Spectroscopic characterization of reaction centers of the (M)Y210W mutant of the photosynthetic bacterium Rhodobacter sphaeroides

The tyrosine-(M)210 of the reaction center of Rhodobacter sphaeroides 2.4.1 has been changed to a tryptophan using site-directed mutagenesis. The reaction center of this mutant has been characterized by low-temperature absorption and fluorescence spectroscopy, time-resolved sub-picosecond spectroscopy, and magnetic resonance spectroscopy. The charge separation process showed bi-exponential kinetics at room temperature, with a main time constant of 36 ps and an additional fast time constant of 5.1 ps. Temperature dependent fluorescence measurements predict that the lifetime of P* becomes 4–5 times slower at cryogenic temperatures. From EPR and absorbance-detected magnetic resonance (ADMR, LD-ADMR) we conclude that the dimeric structure of P is not significantly changed upon mutation. In contrast, the interaction of the accessory bacteriochlorophyll BA with its environment appears to be altered, possibly because of a change in its position

Two different charge-separation pathways in photosystem II

Charge separation is an essential step in the conversion of solar energy into chemical energy in photosynthesis. To investigate this process, we performed transient absorption experiments at 77 K with various excitation conditions on the isolated Photosystem II reaction center preparations from spinach. The results have been analyzed by global and target analysis and demonstrate that at least two different excited states, (Ch

Inibition of soluble guanylate cyclase by ODQ

The heme in soluble guanylate cyclases (sGC) as isolated is ferrous, high-spin, and 5-coordinate. [1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one] (ODQ) has been used extensively as a specific inhibitor for sGC and as a diagnostic tool for identifying a role for sGC in signal transduction events. Addition of ODQ to ferrous sGC lends to a Soret shift from 431 to 392 nm and a decrease in nitric oxide (NO)stimulated sGC activity. This Soret shift is consistent with oxidation of the ferrous heme to ferric heme, The results reported here further define the molecular mechanism of inhibition of sGC by ODQ. addition of ODQ to the isolated sGC heme domain [beta 1(1-385)] gave the same spectral changes as when sGC was treated with ODQ. EPR and resonance Raman spectroscopy was used to show that the heme in ODQ-treated beta 1(1-385) is indeed ferric. Inhibition of the NO-stimulated sGC activity by ODQ is due to oxidation of the sGC heme and not to perturbation of the catalytic site, since the ODQ-treated sGC has the same basal activity as untreated sGC (68 +/- 12 nmol min(-1) mg(-1)). In addition, ODQ-oxidized sGC can be re-reduced by dithionite, and this re-reduced sGC has identical NO-stimulated activity as the original ferrous sGC. Oxidation of the sGC heme by ODQ is fast with a second-order rate constant of 8.5 x 10(3) M-1 s(-1). ODQ can also oxidize hemoglobin, indicating that the reaction is not specific for the heme in sGC versus that in other hemoproteins

The addition bias in Dutch and Spanish phonological speech errors: The role of structural context

Item does not contain fulltextIn speaking, both content (words, segments) and structure (syntactic trees, metrical structures) need to be produced. There is controversy about which aspects of metrical structure are generated during phonological encoding. In particular, do speakers retrieve a unit that represents CV structure? The present study shows that the incorporation of units for syllable CV structures in a connectionist model of phonological encoding enables us to explain empirical patterns of speech errors. The model accounts for the finding of a bias towards additions of segments. First, corpus analyses in Dutch and Spanish showed an addition bias in both languages. A second corpus analysis showed that in apparently noncontextual deletion errors the resulting CV structure is often identical to that of syllables in the immediate context. Furthermore, simulations are reported with two connectionist models, one that incorporates representations for CV structures (Dell, 1988, Journal of Memory and Language) and one that does not (Dell, 1986, Psychological Review). Only the first model correctly simulated the empirically obtained addition bias, as well as the general pattern of substitution errors. Further simulations showed that the model predicts effects of the structural context. A final simulation tested a second explanatory mechanism for the addition bias: Feedback from segments to syllables

Distributed garbage collection for mobile actor systems: The pseudo root approach

Abstract. Automatic distributed garbage collection (GC) gives abstraction to grid application development, promoting code quality and improving resource management. Unreachability of active objects or actors from the root set is not a sufficient condition to collect actor garbage, making passive object GC algorithms unsafe when directly used on actor systems. In practical actor languages, all actors have references to the root set since they can interact with users, e.g., through standard input or output streams. Based on this observation, we introduce pseudo roots: a dynamic set of actors that can be viewed as the root set. Pseudo roots use protected (undeletable) references to ensure that no actors are erroneously collected even with messages in transit. Following this idea, we introduce a new direction of actor GC, and demonstrate it by developing a distributed GC framework. The framework can thus be used for automatic life time management of mobile reactive processes with unordered asynchronous communication. This report is an extended version of [42]. It provides more information about how we built a distributed garbage collector with the help of the pseudo root approach. It also shows experimental results for local GC.