Scission point

Figure 2. Specificities of Endoglucanases (EGA, EGB, EGC, EGD) from Clostridium thermocellum cloned in E. coli (10). The substrates (MeUmb-Glc , n = 2-5, MeUmbLac) are depicted (symbols A, (3-1,4 galactopyra-nosyl , / -1,4 glucopyranosyl , 4-methylumbelliferyl) and the arrows indicate scission points as determined by HPLC (1).

In an attempt to separate the domains from the cores, we used limited degradation with several proteases. CBH I (65 kda) and CBH II (58 kda) under native conditions could only be cleaved successfully with papain (15). The cores (56 and 45 kda) and terminal peptides (11 and 13 kda) were isolated by affinity chromatography (15,16) and the scission points were determined unequivocally. The effect on the activity of these enzymes was quite remarkable (Fig. 7). The cores remained perfectly active towards soluble substrates such as those described above. They exhibited, however, a considerably decreased activity towards native (microcrystalline) cellulose. These effects could be attributed to the loss of the terminal peptides, which were recognized as binding domains, whose role is to raise the relative concentration of the intact enzymes on the cellulose surface. This aspect is discussed further below. The tertiary structures of the intact CBH I and its core in solution were examined by small angle X-ray scattering (SAXS) analysis (17,18). The molecular parameters derived for the core (Rj = 2.09 mm, Dmax = 6.5 nm) and for the intact CBH I (R = 4.27 nm, Dmax = 18 nm) indicated very different shapes for both enzymes. Models constructed on the basis of these SAXS measurements showed a tadpole structure for the intact enzyme and an isotropic ellipsoid for the core (Fig. 8). The extended, flexible tail part of the tadpole should thus be identified with the C-terminal peptide of CBH I. 

Figure 11.1 suggests fission proceeds in two steps, the ascent to the saddle point and the passage through the scission point. We shall present our discussion of fission from this point of view. We shall assert that like chemical reactions, the reaction probability is determined by the passage through the transition state. We shall also assert, more controversially, that the distribution of fission product energies, masses, and so forth is determined at or near the scission point. 

It is clear from these basic facts and our picture of fission that spontaneous fission is a barrier penetration phenomenon similar to a or proton decay. The nucleus tunnels from its ground state through the fission barrier to the scission point. Therefore, we would expect the spontaneous fission (SF) half-life to have the form... 

Up to this point, we have focused on describing the factors that control the probability of fission to occur. Now we will focus our attention on the distributions of the products in mass, energy, charge, and so forth. In doing so, we will mostly be discussing scission point or postfission phenomena. Our treatment of these phenomena is, of necessity, somewhat superficial, and the reader is referred to the excellent monograph of Vandenbosch and Huizenga (1973) for a more authoritative account. 

One reaches the scission point in a stretched neck position (i.e., at a lower point of the Coulomb barrier—thus lower kinetic energy for the fragments) while the other one reaches the scission point practically in a touching-spheres-position (i.e.. 

Wetherill GW (1975) diometiic chronology of the early solar system. Ann Rev Nud Sci 25 283 Wilczynski J, Volkov W, Decowski P (1967) Some features of the mechanism of many-neutron-transfer reactions. Sov J Nud Phys 5 672 Yad Fiz 5 942 Wilkins BD, Steinberg EP, Chasman RR (1976) Scission-point model of nuclear fission based on deformed-shell effects. Phys Rev G 14 1832 Wilkinson DH, Wapstra AH, Ulehla I et al (1993) Discovery of the transfermium dements, Report of the Transfermium Working Group of lUPAC and lUPAP II, Introduction to discovery profiles. III, Discovery profiles of the transfermium elements. Pure Appl Chem 65 1757, 1764 Willard JE (1953) Chemical effects of nuclear transformations. Ann Rev Nud Sci 3 193... 

The dip between the two peaks is most pronounced for the fission of and decreases with increasing mass or rather with increasing fissility parameter (Z M) of the fissioning nucleus. This is in agreement with the expectation that shell effects will become less important with increasing excitation energy of the nucleus at the scission point. 

It can be seen that the LDM barrier is either absent or very small, and that the total barrier is due almost exclusively to electronic shell effects. The total barrier has a doublehumped structure, with the outer hump corresponding to the LDM saddle point, which also happens to be the scission point (indicated by an empty vertical arrow). The inner hump coincides with the peak of the shell-effect term, and is associated with the rearrangement... 

Bottom panel LDM energy (surface plus Coulomb, dashed curve) and total potential energy (LDM plus shell-corrections, solid curve) as a function of fragment separation d. The empty vertical arrow marks the scission point. The zero of energy is taken at / = 0. A number (—1.58 eV or —0.98 eV), or a horizontal solid arrow, denotes the corresponding dissociation energy. 

Figure 9.18 shows the situation where Seg-0 has grown at a scission point on Seg-2 after scission of the latter at birth conversion 02 < 0o, to be selected as a random number between 0 and Go- The growth direction of Seg-2 has then to be selected. There is a probability of that it is in the direction indicated, to the LHS. Its length is determined by comparing lengths with Eqs. (139) and (171). In this case no sets-... 

The LHS part of the branching algorithm is also slightly modified. The probability Prs of Seg-0 being started on a scission point at Seg-2 again follows from Eq. (172), but it is constant in a CSTR. The residence time 02, now longer than 0o, is sampled from a conditional probability distribution similar to Eq. (148) ... 

If the superimposed sextet is subtracted from the original spectrum a triplet remains which must be due to a scission product of the above molecule. The exclusive breakage of bonds 3 and 4 can be ruled out immediately since it would only lead to radicals of type III. Breakage of bonds 6 and 7 was eliminated after comparison with capron deuterated in the imino groups breakage of bond 1 would not lead to a radical with a triplet spectrum. After inspection of all other possibilities, including secondary radicals, there remain as likely scission points only the bonds 2 and 5. After having studied a and e methyl substituted caprolactam the authors [10] finally were able to state that in a stressed 6 polyamide molecule both the bonds 2 and 5 do break and with equal probability. The rupture of a PA 6 molecule, therefore, leads to three primary radicals ... 

In Sections I and II of this Chapter primary and secondary free radicals have been treated as microprobes which characterize occurrence and molecular environment of chain breakages. As shown in Chapter 6 the primary mechano-radicals are always chain end radicals which are mostly unstable. At a rate depending on temperature these radicals will transfer the free electrons and thus convert to secondary radicals. This reaction and also subsequent conversion and decay reactions including recombination are relevant with respect to an interpretation of the fracture process in two ways. Firstly these reactions interfere with a determination of the concentration and molecular environment of the original chain scission points. Secondly they change the physical properties of other network chains through the introduction of... 

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