Flash photolysis of the carbon monoxide compounds of wild-type and mutant variants of cytochrome bo from Escherichia coli

The carbon monoxide compounds of the fully reduced and mixed valence forms of cytochrome bo from Escherichia coli were laser photolysed under anaerobic conditions at room temperature. The carbon monoxide recombined with characteristic rate constants of 50 s-1 or 35 s-1 in the fully reduced and mixed valence forms, respectively. Rates of CO recombination with the fully reduced enzyme were examined in a variety of mutant forms of cytochrome bo, produced by site-directed mutagenesis. A method was developed to deconvolute cytochromes bo and bd, leading to some reassessment of histidine ligands to the metals. Significant changes in the rate constant of recombination of carbon monoxide occurred in many of these mutants and these results could be rationalised generally in terms of our current working model of the folding structure of subunit I. In the mixed valence form of the enzyme the transient photolysis spectra in the visible region are consistent with a rapid electron redistribution from the binuclear centre to the low-spin haem. This electron transfer is biphasic, with rate constants of around 10(5) and 8000 s-1. The process was also examined in the His-333-Leu mutant, in which a putative histidine ligand to CuB is replaced by leucine, and which results in the loss of the CuB. It appeared that rapid haem-haem electron transfer could still occur. The observation that CuB is apparently not required for rapid haem-haem electron transfer is consistent with the recently proposed model in which the two haems are positioned on opposite sides of transmembrane helix X in subunit I of the oxidase.     

The UK Renewable Energy programme

The UK Government has supported a programme on Renewable Energy since 1975. In 1988 progress was reviewed and this led to some redirection of the programme with greater emphasis on the commercial development of renewables, as published in Energy Paper 55.The climate in which Renewable Energy operates has been influenced recently by a number of factors including privatisation of the UK electricity supply industry, when the Government introduced the New Fossil Fuel Obligation and publication of the Government's 1990 White Paper on the Environment which noted that the Government is working towards a figure of new renewable energy generating capacity of 1000 MW by 2000.The paper aims to give an overview of the present state of development of the renewable technologies in the UK and to discuss some of the factors - technical and non-technical - which will affect their likely future development.     

Reflection and transmission of plane waves at an interface between two fluids

The propagation, reflection, and transmission of a plane wave through a column of two fluids with a material discontinuity is studied by three methods: a mixed finite element formulation with both pressure and velocity at a point taken as independent variables, and a scaled and an un-scaled acoustic pressure formulation in which only the pressure at a point is taken as an independent variable. It is found that when mass densities of two fluids are close to each other, the un-scaled acoustic pressure formulation gives reasonable results. However, when the speeds of sound in two fluids are close to each other but their mass densities are quite different, and for cases where the first fluid has high impedance relative to that of the second fluid, a mixed or scaled pressure formulation is necessary. Without the mixed or scaled pressure formulation, the continuity conditions at the interface between two fluids are not well satisfied for the un-scaled pressure formulation. The consideration of viscosity of the two fluids and using a dispersion correction method in the time integration scheme in the mixed formulation slightly improves results.     

Intra-seasonal NDVI change projections in semi-arid Africa

Early warning systems (EWS) tend to focus on the identification of slow onset disasters such famine and epidemic disease. Since hazardous environmental conditions often precede disastrous outcomes by many months, effective monitoring via satellite and in situ observations can successfully guide mitigation activities. Accurate short term forecasts of NDVI could increase lead times, making early warning earlier. This paper presents a simple empirical model for making 1 to 4 month NDVI projections. These statistical projections are based on parameterized satellite rainfall estimates (RFE) and relative humidity demand (RHD). A quasi-global, 1 month ahead, 1° study demonstrates reasonable accuracies in many semi-arid regions. In Africa, a 0.1° cross-validated skill assessment quantifies the technique's applicability at 1 to 4 month forecast intervals. These results suggest that useful projections can be made over many semi-arid, food insecure regions of Africa, with plausible extensions to drought prone areas of Asia, Australia and South America.     

New mechanisms in photo-assisted MOVPE of II-VI semiconductors

The first detailed comparison has been made of the metalorganic vapor phase epitaxy growth rates of CdTe, ZnTe, and ZnSe, measured in situ with laser reflectometry. The comparison also includes the photo-assisted growth with visible radiation from an argon ion laser. Using a standard Group II precursor (DMCd or DMZn.TEN) partial pressure of 1.5 × 10⁻⁴ atm, VI/II ratio of 1 and DIPM (M = Te, Se) the maximum growth rates are in the region of 10 to 15 AU/ s. Decrease in growth rates of ZnTe at higher temperatures or higher laser powers have been attributed to the desorption from the substrate of unreacted Te precursor. The behavior of DTBSe is quite different from DIPSe for both pyrolytic and photo-assisted growth. The maximum growth rate is around 1 AU/ s with very little photo-enhancement, except at 300°C. Secondary ion mass spectroscopy analysis of hydrogen concentration in the ZnSe layers shows high concentrations, up to 5.9 × 10¹⁹ atoms cm⁻³ for DTBSe grown ZnSe under pyrolytic conditions. These results show that the growth kinetics play an important part in the incorporation of hydrogen and passivation of acceptor doped material.