Kinetics and mechanism of CO substitution of [η-CpFe(CO)3]PF6 with PPh3 in the presence of substituted pyridine N-oxide

The kinetics of substitution reactions of [η-CpFe(CO)₃]PF₆ with PPh₃ in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH)₄, 4-Ph, 3-Me, 2,3-(CH)₄, 2,6-Me₂, 2-Me), the reactions yeild the same product [η⁵-CpFe(CO)₂PPh₃]PF₆, according to a second-order rate law that is first order in concentrations of [η⁵-CpFe(CO)₃]PF₆ and of R-PyO but zero order in PPh₃ concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH^≠ = 13.4 ± 0.4 kcal mol⁻¹, ΔS^≠ = −19.1 ± 1.3 cal k⁻¹ mol⁻¹ for 4-PhPyO; ΔH^≠ = 12.3 ± 0.3 kcal mol⁻¹, ΔS^≠ = 24.7 ±1.0 cal K⁻¹ mol⁻¹ for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing ¹⁷O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed.     

Photosynthetic performance of a helical tubular photobioreactor incorporating the cyanobacterium spirulina platensis

The photosynthetic performance of a helical tubular photobioreactor ("Biocoil"), incorporating the filamentous cyanobacterium Spirulina platensis, was investigated. The photobioreactor was constructed in a cylindrical shape (0.9 m high) with a 0.25-m(2)basal area and a photostage comprising 60 m of transparent PVC tubing of 1.6-cm inner diameter (volume = 12.1 L). The inner surface of the cylinder (area = 1.32 m(2)) was illuminated with cool white fluorescent lamps; the energy input of photosynthetically active radiation(PAR, 400 to 700 nm) into the photobioreactor was 2920 kJ per day. An air-lift system ncorporating 4%CO(2) was used to circulate the growth medium in the tubing. The maximum productivity achieved in batch culture was 7.18 g dry biomass per day [0.51 g . d biomass/L . day, or 5.44 g . d biomass/m(2)(inner surface of cylindrical shape)/day] which corresponded to a photosynthetic (PAR) efficiency of 5.45%. The CO(2) was efficiently removed from the gaseous stream; monitoring the CO(2) the outlet and inlet gas streams showed a 70% removal of CO(2) from the inlet gas over an 8-h period with almost maximum growth rate. (c) 1995 John Wiley & Sons, Inc.     

Myoglobin discriminates between O2, NO, and CO by electrostatic interactions with the bound ligand

 Most biological substrates have distinctive sizes, shapes, and charge distributions which can be recognized specifically by proteins. In contrast, myoglobin must discriminate between the diatomic gases O₂, CO, and NO which are apolar and virtually the same size. Selectivity occurs at the level of the covalent Fe-ligand complexes, which exhibit markedly different bond strengths and electrostatic properties. By pulling a water molecule into the distal pocket, His64(E7)¹ inhibits the binding of all three ligands by a factor of ∼10 compared to that observed for protoheme-imidazole complexes in organic solvents. In the case of O₂ binding, this unfavorable effect is overcome by the formation of a strong hydrogen bond between His64(E7) and the highly polar FeO₂ complex. This favorable electrostatic interaction stabilizes the bound O₂ by a factor of ∼1000, and the net result is a 100-fold increase in overall affinity compared to model hemes or mutants with an apolar residue at position 64. Electrostatic interaction between FeCO and His64 is very weak, resulting in only a two- to three-fold stabilization of the bound state. In this case, the inhibitory effect of distal pocket water dominates, and a net fivefold reduction in K _CO is observed for the wild-type protein compared to mutants with an apolar residue at position 64. Bound NO is stabilized ∼tenfold by hydrogen bonding to His64. This favorable interaction with FeNO exactly compensates for the tenfold inhibition due to the presence of distal pocket water, and the net result is little change in K _NO when the distal histidine is replaced with apolar residues. Thus, it is the polarity of His64 which allows discrimination between the diatomic gases. Direct steric hindrance by this residue plays a minor role as judged by: (1) the independence of K _O2, K _CO, and K _NO on the size of apolar residues inserted at position 64, and (2) the observation of small decreases, not increases, in CO affinity when the mobility of the His64 side chain is increased. Val68(E11) does appear to hinder selectively the binding of CO. However, the extent is no more than a factor of 2–5, and much smaller than electrostatic stabilization of bound O₂ by the distal histidine. Received, accepted: 23 May 1997     

Infrared spectra of carbon monoxide bound to mitochondria from diverse species and tissues reveal structurally similar cytochrome c oxidase dioxygen reaction sites

Infrared bands for CO bound to mitochondria from bovine and porcine hearts, bovine brain, rat kidney, and blowfly flight muscle and to intact blowfly flight muscle have been measured in the carbon-oxygen stretch region. Each spectrum contains a narrow band near 1963 cm-1 similar to the major band found earlier for the carbonyl cytochrome c oxidase purified from bovine heart. A second band near 1959 cm-1 ascribed to a less stable conformer of the purified oxidase carbonyl is also detected in mitochondria. These spectra support very similar CO (and O2) binding sites among all the oxidases examined whether the enzyme is purified or is still within mitochondria or intact tissue and therefore suggest that the reduced heme A ligand binding site has been highly conserved during evolution.     

