Cross-links, covalent ionic

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... 

The thermal stability of all membranes was determined via thermogravimetry under a 65-70 % O2 atmosphere with a heating rate of 20 K/min [54]. Firstly, it was investigated whether the type of cross-linking influences the thermal stability of the membranes. Therefore, in Fig. 4.13, the TGA traces of the membranes 1925C (covalently cross-linked), 1927A (ionically cross-linked), and 1943 (covalent-ionically cross-linked) are presented together with B2. 

The most important physical methods are physical and ionic adsorption on a water-insoluble matrix, inclusion and gel entrapment, and microencapsulation with a liquid or a solid membrane. The most important chemical methods include covalent attachment to a water-insoluble matrix, cross-hnking with the use of a multifunctional, low-molecular weight reagent, and co-cross-linking with other neutral substances, for example proteins. 

Cross-linked gel-type functional polymers (CFPs) are organic materials built up with interconnected polymer chains [1]. Pendants hanging from the polymer chains may render CFPs reactive materials particularly suitable for anchoring metal centres removed from a liquid phase, by means of covalent or ionic bonds [2] ... 

Kokufuta, Zhang and Tanaka developed a gel system that undergoes reversible swelling and collapsing changes in response to saccharides, sodium salt of dextran sulfate (DSS) and a-methyl-D-mannopyranoside (MP) [126]. The gel consists of a covalently cross-linked polymer network of W-isopropylacrylamide into which concanavalin A (ConA) is immobilized. As shown in Fig. 31, at a certain temperature the gel swells five times when DSS ions bind to ConA due to the excess ionic pressure created by DSS. The replacement of the DSS by non-ionic MP brings about collapse of the gel. The transition can be repeated with excellent reproducibility. 

Larger proteins often contain more than one polypeptide chain. These multi-subunit proteins have a more complex shape, but are still formed from the same forces that twist and fold the local polypeptide. The unique three-dimensional interaction between different polypeptides in multi-subunit proteins is called the quaternary structure. Subunits may be held together by noncovalent contacts, such as hydrophobic or ionic interactions, or by covalent disulfide bonds formed from the cysteine residue of one polypeptide chain being cross-linked to a cysteine sulfhydryl of another chain (Fig. 15). 

Mode of immobilization. Immobilization can be effected either chemically, by covalent bonding of the biocatalyst on a surface (Figure 5.6, option 1), by adsorption, or by ionic interactions between catalyst and surface (option 2), as well as by cross-linking of biocatalyst molecules for the purpose of enlargement (option 3), or physically by encapsulation in matrices or by embedding in a membrane (option 4). 

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