Secondary cleavages

Figure 2 Locations of cleavage of RGO s by RG-hydrolase. Differently dotted arrows indicate different cleavage options. A short arrow indicates a secondary cleavage, see text. Explanation of symbols, see table 1. Numbers refer to degree of polymerisation.

However, this hypothesis does not explain why Cj, is not equal to C, j, forp = 1-6. If the mechanism described in Scheme 3.14 is considered, a successive P-alkyl transfer should occur. When zirconium is in position (P-rl) ethane and C, j 2 (instead of C j,) hydrocarbons are produced. When zirconium is in position (P+2) propane and C, j 3 hydrocarbon are produced, and so on. This hypothesis involves only an overproduction of ethane and of propane, but no overproduction of C4 to Cis. These results imply a secondary cleavage for p = 1-6. 

This hypothesis of secondary cleavage is confirmed in the hydrogenolysis of butane (Figure 3.22). At low conversion, one observes the formation of ethane. 

The 1 sO-incorporation experiment was repeated for a reaction in which transpeptidation and secondary cleavage are not observed the HIV protease-catalyzed cleavage of Ac-Ser-Gln-Asn-Tyr-Pro-Val-Val-NH2 at the Tyr-Pro bond.174 The substrate was subjected to hydrolysis in 78% enriched H2180 and the remaining substrate isolated after > 80% had converted to products. 180 was found in the Tyr-Pro carbonyl group. The incorporation had to have arisen from the direct attack of H2lsO on the substrate to generate a tetrahedral adduct. 

Although the fragmentation required to produce a methyl radical and a primary carbocation did not occur in the case of octane, the loss of a methyl radical does occur here because the resulting carbocation. witli m/z 71, is secondary. Cleavage on the other side of the branch results in the formation of a primary radical and a secondary carbocation, with mlz 43. This is the most favorable fragmentation, so m/z 43 is the base ion. 

The product is 3 -[32P]-labelled DNA essentially free of salt and ready for strand separation. Where secondary cleavage with a restriction enzyme is to follow it may be necessary to renature the sample. This may be achieved as follows ... 

Cleavage at branched positions is the favored process of hydrocarbons and in this case leads to three of the most intense peaks in the spectrum, miz 43, 57, and 113. These secondary carbonium ions are primary cleavage products of the molecular ion. The fourth intense peak in this spectrum, at mk 71, is a typical example of a secondary cleavage ion. Elimination of a stable alkene molecule provides the driving force for this fragmentation. Note that no structures are drawn since it is likely that considerable rearrangement occurs in these hydrocarbon ions. 

Utilization of the primary site produces the --26 kD peptide, and the secondary cleavage site yields a 25 kD peptide lacking the basic N-terminus of LHCP. See Discussion for the possible role of these peptides in LHC (after 14) and upon phosphorylation (after 14,17). 

Determination of the contribution of each for any particular ketone is complicated by the possible occurrence of the secondary cleavages ... 

Benzilketals are a special case. They are cleaved in a similar way but they are more efficient due to a somewhat higher extinction coefficient (e 250 M cm ) and the occurrence of a secondary cleavage ... 

Minor cleavage also occurs at GlpC2 and secondary cleavage occurs at 41-3 X-ray crystallographic structure (orthorhombic) in the vicinity of Pb(II) bind- ... 

Jog formation during discontinuous cleavage. After arrest, the crack reinitiates at X (a) and advances on facet B (b). The crack overshoots the cleavage step (c), resulting in jog formation by secondary cleavage (d) and finally advances uniformaly (e) [115]. 

Octaphenylcyclotetrasilane reacts with triphenylsilyllithium to give, subsequent to acid hydrolysis, decaphenylcyclopentasilane and a product tentatively identified as l//-nonaphenyltetrasilane (44). Excess triphenylsilyllithium and extended reaction time gave hexaphenyldisilane and penta-phenyldisilane (45) in moderate yields. The products isolated from these reactions establish that secondary cleavages have taken place under the conditions used. 

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