The effect of ethoxyzolamide, an analogue of insect juvenile hormone, nor-adrenaline and iodine on changes in the optical path difference in the excretory cells and oesophagus during exsheathment in Haemonchus contortus

Davey K. G., Sommerville R. I. and Rogers W. P. 1982. The effect of ethoxyzolamide, an analogue of insect juvenile hormone, nor-adrenaline and iodine on changes in the optical path difference in the excretory cells and oesophagus during exsheathment in Haemonchus contortus. International Journal for Parasitology12: 509–513. Ethoxyzolamide, an inhibitor of carbonic anhydrase, markedly inhibits exsheathment of Haemonchus when the larvae are subsequently exposed to an exsheathing stimulus of CO₂ at 38.5°C. Ethoxyzolamide at 2 × 10^?5M does not prevent the increase in optical path difference in the oesophageal region which normally accompanies exsheathment, but markedly inhibits the increase in optical path difference in the excretory cells. An analogue of juvenile hormone (JHA; the methyl ester of 3,7,11 trimethyl-7,11-dichloro-2-dodecenic acid) does not affect the optical path difference in either the oesophagus or the excretory cells of ensheathed worms. When worms are artificially desheathed by exposure to NaOCl, a procedure which mimics the effect of CO₂ upon the oesophagus, but which does not affect the excretory cells, subsequent exposure to JHA at room temperature increases the optical path difference in the excretory cells. This increase is enhanced by subsequent incubation of the worms at 38.5°C at 30–60 min and further enhanced when CO₂ is present during the incubation at 38.5°C. The stimulation of the excretory cells by JHA is inhibited by ethoxyzolamide at 2 × 10^?5M. Noradrenaline at 10^?3M has no effect on ensheathed larvae, but causes an increase in optical path difference in the excretory cells of larvae desheathed with NaOCl. This increase is inhibited by ethoxyzolamide. A brief exposure to I₂ blocks the response of the excretory cells of both CO₂ and JHA, but does not significantly reduce the effect of nor-adrenaline. On the basis of these and previous results, it is proposed that both CO₂ and JHA stimulate a hypothetical CO₂ receptor which leads to the release of nor-adrenaline. The noradrenaline in turn stimulates, either directly or indirectly, the excretory cells.     

Photosynthetic induction in wheat protoplasts and chloroplasts. Autocatalysis and light activation of enzymes

Abstract Unlike wheat chloroplasts, wheat protoplasts showed a pronounced restoration of the induction phase after a short period of darkness. This difference was used to investigate the relative roles of light-induced reductive activation of enzymes and the auto-catalytic increase in the level of substrates in the control of the rate of photosynthesis during induction. Light activation and dark inactivation of ribulose 5-phosphate kinase, fructose 1,6-biphosphatase and NADP⁺-specific glyceraldehydephosphate dehydrogenase were measured. In this respect there was no appreciable difference between protoplasts and chloroplasts. In contrast, the level of photosynthetic intermediates remained constant in darkened isolated chloroplasts, but declined rapidly in chloroplasts isolated from darkened protoplasts. When fructose 1,6-bisphosphatase was pre-activated by treating protoplasts with dithiothreitol the lag was only slightly shortened. These results are discussed in terms of control of the rate of the photosynthesis during the lag by substrates rather than limitation imposed by activity of any of the enzymes measured.     

Regulation by Ammonium of Variation in Heterotrophic CO2 Fixation by Euglena gracilis During Growth Cycles*

SYNOPSIS Heterotrophic (dark) CO₂ fixation by Euglena gracilis strain Z varies with phase of batch culture growth and mode of nutrition. Increases in the fixation during growth cycles correlate closely with the depletion of exogenous NH₄* from the medium during growth. It is demonstrated that exogenous NH₄⁺ regulates a component of heterotrophic CO₂ fixation and that another component is independent of NH₄⁺. This is true for cells grown heterotrophically (glucose, dark), autotrophically (CO₂, light) and for a permanently bleached strain (E. gracilis SB3). Some kinetics of the NH₄⁺ regulation are presented.