Covalently attached complexes

Figure 11 Dependence on driving force of first-order rate constant for back electron transfer from colloidal Sn02 films to covalently attached complexes. The variations indicate that the reactions occur in the Marcus normal region. The identities of the molecular redox couples, listed from highest driving force to lowest, are, Rulll/n (5-Cl-phen)2 (phos-...

The ability of clathrochelates to form ion pairs and covalently attached complexes is utilized in biochemistry [315-321], The stereoselectivity of the redox reactions of plastocyanine and horse heart cytochrome C with several cage complexes was reported in Ref 319. Studies on stereoselective electron transfer in different systems provide information on the importance of close ion pair association of a cage complex with protein in chiral discrimination. 

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

Fig. 5. (a) Antibody 2 is added to test solution where some or all of becomes complexed with target, T. (b) A fiber-optic probe containing covalently attached target is insetted into the solution. Unbound binds to probe, (c) The probe is inserted into solution containing en2yme-modified detector antibody which binds to probe if any is attached, (d) The probe is inserted into a solution producing a fluorescent compound, which is then... 

If the inhibitor combines irreversibly with the enzyme—for example, by covalent attachment—the kinetic pattern seen is like that of noncompetitive inhibition, because the net effect is a loss of active enzyme. Usually, this type of inhibition can be distinguished from the noncompetitive, reversible inhibition case since the reaction of I with E (and/or ES) is not instantaneous. Instead, there is a time-dependent decrease in enzymatic activity as E + I El proceeds, and the rate of this inactivation can be followed. Also, unlike reversible inhibitions, dilution or dialysis of the enzyme inhibitor solution does not dissociate the El complex and restore enzyme activity. 

Cations which are covalently attached to the allyl anion part by a cr-bond and have sufficient Lewis acid properties offer the broadest versatility and highest levels of stereocontrol, since the C—C bond-forming step can occur in a pericyclic process9 accompanied by allylic inversion. It is reasonable to assume the prior assembly of both reaction partners in an open-chain complex, in which usually the (F )-oxonium ion, avoiding allylic 1,3-strain10, is predominant. 

Protein modification by the covalent attachment of ubiquitin chains serves as a signal to mark proteins for the degradation by a multicatalytic proteinase complex called the proteasome. Thus the ubiquitin proteasome system (UPS) controls the stability of proteins in a... 

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