Intramolecular cross-linking kinetics

The second important problem is a kinetical description of intramolecular cross-linking. Kinetic characteristics can vary in wide range depending on the nature of cross-linking agents, properties of the polymeric chain and experimental conditions. ... 

Kinetics of Intramolecular Cross-Linking and Conformational Properties of Cross-Linked Chains... 

The kinetic problem for the intramolecular cross-linking reactions in general form was not yet solved. Only some particular cases, i.e. the cvclization of macromolecules, the intramolecular catalysis and diffusion-controlled collision of two reactive groups were studied theoretically bv Xorawetz, Sisido and Fixman... 

Thus this model study permitted to elucidate the main regularities of the process of intramolecular cross-linking. The most interesting of them are, firstly, the existence of a uniform relationship between the kinetics of the reaction and the equilibrium properties of partially cross-linked chains and secondly, the independence of the kinetics of cross-linking on the character of reactive groups distribution on the initial stage of the reaction. 

Thus the approach described can be apolied to the description of the kinetics of intramolecular cross-linking and the conformational properties of cross-linked products. 

Based on these results, the possibility of a kinetic mechanism for the sequence-specific cross-linking of 19 and 20, and similarly, 21 and 22, was mled out, which left the thermodynamic mechanism that requires the selective stabilization of the sequence-matched, cross-linked products. The observed sequence specificity is likely to be due to the stabilization of the products by intramolecular H bonds... 

The practice of using an insoluble polymer to isolate and kinetic-ally stabilize a reactive intermediate has been addressed in several reports, most commonly using DVB cross-linked polystyrene as a support. In these cases, the three dimensional structure of the polymer and rigidity of the polymer backbone diminish intramolecular reactivity between two sites on the same polymer bead. Physical constraints preclude any significant reaction between two different polymer beads. Similar, less dramatic reduced intramolecular reactivity has also been noted for reactive intermediates bound to linear polystyrene. For example, o-benzyne bound to linear polystyrene has been shown by Mazur to have enhanced stability relative to non-polymer-bound -benzyne (35). In this case, o-benzyne was generated by lead tetraacetate oxidation of a 2-aminobenzotriazole precursor, 1. Analysis of the reaction products after cleaving the benzyne derived products from the polymer by hydrolysis showed a 60% yield of aryl acetates was obtained (Equation 11). In contrast, the monomeric aryne forms only coupled products under similar conditions. Further comparisons of the reactivity of -benzyne bound to insoluble 2% or 20%... 

It should be pt ted out in conclusion that intramolecular ctoss-linldng is an autoaccelerated reaction, and that the initial rate and the degree of autoacceleration increase with an increase in chain Imgth. Another inqiortant condusion is the existence of a uniform relationsliip between the kinetics of tire reaction and the equQibrium j perties of partially cross-linked drains. 

The kinetics of electron transfer reactions between spinach plastocyanin and [Fe(CN)6] ", [Co(phen)3] , and Fe(II) cytochrome c have been studied as a function of ionic strength. Applications of the equations of Van Leeuwen support the proposal of two sites of electron transfer, with [Co(phen)3] binding near residues 42-45 and the interaction of [Fe(CN)6] at a hydrophobic region near the copper ion. Pulse radiolysis has been employed to measure the rates of electron transfer from Ru(II) to Cu(II) in plastocyanins from Anabaena variabilis and Scenedesmus obliquus which have been modified at His-59 by [Ru(NH3)5] . The small intramolecular rates (<0.082 and <0.26 s , respectively) over a donor-acceptor distance of 12 A indicate that electron transfer from the His-59 site to the Cu center is not a preferred pathway. A more favorable route, via the acidic (residues 42-44) patch ( 14 A to Cu), is supported by the rate of >5 x 10 s for the reduction of PCu(II) by unattached [Ru(NH3)5im] . The intramolecular electron transfer from Fe(II) in horse cytochrome c to Cu(II) in French bean plastocyanin ( 12 A from heme edge to Cys-84 S), in a carbodiimide cross-linked covalent complex, proceeds with a rate of 1.05 x 10 s . The presence of the... 

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