.     

Cyanobacterial H<Subscript>2</Subscript> production — a comparative analysis

Several unicellular and filamentous, nitrogen-fixing and non-nitrogen-fixing cyanobacterial strains have been investigated on the molecular and the physiological level in order to find the most efficient organisms for photobiological hydrogen production. These strains were screened for the presence or absence of hup and hox genes, and it was shown that they have different sets of genes involved in H₂ evolution. The uptake hydrogenase was identified in all N₂-fixing cyanobacteria, and some of these strains also contained the bidirectional hydrogenase, whereas the non-nitrogen fixing strains only possessed the bidirectional enzyme. In N₂-fixing strains, hydrogen was mainly produced by the nitrogenase as a by-product during the reduction of atmospheric nitrogen to ammonia. Therefore, hydrogen production was investigated both under non-nitrogen-fixing conditions and under nitrogen limitation. It was shown that the hydrogen uptake activity is linked to the nitrogenase activity, whereas the hydrogen evolution activity of the bidirectional hydrogenase is not dependent or even related to diazotrophic growth conditions. With regard to large-scale hydrogen evolution by N₂-fixing cyanobacteria, hydrogen uptake-deficient mutants have to be used because of their inability to re-oxidize the hydrogen produced by the nitrogenase. On the other hand, fermentative H₂ production by the bidirectional hydrogenase should also be taken into account in further investigations of biological hydrogen production.Abbreviations Chl chlorophyll - MV methyl viologen     

Inhibition of methanogenesis by sulphate reducing bacteria competing for transferred hydrogen

A methanogenic bacterial consortium was obtained after inoculation of benzoate medium under N₂/CO₂ atmosphere with intertidal sediment. A hydrogen donating organotroph andMethanococcus mazei were isolated from this enrichment. H₂-utilising sulphate reducing bacteria were isolated under H₂/CO₂ in the absence of organic electron donors. TheMethanococcus was able to produce methane in yeast extract medium under N₂/CO₂ if the H₂ donating organism was present, and sulphate reduction occurred if the hydrogen utilising sulphate reducing bacteria were grown with the H₂ donating organism. The ability of the H₂ utilising sulphate reducing bacteria to inhibitMethanococcus competitively was shown in cultures containing both of these H₂ utilising bacteria.Abbreviations HDO hydrogen donating organism - SRB sulphate reducing bacteria - HSRB hydrogen utilising sulphate reducing bacteria     

Reduced glutathione esters—antidotes to toxicity. Cytotoxicity induced by hydrogen peroxide, 1-chloro-2,4-dinitrobenzene,and menadione in murine P388D1 macrophages in vitro

Repletion of depleted cellular reduced glutathione (GSH) levels in oxidative stress and exposure to arylating agents is a strategy for the development of antidotes to chemical toxicity. The effect of GSH, reduced glutathione ethyl monoester (GSHEt), and reduced glutathione ethyl diester (GSHEt₂) on the cytotoxicity of hydrogen peroxide, 1-chloro-2,4-dinitrobenzene (CDNB), and menadione to P388D₁ macrophages in vitro was investigated. The median toxic concentration TC₅₀ values of the toxicants were hydrogen peroxide 24 ± 2 mM (N = 19), CDNB 63 ± 6 μM (N = 18), and menadione 30 ± 4 μM (N = 22). Reduced glutathione, GSHEt, and GSHEt₂ were poor antidotes to hydrogen peroxide toxicity. Indeed, the observed antidote effects were attributed to the nonenzymatic reaction of the GSH derivatives with hydrogen peroxide in the extracellular medium. Reduced glutathione ethyl diester was a more potent antidote of CDNB- and menadione-mediated toxicity than GSHEt and GSH. For cell incubations with the approximate median toxic concentration TC₅₀ values of hydrogen peroxide, CDNB, and menadione, the respective median effective antidote concentration EC₅₀ values were GSHEt 23.8 ± 4.1 mM (N = 9), 3.6 ± 0.6 mM (N = 11), and 226 ± 93 μM (N = 12); and GSHEt₂ 20.4 ± 1.9 mM (N = 6), 603 ± 2 μM (N = 9), and 7.6 ± 2.3 μM (N = 12). Reduced glutathione ethyl diester was a potent antidote to CDNB- and menadione-induced toxicities but not to hydrogen peroxide-induced toxicity under acute intoxication conditions. © 1996 John Wiley & Sons, Inc.     

