Extended fragments

All of these features contrast with the structure of the second extender-fragment complex, shown in Fig. 9.9b. Here, the extender forces itself into the S2 pocket, but the disulfide linker then curves back to place the thiophene sul-fone into the S4 pocket. The sulfone makes some of the same hydrogen bonds as the salicylic acid and the aspartyl residue in the tetrapeptide but with completely different chemistry. The flexibility of caspase-3 to accommodate different... 

For T = 573 K, log(n,/nn) log(nIa/n,Ia) ss 0, that is, the surface densities of BASs of the I and II types are approximately equal and independent of coverage. Also, log(nIa/nin) = log(nna/nin) = 12, that is, the surface density of BASs of the III type is too low to be of any practical interest in catalysis. Consideration of the extended fragment of a dehydroxylated surface plus five water molecules, where a surface H30+ ion receives additional possibilities for solvation, did not qualitatively change the conclusion. 

Even more extended fragmentation cascades have been reported. For example, Grob reported formation of acyclic nitriles from the Beckmann fragmentation of bicyclic Ts-oximes (Figure 8.23). Electron flow that led to the scission of two C-C and one N-O bond started from the two strong donors (the appropriately oriented lone pairs of the two oxygen atoms in the tetrahedral intermediate). 

Retrosynthetic analysis indicated that extended fragments of the O-SP can best be constructed by block synthesis using tetrasaccharide units. Of the four... 

Figure 2.36 sketches this extended fragmentation model. 

Fig. 2.36. Extended fragmentation model (taken from [307]). For explanation see the text in this section...

From now on the number of clusters involved in the Type I and Type II dissociation process will be called ni t) and ri2 i), respectively. The number of larger clusters, which are excited by the pump pulse and dissociate into the recorded mass channel, is characterized by a single simplifying function Hence, this extended fragmentation model leads to the following system of coupled differential equations ... 

To describe this more complicated behavior, the extended fragmentation model given in Sect. 2.2.2 is utilized as a basis. It takes into account the population density and fragmentation characteristics of Type I and Type II clusters. Since for delay times At < 0 and At > 0 the pump and probe pulses play different roles a further fragmentation channel has to be added to the extended model. Hence, the model used here contains four different fragmentation processes with four time constants tq, n, T2, and ts. Here, To characterizes the fragmentation behavior of the relevant Type I clusters. 

The real-time spectra indeed reveal this predicted behavior (see Fig. 4.11). The spectra were analyzed within the extended fragmentation model (see Sect. 2.2.2). For clusters with an odd number of atoms a fairly large number of Type II clusters is observable(Fig. 4.12 a), which is in excellent agreement with the reaction equations given above. The real-time spectra of clusters with an even number of sodium atoms show a decay which can be described... 

Fig. 4.15. Real-time spectra of the two-color experiment for different cluster sizes n (taken from [307]). For At Epump = 1.47eV, Eprobe = 2.94 eV for At>0 Epump = 2.94 eV, F probe = 1.47eV. Dots, experimental data lines, fitted curves based on the extended fragmentation model...

To describe the shape of the curves, the extended fragmentation model (Sect. 2.2.2) was again applied. The analysis was performed analogously to that for the sodium clusters (Sect. 4.3). The function n on t) was fitted to the experimental data by means of a least-squares routine. This function is shown in Figs. 4.15 and 4.16 as solid lines. Obviously there is an excellent agreement between measured and fitted curves. 

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