S state model

Mn(III) r Mn(IV) transition, 33 226-227 Mn-K edge spectra, 33 228-229 Oj-evolving complex, 33 221 proton release pattern, 33 235 rebinding, 33 223 spin echo studies, 33 233-234 S state model, 33 221 temperature dependence of EPR signals, 33 231-232... 

The mechanism of 02 evolution in photosynthesis has been the focus of much research. In a classic experiment, Joliot and co-workers (6) showed that 02 is evolved in a periodic pattern when a dark-adapted sample is given a series of short, saturating light flashes (Figure 1). This pattern of 02 evolution was explained by Kok and co-workers (7, 8) by the S-state model shown in equation 2 ... 

In a classical flash study, Kok et al. showed that the 02-evolving complex of photosystem II is oxidized sequentially in a four-quantum, four one-electron oxidative process, the steps being named S0-S4, with S0 representing the totally reduced form (186). In the S state model (Scheme 1), S4 is only transiently stable, rapidly converting to S0 with concomitant 02 evolution. 

The evolution of O2 from water has been shown to occur every 4th flash, when flashes of saturating intensity, short enough to allow only one turnover of the PS II reaction centres, are fired, separated by a dark period long enough to permit the reoxidation of the electron acceptors on the reducing side of PS II [7]. This observation has been the basis of the S states model. Each flash promotes the transition from the state S to S +, in the sequence [8,9] ... 

The stoichiometries of the two reactions above raise an interesting mechanistic question how is the one-electron photochemistry in PS II coupled to the four-electron water-splitting chemistry The now-classical single turnover flash experiments of Joliot and co-workers [2], which showed period four oscillation in the O2 yield with flash number, provided a clearcut answer the PS II units function independently in accumulating the four oxidizing equivalents required to split water. This observation was quickly confirmed [3,4] and Kok provided the S-state model which is now widely used to summarize the situation ... 

A substantial body of work had associated manganese with water splitting [5,10,109] and with the development of Kok s S-state model it was generally assumed that the S-state transitions correspond to valence changes in a functional manganese... 

In the original formulation of the S-state model, Kok et al. postulated that the dark resting state of the OEC was a 3 1 mixture of S, and Sq, respectively [3]. This... 

S-state model showing the redox state of the oxy n evolving system... 

Figure 5 shows a detail of the component s state model. The full model consists of 127 places, 182 transitions and 406 connecting arcs. It is scalable and can be easily detailed to depict more complex functional interactions and/or maintenance dependencies. 

TABLE 1. Initial population of S-states and flash parameters calculated by fitting the classical Kok S-state model to the data in Fig. 3 using a non-linear least-squares method. 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Fig. 8. Steady-state model for the earth s surface geochemical system. The kiteraction of water with rocks ki the presence of photosynthesized organic matter contkiuously produces reactive material of high surface area. This process provides nutrient supply to the biosphere and, along with biota, forms the array of small particles (sods). Weatheriag imparts solutes to the water, and erosion brings particles kito surface waters and oceans.

Wells, S. A. (1991) "Two-Dimensional, Steady-State Modeling of Cake Filtration in a Laterally Unconfined Domain," Fluid/Partide Separation Journal, Vol. 4, No. 2, June, 107-116... 

Takemasa, Y., S.Togati, and Y. Aral. 1996. Application of an unsteady-state model for predicting vertical temperature distribution to an existing atrium. ASHRAE Transactions, vol. 102, no. 1. 

Expert Judgment quantifies an expert s state of knowledge or perceptions of the likelihood of an incident. This knowledge may be based on historical data, insights gained from models, experience, or a combination of these factors. 

California, the Energy Commission in 1983 adopted stringent standards for refrigerators, freezers, central air conditioners, and heat pumps. Following California s lead, several other states became interested in adopting appliance efficiency standards, since there was now a state model on which they could draw. By the end of 1986, six states had adopted new standards for one or more products. 

Desulfurization of FCC feedstocks reduces the sulfur content of FCC products and SOX emissions. In the United States, road diesel sulfur can be 500 ppm (0.05 wt%). In some European countries, for example in Sweden, the sulfur of road diesel is 50 ppm or less. In California, the gasoline sulfur is required to be less than 40 ppm. The EPA s complex model uses sulfur as a controlling parameter to reduce toxic emissions. With hydroprocessed FCC feeds, about 5% of feed sulfur is in the FCC gasoline. For non-hydroprocessed feeds, the FCC gasoline sulfur is typically 10% of the feed sulfur. 

The results for the reaction of (S)-2-phenylpropanal and (Z)-2-butenylboronate may be reconciled with this transition state model if it is assumed that the phenyl substituent is smaller than methyl in the pair of transition states 6 and 7. analogous to 4 and 5. This is possible if the lowest energy transition state is one in which the phenyl group eclipses C(cc)-H, such that a flat, sterically undemanding surface is presented to the incoming (Z)-2-butenylboronate. Similar... 

A Zimmerman-Traxler transition state model is postulated in order to rationalize the ul topicity of this aldol addition [i.e., the (S)-enolate preferentially attacks the 7 e-face of the aldehyde]33. In the two alternative transition states 3a [ul topicity (S)jRe] and 3b [Ik topicity (S)/Si, the substituents at the stereogenic center of the enolatc are oriented in such a way that... 

The preference of the (5, .S )-boron cnolatc to attack almost exclusively the Si-face of an aldehyde is rationalized by assuming the Zimmerman-Traxler transition state model. It is postulated that the methyl group of the propyl residue directs the 3-elhylpenlane-3-thiol group towards the borolane moiety, the chirality of which is thus effectively transferred34. 

As outlined in Section D.2.3.5., the stereochemical outcome of the addition of nucleophilic reagents to chiral aldehydes or ketones is rationalized most plausibly by the Cram-Felkin-Anh model. On the other hand, the corresponding reactions of oxygen- or nitrogen-heterosub-stituted aldehydes or ketones may be interpreted either by the same transition state hypothesis or, alternatively, by Cram s cyclic model. 

Step 2. The qualitative value of the desired change is propagated through the steady-state model equations of the plant equipment, following the constraint propagation procedure of Steele (1980). Manipulations that cause the desired change and that are feasible are identified as White Knights and are constrained to lie before the situation of interest s, in accordance with the truth criterion. 

---