High‐Performance Hydrogen Evolution by Ru Single Atoms and Nitrided‐Ru Nanoparticles Implanted on N‐Doped Graphitic Sheet

The most efficient electrocatalyst for the hydrogen evolution reaction (HER) is a Pt‐based catalyst, but its high cost and nonperfect efficiency hinder wide‐ranging industrial/technological applications. Here, an electrocatalyst of both ruthenium (Ru) single atoms (SAs) and N‐doped‐graphitic(G_N)‐shell‐covered nitrided‐Ru nanoparticles (NPs) (having a Ru‐N_x shell) embedded on melamine‐derived G_N matrix { 1 : [Ru(SA)+Ru(NP)@RuN_x@G_N]/G_N}, which exhibits superior HER activity in both acidic and basic media, is presented. In 0.5 m H₂SO₄/1 m KOH solutions, 1 shows diminutive “negative overpotentials” (?η = |η| = 10/7 mV at 10 mA cm^?2, lowest ever) and high exchange current densities (4.70/1.96 mA cm^?2). The remarkable HER performance is attributed to the near‐zero free energies for hydrogen adsorption/desorption on Ru(SAs) and the increased conductivity of melamine‐derived G_N sheets by the presence of nitrided‐Ru(NPs). The nitridation process forming nitrided‐Ru(NPs), which are imperfectly covered by a G_N shell, allows superb long‐term operation durability. The catalyst splits water into molecular oxygen and hydrogen at 1.50/1.40 V (in 0.1 m HClO₄/1 m KOH), demonstrating its potential as a ready‐to‐use, highly effective energy device for industrial applications.     

Nitrogen fixation by hydrogen-utilizing bacteria

Seventeen strains of nitrogen-fixing bacteria, isolated from different habitats on hydrogen and carbon dioxide as well as on other substrates, morphologically resembled each other. All strains, including Mycobacterium flavum 301, grew autotrophically with hydrogen. The isolate strain 6 was sensitive to oxygen when dependent on N₂ as nitrogen source, a consequence of the sensitivity of its nitrogenase towards oxygen. At the same time, strain 6 was sensitive to hydrogen when growing autotrophically on N₂ as nitrogen source, but hydrogen did not affect acetylene reduction by these cells.Abbreviations MPN Most probable number - BS medium basal salts medium     

Isolation and characterization of hydrogenase-negative mutants of Azorhizobium caulinodans ORS571

Hydrogenase-negative (Hup^-) mutants of Azorhizobium caulinodans ORS571 were isolated by means of Tn5 mutagenesis. The colony test used for screening for Hup^- strains was based on the absence of reduction of triphenyltetrazolium chloride with hydrogen. Suspensions from cultures of the mutant strains grown under derepressing conditions did not use hydrogen with methylene blue or oxygen as the hydrogen acceptor. The mutants were shown to carry single Tn5 insertions at different locations in the A. caulinodans genome. Molar growth yields (corrected for poly--hydroxybutyrate formation) in chemostat cultures of the mutants were similar to those of the wild type. Molar growth yields of the mutants were not increased by passing additional hydrogen through chemostat cultures, which is in agreement with the hydrogenase-negative phenotype of the mutants. H₂/N₂ ratios (mol H₂ formed per mol N₂ fixed) were calculated from the hydrogen content of the effluent gas and the N-content of the bacterial dry weight. Low H₂/N₂ ratios (between 1.2 and 1.9) were found in both energy-limited (oxygen or succinate) cultures and in cultures limited by the supply of an anabolic substrate (Mg²⁺). ATP/2e values (mol ATP used at the transport of 2e to nitrogen or H⁺) were calculated from the H₂/N₂ ratios and the molar growth yields of nitrogen-fixing and ammonia-assimilating cultures. ATP/2e values were between 7 and 11. It was concluded that the calculated ATP/2e values comprise not only 4 mol ATP used at the transport of 2e through nitrogenase but also energy equivalents needed for reversed electron flow from NADH to the low-potential hydrogen donor used by nitrogenase.     

