Bound complex mechanism

Atoms may also combine by a mechanism called the bound complex mechanism, in which an atom. A, forms a van der Waals cluster, AM, followed by reaction of the cluster with another atom... 

For weakly bound complexes the energy transfer and the bound complex mechanisms give similar order of magnitude estimates for k(TJ, so that it is difficult to distinguish which mechanism predominates. In fact, in these cases probably both mechanisms are simultaneously operative. If experimental rate coefficient data are not available, the simple modeling approach provides a rapid method of estimating overall termolecular rate coefficient from the contributions of each mechanism that will usually be satisfactory, because atom combination reactions are usually relatively slow, and even if the rate coefficient is not accurately known only minor errors will be made. If the A-M interaction is stronger, however, it may be necessary to estimate the lifetime of the complex by other means if rate data are not available. 

RRKM calculations based on a bound complex mechanism (ONF2) were performed in support of the revised mechanism [18]. However, no explanation was given for the observed NF luminescence. 

The rhodamine B-bound complex of Ir1 (387) shows only minor alterations in the absorption spectrum of bound rhodamine B as opposed to free dye however, its fluorescence is strongly quenched.626 Fluorescence is intense when the rhodamine dye is attached to an Ir111 center. The authors conclude that the excited-state quenching mechanism is via electron transfer. 

Thus, the hemicarcerands differ161 from the related carce-rands, which are similar in shape and constitution, but from which, guests cannot escape without the breaking of covalent bonds. The mechanically bound complexes formed between carcerands and their guests are termed carceplexes. 

Irreversible CYP inhibition can arise from different chemical mechanisms. However, a common initial step is the metabolic activation of a substrate into a reactive metabolite that is trapped within the active site of the CYP to form a tightly bound complex causing a long-lasting inactivation of enzyme activity. Enzymatic activity can be restored only through the new synthesis of the enzyme. For this reason, irreversible CYP inhibition is often referred to as mechanism-based inhibition , metabolite-based inhibition or suicide inhibition . 

The more tightly bound substrates have the lower values of Km. But beware The condition that k3 is negligible compared to k2 may be met with some (poorer) substrates but may not always be met by others. From Eq. 9-15 we see that Km is always greater than or equal to Ks, but it may be less than Ks for more complex mechanisms. 

The concept of the enzyme-substrate complex is the foundation stone of enzyme kinetics and our understanding of the mechanism of enzyme catalysis. In honor of its introducer, this noncovalently bound complex is often termed the Michaelis complex. 

Figure 23.12. Bidirectional mismatch repair in E. coli. MutS binds to mismatches formed by errors made during replication. MutL binds to the MutS-bound complex. MutH incises the newly synthesized strand next to the unmethylated GATC sequence. The green arrows indicate signally between the two sites of activity. DNA helicase II unwinds the DNA and exonucleases excise the DNA from the point of incision up to and including the incorrect base. DNA Pol III synthesizes new DNA to replace the excised DNA. (Reproduced with permission from Iyer, R. R., Pluciennik, A., Burdett, V., and Modrich, P. L. DNA mismatch repair Functions and mechanism. Chem. Rev. 106, 302-323, 2006.)...

Murahashi and Naota studied the reaction mechanism of cyclopentadienyl ruthenium enolate complex-catalyzed aldol and Michael addition reactions [80-82]. This mechanistic study revealed that the cone angle of the tertiary phosphine ligands largely affects the stability of C- and N-bound complexes [80, 82], Thus, ligation of bulky phosphine ligand would favor the N -bound complexes [80]... 

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