Blue proteins model studies

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Jiang and Zhu (2000) and Qiu and Zhu (2001) have reported the fabrication of multilayered devices composed of stacks of compression-molded disks of alternating compositions. One type of disk is either P(SA-EG) or P[SA-co-TMAgly)-Z>-EG] and the other is a pH-sensitive, protein-loaded blend of, for example, poly(methacrylic acid) and polyethoxazoline. The release of model proteins, myoglobin, bovine serum albumin, and FITC-dextran, and compounds such as brilliant blue have been studied and pulsatile release profiles have been demonstrated (Jiang and Zhu, 2000 Qiu and Zhu, 2001). 

During electron transfer, the Cua site alternates between the fully reduced and the mixed-valence (Cu +Cu ) forms. Interestingly, the unpaired electron in the mixed-valence form seems to be delocalised between the two copper ions. Several theoretical investigations of the electronic structure and spectrum of the Cua dimer have been published [138-144]. In similarity to the blue copper proteins, it has been suggested that the structure and the properties of the Cua site is determined by protein strain. More precisely, it has been proposed [136] that Cua in its natural state is similar to an inorganic model studied by Tolman and coworkers [145]. This complex has a long Cu-Cu bond (293 pm) and short axial interactions (-212 pm). The protein is said to enforce weaker axial interactions, which is conpensated by shorter bonds to the other ligands and the formation of a Cu-Cu bond. This should allow the protein to modulate the reduction potential of the site [136,146]. 

There has been further model work aimed at elucidating the role of copper in copper-proteins. Gagn et al. have reported the preparation of a family of five-co-ordinate adducts of Cu and, while not suggesting these as models for any particular enzyme system, they point out that their existence means that consideration of copper(i) active site structures must include the possibility of five-co-ordination. Spectroscopic studies of blue copper models have included work with compounds containing copper(ii)—sulphur bonds and/or pseudotetrahedrally co-ordinated copper(ii) e.g. refs. 207—212). It appears that the metal sites in cobalt(ii) sub-... 

Blue copper proteins in their oxidized form contain a Cu2+ ion in the active site. The copper atom has a rather unusual tetra-hedral/trigonal pyramidal coordination formed by two histidine residues, a cysteine and a methionine residue. One of the models of plastocyanin used in our computational studies (160) is pictured in Fig. 7. Among the four proteins, the active sites differ in the distance of the sulfur atoms from the Cu center and the distortion from an approximately trigonal pyramidal to a more tetrahedral structure in the order azurin, plastocyanin, and NiR. This unusual geometrical arrangement of the active site leads to it having a number of novel electronic properties (26). 

A new line of research involving blue copper proteins deals with their unfolding and their structural heterogeneity. The thermal unfolding of amicyanin was studied with calorimetric and spectroscopic methods. It was found to be irreversible and the kinetic data were analyzed in a three state model. 

The other way to study the "conductivity of protein molecules towards electron tunneling is to investigate the quenching of luminescence of electron-excited simple molecules by redox sites of proteins [95,96]. Experiments of this sort on reduced blue copper proteins have involved electron-excited Ru(II)(bpy)3, Cr(III)(phen)3, and Co(III)(phen)3 as oxidants. The kinetics of these reactions exhibit saturation at protein concentrations of 10 3 M, suggesting that, at high protein concentrations, the excited reagent is bound to reduced protein in an electron transfer precursor complex. Extensive data have been obtained for the reaction of reduced bean plastocyanin Pl(Cu(I)) with Cr(III)(phen)3. To analyze quenching experimental data, a mechanistic model that includes both 1 1 and 2 1 [Pl(Cu(I))/ Cr(III)(phen)3] complexes was considered [96]... 

Metal salts with other cations are synthesized in a similar manner. Among these studies, the obtaining of coordination compounds of copper(II) in methanol with N2S2 ligand environment should be mentioned. These complexes are of permanent interest due to the modeling of active centers of blue copper proteins on their basis (see Sec. 2.2.5.4) [235-237]. Such complexes were obtained, in particular, by interaction of divalent copper perchlorate and tetrafluoroborate with very exotic ligands 661 and 662 in methanol [238] ... 

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