Hydrogen oxidation and efficiency of nitrogen fixation in succinate-limited chemostat cultures ofRhizobium ORS 571

Rhizobium ORS 571, isolated from stem nodules of the tropical legumeSesbania rostrata is able to grow in the chemostat with molecular nitrogen as sole nitrogen source at a specific growth rate of 0.1 h^-1. Samples from nitrogenfixing cultures showed high acetylene reduction activities: 1,500 nmol ethylene formed per milligram dry weight per hour. Under nitrogen-fixing conditions an uptake hydrogenase is induced. Ammonia-assimilating cultures, without additional hydrogen, did not induce hydrogenase. The addition of hydrogen to succinate-limited nitrogen-fixing cultures resulted in an increase in the molar growth yield on succinate (Y _succinate) from 27 to 35 and a slight decrease in the molar growth yield on oxygen ( ), showing that hydrogen oxidation is less energy-yielding than the oxidation of endogenous substrates. Respiration-driven proton translocation measured with starved cells indicated the functioning of site 1 and 2 of oxidative phosphorylation. Cytochrome spectra showed that cytochromea ₆₀₀, present at high dissolved oxygen tension (d.o.t.) almost completely disappeared at low d.o.t. In flash-photolysis spectra only thea-type cytochrome could be detected as an oxidase in cells both grown at high and low d.o.t. Growth yields in ammonia-assimilating cultures were higher than those measured in nitrogen-fixing cultures. Assuming two sites of oxidative phosphorylation, a molar growth yield on ATP (Y _ATP) of about 3 and 6 was calculated for respecticely nitrogen-fixing and ammonia-assimilating cultures. TheY _ATP under nitrogen-fixing conditions is dependent on the amount of H₂ formed per mol N₂ fixed (H₂/N₂ ratio). A method has been described to calculate the total amount of ATP use by nitrogenase during the fixation of 1 mol N₂ (ATP/N₂ ratio) and H₂/N₂ ratios in aerobic nitrogen fixing organisms. This calculation yielded that nitrogen fixation inRhizobium ORS 571 is a high ATP-consuming process. The calculated ATP/N₂ and H₂/N₂ ratios were respectively 42 and 7.5.Abbreviations d.o.t. dissolved oxygen tension A preliminary account of this work was presented at the 5th International Symposium on Nitrogen Fixation, September 1983, Noordwijkerhout, The Netherlands     

Hydrogen oxidation and nitrogen fixation in rhizobia,with special attention focused on strain ORS 571

In this survey we describe the influence of hydrogen oxidation on the physiology ofRhizobium ORS 571. The presence of hydrogen is required for the synthesis of hydrogenase. Carbon substrates do not repress the synthesis of hydrogenase. The respiratory system contains cytrochromes of theb- andc-type. Cytochromea ₆₀₀ is present after growth at high oxygen tensions. The nature of the terminal oxidases functioning at low oxygen tensions has not been established yet. → H⁺/O values with endogenous substrates are between 6 and 7. The results show the presence of two phosphorylation sites: site 1 (ATP/2e=1.0) and site 2(ATP/2e=1.33). By measuring molar growth yields it has been demonstrated that carbon-limited, nitrogen-fixing cultures obtain additional ATP from hydrogen oxidation, and that site 2 of oxidative phosphorylation is passed during hydrogen oxidation. A method is described to calculate ATP/N₂ values (the total amount of ATP used by nitrogenase during the fixation of 1 mol N₂) and H₂/N₂ ratios (mol hydrogen formed per mol N₂ fixed) in aerobic organisms. ForRhizobium ORS 571 the ATP/N₂ value is about 40 and the H₂/N₂ ratio is between 5 and 7.5. Cells obtained from oxygen-limited nitrogen-fixing cultures contain 30–40% poly-β-hydroxybutyrate, which explains the high molar growth yields found. Hydrogen has not been detected in the effluent gas of these cultures, which may point to reoxidation of the hydrogen formed at nitrogen fixation. Calculations show that the effect of hydrogen reoxidation on the efficiency of nitrogen fixation (g N fixed × mol⁻¹ substrate converted) is not very large and that the actual H₂/N₂ ratio is of much more importance. After addition of hydrogen to succinate-limited, ammonia-assimilating cultures, an initial increase of the Y_succinate value (g dry wt × mol⁻¹ succinate) is followed by a gradual decrease. This is accompanied by a large decrease of the value, and an increased permeability of the cytoplasmic membrane to protons. The results may be explained by a transition of the culture from an energy-limited state to a carbon-limited state.     

