Simultaneous Irreversible Side Reaction

Release of the assembled peptide and removal of side-chain protecting groups are key steps in peptide synthesis. However, the treatment of a peptidyl-resin with a cleavage cocktail does not initiate just one simple reaction, but several simultaneous reactions that can compete and interfere with each other. Suitable scavengers need to be used to avoid unwanted irreversible side reactions that can modify the assembled peptide. 

The result of this complex system of reversible and irreversible side reactions is the simultaneous consumption and partial recovery of both components of the initiation system, a reduction in its activity, the formation of irregular (thermally or hydrolytically instable) stractures in polymer chains, and last-but not least-the creation of branched stractures. Such branching leads, in the case of multifunctional activators, to partial gelation. 

A second, even more worrying problem is the side reaction, the formation of condensation products. This process is essentially irreversible in most cases. The condensation products can arise either from the aldol product or directly through a Knoevenagel-Mannich type reaction where the enamine reacts with an imininm ion [26, 81, 82]. The condensation process requires only an external Brpnsted acid, whereas the aldol process appears to require simultaneous activation of the carbonyl electrophile by an internal Brpnsted acid/hydrogen bond donor (Scheme 15). 

Assuming a completely effective washing without dissociation of the complex, the initial free antigen concentration, [H] , at the start of the incubation with Ab-E is zero. Rodbard and Feldman (1978) solved eqs. 32-37 simultaneously, using numerical methods, with the computer system developed by Reece and Knott (1973) and confirmed the intuitive expectations that this assay performs best if the reactions are irreversible (or have high affinities) and that the side-reactions diminish the precision and detectability of the assay. The formation of the complex was found to be a monotonic... 

Although the oxidation of tertiary phosphines by these catalytic processes has minimal useful application, it needs to be considered as a problematic side reaction in homogeneous catalysis. Much effort is being currently expended to immobilize platinum metal phosphine complexes on heterogenized tertiary phosphine supports, and irreversible oxidation at phosphorus on these supports effectively destroys the supported catalyst. Recent observations that the compound Rh6(CO)i6 catalyzes the oxidation of tertiary phosphines correlate with the report that phosphine oxidation occurs with molecular oxygen on Rh6(CO)i6 bound to diphenylphosphino-functionalized poly(styrenedivinylbenzene). Thus, in order to use these phosphinated polymer-supported rhodium catalysts, one needs either to rigorously exclude oxygen, or to find a way to inhibit the simultaneous catalyzed phosphine oxidation. 

Simultaneous determination of and a. If it is not possible to keep the gas-side resistance negligible, it is still possible to determine k(j and a by use of a fast irreversible pseudo-mth order reaction (Ha >> 1 and /l+Ha Ha). Thus the rate of absorption expressed by Eqn. (20) becomes,... 

Living ionic polymerization is frequently used to synthesize amphiphilic block copolymers in the absence of irreversible chain transfer and chain termination, where aU polymer chains are instantaneously initiated and grow simultaneously [30]. The presence of counterions in the reaction medium associated witli the propagating chains preserves the electroneutrality of the entire system. However, these living polymerization methods are affected significantly by the nature of the solvent and the presence of water, carbon dioxide, and/or impurities. Moreover, they have limited apphcations for the synthesis of block copolymers with functional groups as side chains. 

The electrochemical reactions at the anode and cathode sides take place simultaneously, producing electricity, water, and some heat owing to the irreversibilities associated with the electrode reactions, and charge conducting... 

---