Deactivation process, cascade

Figure 2, Diagram of the cascade deactivation process note the compe-tit n between colUsional stabilization arid unimolecular reaction...

Photodecomposition. Since the last review of photochemistry of HFA (61), there has been a great deal of effort expended in the study of the primary processes and decomposition modes of HFA. The photodecomposition products observed appear to be carbon monoxide and hexafluoroethane exclusively. The trifluoroacetyl radical, CF3CO, must be very unstable. As in acetone, it has been proposed that the decomposition processes must overcome an energy barrier, as temperature-dependent quantum yields were observed (252). A detailed mechanism that takes into account a vibrational deactivation cascade has been proposed by several authors (34,35,97,252). 

In comparison with conventional electrical discharge processes, LPCAT is a very different process in that its activation of carrier gas and the creation of polymerizable species by the activated carrier gas are temporally and spatially separated. When discharge power is applied to the cascade arc generator, the plasma of carrier gas (usually argon) is produced in the cascade arc column and the luminous gas phase is blown into a vacuum chamber where monomers are introduced. The deactivation of the reactive species, some of which lead to the creation of polymerizable species in the luminous gas phase, occurs within the relatively narrow beam of an argon luminous gas jet. The higher the flow rate of Ar, the narrower is the beam and the longer the luminous gas flame. 

The CHsCHF F Unimolecular Decompositioii Mechanism. The large range of excitation energies for the nascent [CHgCHF Flt from Reaction 24 leads to a complex decomposition mechanism in which secondary and tertiary imimolecular processes compete with coUisional deactivation. As shown in Reactions 28-38, this cascading sequence continues until the excess energy is insufficient to induce further decomposition (70). 

However, in most cases, covalent modifications can be either activation or deactivation and are normally energy-dependent and reversible, but catalyzed by separate enzymes. These include phosphorylation/dephosphorylation, adenylylation (nucleotidyla-tion)/deadenylylation and ADP-ribosylation (Stadtman and Chock, 1978). This cyclic interconversion of key enzymes between covalently modified and unmodified forms is a mechanism of singular importance in cellular regulation. The interconversion of an enzyme between the active and inactive forms involving separate modification and demodification (i.e. different converter enzymes) is a dynamic process that may lead to a steady state in a cascade system. The covalent interconversion of regulatory enzymes is characterized by ... 

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