1H nuclear magnetic resonance studies of N-acetyl-L-proline N-methylamide. Molecular conformations,hydrogen bondings,and thermodynamic quantities in various solvents

Concentration and temperature dependences of the ¹H nmr spectra of N-acetyl-L -proline N-methylamide were observed in various solvents [CCl₄, CDCl₃, (CD₃)₂CO, (CD₃)₂SO, H₂O, and D₂O]. The fraction of the cis isomer (with respect to the bond between the acetyl carbonyl carbon and prolyl nitrogen atoms) depends greatly on the solvent used; the fraction of the cis isomer is higher in polar solvents than in nonpolar solvents. It depends also on concentration and temperature in nonpolar solvents but not in polar solvents. In nonpolar solvents the trans isomer mostly exists in the γ-turn structure with an intramolecular hydrogen bond and the cis isomer tends to form molecular aggregates by intermolecular hydrogen bonds. In polar solvents both the cis and trans isomers exist in monomeric forms which interact with solvent molecules. The pH dependences of the N-methyl proton resonances indicate that the γ-turn structure of the trans isomer is present also in aqueous solution, though its population is difficult to determine. Apparent enthalpy and entropy changes for the conversion of the trans isomer to cis isomer are evaluated for various solvents. The results are discussed in terms of the intra- and intermolecular hydrogen bondings.     

The role of water in the crystal structure of N-acetyl-L-4-hydroxyproline

The crystal structure of N-acetyl-L -4-hydroxyproline (Hyp) was determined by direct methods. (The crystal is orthorhombic with the space group P2₁2₁2₁.) The acetyl group is in the trans conformation and the pyrrolidine ring puckers at C^γ (C^sC^γ envelope), as in most Hyp residues. According to the rotation angle ψ = ?30°, the N-acetyl-L -4Hyp has the same conformation as an α-helix of prolyl residues. The crystal packing is stabilized by hydrogen bonds between three different molecules and the same molecule of water. One of the water bridges involves the carbonyl of the N-acetyl group of one molecule and the hydrogen atom of the 4-OH group of another. Such an arrangement has been proposed to explain the high stability of (Gly-L -Pro-L -4Hyp)_n. A second bridge involves the two hydrogens of the water molecule and the carbonyl groups of two neighbouring molecules, as already proposed in a dihydrated model of collagen. These experimental features, which are discussed in relation to the different models of collagen, allow us to propose an hypothetical arrangement for the water molecule which is strongly retained in the triple helix of (Gly-L -Pro-L -4Hyp)_n.     

Increase of Nitrogenase Activity in the Blue-Green Alga Nostoc muscorum (Cyanobacterium)

Preincubation of the blue-green alga (cyanobacterium) Nostoc muscorum under hydrogen or argon (nongrowing conditions, neither CO₂ nor N₂ or bound nitrogen present) in the light resulted in a two- to fourfold increase of light-induced hydrogen evolution and a 30% increase of acetylene reduction. Preincubation under the same gases in the dark led to a decrease of both activities. Cultivation of algae under a hydrogen-containing atmosphere (N₂, H₂, CO₂) increased neither hydrogen nor ethylene evolution by the cells. Formation of both ethylene and hydrogen is due to nitrogenase activity, which apparently was induced by the absence of N₂ or bound nitrogen and not by the presence of hydrogen. Inhibitors of protein biosynthesis prevented the increase of nitrogenase activity. Hydrogen uptake by the cells was almost unaffected under all of these conditions. With either ammonia or chloramphenicol present, nitrogenase activity decreased under growing conditions (i.e., an atmosphere of N₂ and CO₂). The kinetics of decrease were the same with ammonia or chloramphenicol, which was interpreted as being due to rapid protein breakdown with a half-life of approximately 4 h. The decay of nitrogenase activity caused by chloramphenicol could be counteracted by nitrogenase-inducing conditions, i.e., by the absence of N₂ or bound nitrogen. A cell-free system from preconditioned algae with an adenosine 5′-triphosphate-generating system exhibited the same increase or decrease of nitrogenase activity as the intact cell filaments, indicating that this effect resided in the nitrogenase complex only. We tentatively assume that not the whole nitrogenase complex, but merely a subunit or a special protein with regulatory function, is susceptible to fast turnover.     

