Electron transport to nitrogenase in Azotobacter vinelandii

The enzyme nitrogenase requires MgATP, an anaerobic environment and an electron donor with a low redox potential for activity. The experiments described in this thesis deal with the electron transport to nitrogenase in Azotobacter vinelandii . It has been shown previously that the flow of reducing equivalents to nitrogenase is regulated by the Δψcomponent of the proton motive force. Short-term inhibition of nitrogenase activity by externally added NH4 +would be caused by lowering the Δψ. In Chapter 2 it has been shown that the extent of inhibition by NH 4 Cl is variable and depends upon the incubation conditions of the cells. Conditions are described, where nitrogenase activity is hardly inhibited by addition of NH 4 Cl and also conditions, where uptake of NH 4 Cl results in complete inhibition of nitrogenase activity. These results are discussed in Chapter 6. In addition to the membrane potential glutamine is proposed as another regulator of electron transport to nitrogenase.In Chapter 3 it has been shown, that whole cell nitrogenase activity is determined by the generation of reducing equivalents for nitrogenase. The physiological electron transport system to nitrogenase is very effective compared to the electron donor dithionite often used in invitro experiments. It has been shown that whole cell nitrogenase activity invivo can be twice the activity measured invitro . The consequences of this finding are discussed with respect to the mechanism for nitrogenase catalysis invitro and invivo .In Chapter 4 it has been shown, that three different flavodoxins can be isolated from A.vinelandii cells. Experimental evidence indicates that only flavodoxin II is involved in N 2 -fixation. The concentration of flavodoxin II is tenfold higher in N 2 -fixing cells compared to cells grown on NH 4 Ac. And its synthesis seems to be under the same regulatory control as the nitrogenase proteins.In Chapter 5 evidence is presented that a membrane bound NADPH dehydrogenase and two membrane bound polypeptides of relative molecular mass 29000 and 30000 probably play a role in electron transport to nitrogenase. Furthermore In Chapter 5 it has been demonstrated that there is a linear relationship between nitrogenase activity and the rate of respiration of A.vinelandii cells. It is proposed that the generation of reducing equivalents for nitrogenase is directly controlled by electron transfer activity in the respiratory chain.In Chapter 6 the new findings on the electron transport to nitrogenase are summarized in a scheme. In the scheme the electron carrier flavodoxin II is reduced by a membrane bound NADPH dehydrogenase only when the respiratory chain is functioning

Selection of dominant multi-exciton transitions in disordered linear J-aggregates

We show that the third-order optical response of disordered linear J-aggregates can be calculated by considering only a limited number of transitions between (multi-) exciton states. We calculate the pump-probe absorption spectrum resulting from the truncated set of transitions and show that, apart from the blue wing of the induced absorption peak, it agrees well with the exact spectrum.Comment: 8 pages, 2 figures, accepted to Journal of Luminescenc

Pressure dependence of diffusion in simple glasses and supercooled liquids

Using molecular dynamics simulation, we have calculated the pressure dependence of the diffusion constant in a binary Lennard-Jones Glass. We observe four temperature regimes. The apparent activation volume drops from high values in the hot liquid to a plateau value. Near the critical temperature of the mode coupling theory it rises steeply, but in the glassy state we find again small values, similar to the ones in the liquid. The peak of the activation volume at the critical temperature is in agreement with the prediction of mode coupling theory

Continuum theory of vacancy-mediated diffusion

We present and solve a continuum theory of vacancy-mediated diffusion (as evidenced, for example, in the vacancy driven motion of tracers in crystals). Results are obtained for all spatial dimensions, and reveal the strongly non-gaussian nature of the tracer fluctuations. In integer dimensions, our results are in complete agreement with those from previous exact lattice calculations. We also extend our model to describe the vacancy-driven fluctuations of a slaved flux line.Comment: 25 Latex pages, subm. to Physical Review

A review of applications of the Bayes factor in psychological research

The last 25 years have shown a steady increase in attention for the Bayes factor as a tool for hypothesis evaluation and model selection. The present review highlights the potential of the Bayes factor in psychological research. We discuss six types of applications: Bayesian evaluation of point null, interval, and informative hypotheses, Bayesian evidence synthesis, Bayesian variable selection and model averaging, and Bayesian evaluation of cognitive models. We elaborate what each application entails, give illustrative examples, and provide an overview of key references and software with links to other applications. The paper is concluded with a discussion of the opportunities and pitfalls of Bayes factor applications and a sketch of corresponding future research lines