Proton transfer and polarizability of hydrogen bonds formed between cysteine and lysine residues

(L -Cys)_n + N-base systems and (L -Cys)_n + (L -Lys)_n systems were studied by ir spectroscopy. It is shown that in the water-free systems, SH ?N ? S^? ?H⁺N hydrogen bonds are formed. With the (L -Cys)_n + N-base systems, both proton-limiting structures in the SH ?N ? S^? ?H⁺N bonds have equal weight when the pK_a of the protonated N-base is 2 pK_a units larger than that of (L -Cys)_n. The same is true with the water-free (L -Cys)_n + (L -Lys)_n system. Thus, with regard to the type of proton potentials present, these hydrogen bonds are proton-transfer hydrogen bonds showing very large proton polarizabilities. This is confirmed by the occurrence of continua in the ir spectra. Small amounts of water open these hydrogen bonds and increase the transfer of the proton to (L -Lys)_n. In the (L -Lys)_n + N-base systems, with increasing proton transfer the backbone of (L -Cys)_n changes from antiparallel β-structure to coil. In (L -Cys)_n + (L -Lys)_n, the conformation is determined by the (L -Lys)_n conformation and changes depending on the chain length of (L -Lys)_n. Finally, the reactivity increase in the active center of fatty acid synthetase, which should be caused by the shift of a proton, is discussed on the basis of the great proton polarizability of the cysteine–lysine hydrogen bonds.     

Solvent effects on the conformational preferences of model peptoids. MP2 study

The influence of aqueous environment on the main‐chain conformation (ω₀, ?, and ψ dihedral angles) of two model peptoids: N‐acetyl‐N‐methylglycine N’‐methylamide (Ac‐N(Me)‐Gly‐NHMe) ( 1 ) and N‐acetyl‐N‐methylglycine N’,N’‐dimethylamide (Ac‐N(Me)‐Gly‐NMe₂) ( 2 ) was investigated by MP2/6‐311++G(d,p) method. The Ramachandran maps of both studied molecules with cis and trans configuration of the N‐terminal amide bond in the gas phase and in water environment were obtained and all energy minima localized. The polarizable continuum model was applied to estimate the solvation effect on conformation. Energy minima of the Ac‐N(Me)‐Gly‐NHMe and Ac‐N(Me)‐Gly‐NMe₂ have been analyzed in terms of the possible hydrogen bonds and C = O dipole attraction. To validate the theoretical results obtained, conformations of the similar structures gathered in the Cambridge Crystallographic Data Centre were analyzed. Obtained results indicate that aqueous environment in model peptoids 1 and 2 favors the conformation F (? and ψ = ?70º, 180º), and additionally significantly increases the percentage of structures with cis configuration of N‐terminal amide bond in studied compounds. Copyright © 2014 European Peptide Society and John Wiley & Sons, Ltd.     

Conformational energy calculations of enzyme-substrate complexes of lysozyme. I. Energy minimization of monosaccharide and oligosaccharide inhibitors and substrates of lysozyme

Conformational energy calculations were performed on monosaccharide and oligosaccharide inhibitors and substrates of lysozyme to examine the preferred conformations of these molecules. A grid-search method was used to locate all of the low-energy conformational regions for N-acetyl-β-D -glycosamine (NAG), and energy minimization was then carried out in each of these regions. Three stable positions for the N-acetyl group have ben located, in two of which the plane of the amide unit is normal to the mean plane of the pyranosyl ring. Nine local energy minima were located for the —CH₂OH group. The positions of the two vicinal cis —OH groups are determined predominantly by interactions with either the —CH₂OH or the N-acetyl group. The most stable conformations of β-N-acetylmuramic acid (NAM) were determined from the study of the low-energy conformations of NAG. In the two stable orientations for the D -lactic acid side chain, the O—C—C′ plane (C′ being the carbon atom of the terminal carboxyl group) was found to be normal to the mean plane of the pyranosyl ring. The low-energy positions for the COOH group of NAM are determined mainly by interactions with neighboring groups. The conformational preferences of the α-anomers of NAG and NAM were also explored. The calculated conformation of the N-acetyl group for α-NAG was quite close to that determined by X-ray analysis. Two of the three lowest energy conformations of α-NAM are similar to the corresponding conformations of the β-anomer. A third low-energy structure, which has a hydrogen bond from the NH of the N-acetyl group to the C?O of the lactic acid group, corresponds very closely to the X-ray structure of this molecule. The preferred conformations of the disaccharides NAG–NAG, NAM–NAG and NAG–NAM were also investigated. Two preferred orientations of the reducing pyranosyl ring relative to the nonreducing ring were found for all of these disaccharides, both of which are close to the extended conformation. In one of these conformations, a hydrogen bond can form between the OH group attached to C₃ of the reducing sugar and the ring oxygen of the preceding residue. Each conformation can be stabilized further by a hydrogen bond between the CH₂OH (donor) of residue i + 1 and the C?O of residue i (acceptor). The interactions that determine conformations for all oligosaccharides containing both NAG and NAM are shown to be exclusively intraresidue and nearest neighbor interactions, so that it is possible to predict all stable conformations of oligosaccharides containing NAG and NAM in any sequence.     

Abundance and arrangement of single,double and bifurcated hydrogen bonds in protein α-helices: Statistical analysis of PDB-select

Statistics are collected and analyzed for the possibility of hydrogen bonding in the secondary structures of globular proteins, based on geometric criteria. Double and bifurcated bonds are considered as pairs of admissible H-bonds with two proton donors or two proton acceptors, respectively. Most of such bonds belong to peptide groups in α-helices, with O _i …N _{i + 3} nearly as frequent as O _i …N _{i + 4}; in contrast, most of the 3/10-helical segments are too short to have any. Alternating double and bifurcated bonds in α-helices form an apparently cooperative network structure. A typical α-helical segment perhaps carries two stretches of the H-bond network broken in the middle. The constituent H-bonds are nonlinear: the hydrogen atom is off the straight line connecting the proton donor and proton acceptor atoms. This deflection is larger for H _{i + 3} vs. bond line O _i −N _{i + 3} than for H _{i + 4} vs. O _i −N _{i + 4}, and though the two kinds of bond have about the same length (exceeding those typical of low-molecular compounds), O _i …N _{i + 4} must be stronger than O _i …N _{i + 3}. Double/bifurcated bonds are also not coplanar, i.e., hydrogen atoms are beyond the N…O…N (or O…N…O) plane. The text was submitted by the authors in English.     

Relationship between the structure of diamine platinum(II) complexes and their cytostatic activity as measured on plant roots

The amine substituent effect in compounds [Pt(diamine)Cl₂] on inhibition of maize and cucumber root growth and branching has been investigated. The diamines used were ethylenediamine (en),N-methylethylenediamine (men),N,N-dimethylethylenediamine (N,N-dmen),N,N-dimethylethylenediamine (N,N-dmen),N,N,N,N-tetramethylethylenediamine (tmen), 1,2-propanediamine (1,2-pn), 2-methyl-1,2-propanediamine (ibn), 2,3 dimethyl-2,3-butanediamine (C-tmen), 1,3-propanediamine (1,3-pn), 2,2-dimethyl-1,3-propanediamine (C₂-dm-1,3 pn),N,N-dimethyl-1,3-propanediamine (N,N-dm-1,3-pn). Increased substitution of hydrogen atoms of the amine part with CH₃ groups reduces the cytostatic activity of complexes. The substitution of hydrogen atoms of NH₂ and vicinal CH₂ groups displays similar results. C-2 dimethylation (C-dm-1,3-pn) does not change the activity of the complex compared with (1,3-pn). It was observed that maize and cucumber roots differ in their relative sensitivity to various complexes. All complexes containing pn and their substituted analogs inhibited cucumber root growth weaker than that in maize. A comparison of obtained data with earlier published results concerning antitumor activity of complexes shows that they correlate in a similar manner with increased substitution of amino groups. Therefore, roots may be used as cheap test objects for primary screening of cytostatics. The general tendency of a decrease in cytostatic activity goes parallel with the number of Nor vicinal C-methyl groups and seems to arise from a decrease in hydrogen-bonding potential; however, some other possible reasons are also discussed. The activity discrimination by different species in our experiments and clearly different results forN,N-dimethylation depending on the chelate ring size (en and pn derivatives) on maize cannot be attributed to slower ligand-exchange kinetics from methylation. It is possible to assume that the major role in cytostatic activity of platinum complexes belongs to a cell repair system, i.e. the ability to eliminate platinum diamine fragments from DNA, depending on the number and strength of hydrogen bonds formed by thecis-diamine fragment.     

EFFECT OF TEMPERATURE ON THE SENSITIVITY OF NITROGENASE TO OXYGEN IN TWO HETEROCYSTOUS CYANOBACTERIA1

The effect of temperature and oxygen on nitrogenase activity in two heterocystous cyanobacteria, Anabaena variabilis Kütz. ATCC29413 and Nostoc sp. PCC7120, was investigated. The cyanobacteria were grown under a 12:12 light:dark (L:D) cycle at 27°C and were subsequently exposed to different temperatures (27, 36, 39, and 42°C) at different steady‐state O₂ concentrations (20, 10, 5, 0%). Light response curves of nitrogenase activity were recorded under each of these conditions using an online acetylene reduction assay combined with a sensitive laser photoacoustic ethylene detection method. The light response curves were fitted with the rectangular hyperbola model from which the model parameters N_m, N_d, and α were derived. In both strains, nitrogenase activity (N_tot = N_m + N_d) was the highest at 39°C–42°C and at 0% O₂. The ratio N_tot/N_d was 4.1 and 3.1 for Anabaena and Nostoc, respectively, indicating that respectively 25% and 33% of nitrogenase activity was supported by respiration (N_d). N_tot/N_d increased with decreasing O₂ concentration and with increasing temperature. Hence, each of these factors caused a relative increase in the light‐driven nitrogenase activity (N_m). These results demonstrate that photosynthesis and respiration both contribute to nitrogenase activity in Anabaena and Nostoc and that their individual contributions depend on both O₂ concentration and temperature as the latter may dynamically alter the flux of O₂ into the heterocyst.     

Crystal structure of Boc-Ala-Aib-Ala-Aib-Aib-methyl ester,a pentapeptide fragment of the channel-forming ionophore suzukacillin

t-Buthyoxycarbonyl-L -alanyl-α-aminiosobutyryl-L -alanyl-α-aminoisobutyryl-α-aminoisobutyric acid methyl ester (t-Boc-L -Ala-Aib-L -Ala-Aib-Aib-OMe), C₂₄H₄₃N₅O₈, an end-protected pentapeptide with a sequence corresponding to the 6th through the 10th residues in suzukacillin, crystallizes in the orthorhombic space group P2₁2₁2₁ with a = 11.671, b = 14.534, c = 17.906 Å and z = 4. The molecule exists as a right-handed 3₁₀-helix with a pitch of 6.026 Å. The helix is stabilized by three 4 → 1 hydrogen bonds with the NH groups of Ala(3), Aib(4), and Aib(5) hydrogen bonding to the carbonyl oxygens of t-Boc, Ala(1), and Aib(2), respectively. The helical molecules arrange themselves in a head-to-tail fashion along the a direction in such a way that the NH groups of Ala(1) and Aib(2) hydrogen bond to the carbonyl oxygens of Aib(4) and Aib(5), respectively, of a translationally related molecule. The helical columns thus formed close-pack nearly hexagonally to form the crystal.