Blue copper proteins metal coordination

Simple thermodynamic considerations state that the reduction process is favoured (i.e. more positive cu(ii)/cu(p potential values are obtained) if the electron transfer is exothermic (AH° negative) and if the molecular disorder increases (AS° positive). It is therefore evident that the positive potential value for the reduction of azurin (as well as that of the most blue copper proteins) is favoured by the enthalpic factor. This means that the metal-to-ligand interactions inside the first coordination sphere (which favour the stability of the reduced form over the oxidized form) prevail over the metal complex-to-solvent interactions inside the second... 

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] ... 

A keyword search will provide access to coordinate sets from all species that are currently available as well as various site-directed mutants, type 1 copper proteins substituted with metals other than copper, and ruthenated blue copper protein derivatives. Table 1 provides a summary (with references) of the structures of blue copper-binding domains elucidated using X-ray crystallography and NMR spectroscopy. 

The blue copper proteins azurin, plastocyanin, stellacyanin, and umecyanin incorporate Cu bound to a combination of N/thiolate/thioether ligands. An important feature of these metalloenzymes is the facile copper(II)/(I) couple that these species exhibit, which is linked to the highly strained, asymmetric coordination geometry at the metal center. The synthesis of model complexes for these so-called Type 1 copper proteins has been reviewed. ... 

Another impetus for the study of the coordination chemistry of crown thioethers stems from the role of thioether binding in biological systems such as d-biotin (involving tetrahydrothiophene) (145, 208) and blue copper proteins such as plastocyanin and azurin (involving methionine) (4,13, 73,109,124,185). The binding of Cu(II) and Cu(I) centers to macrocyclic thioethers has led to a greater understanding of Cu-S(thioether) interactions and the stereochemical preferences of these metal centers (91, 95, 99,121,180,181). 

The binding of cyclic thioethers to metal centers has also led to the isolation of complexes in which the coordinative properties of the ligand do not lit the stereochemical preferences of the metal ion(s) (188), Thus, a series of macrocyclic thioether complexes incorporating unusual stereochemistries and/or oxidation states has been generated (188). This is linked to the biological activity of the blue copper proteins and model systems in which the coordination geometry about Cu(II) is strained [in an entatic state (.212,221)] such that the Cu(II)/(I) couple occurs at a particularly positive potential that is, the Cud) state is stabilized. The ability of cyclic thioethers to modify their coordination properties is inherent in this approach (76,108,111). 

Another problem with small models is that molecules from the solution (e.g. water) may come in and stabilise tetragonal structures and higher coordination numbers [224]. It is illustrative that very few inorganic con5)lexes reproduce the properties of the blue copper proteins [66,67], whereas typical blue-copper sites have been constructed in several proteins and peptides by metal substitution, e.g. insulin, alcohol dehydrogenase, and superoxide dismutase [66]. This shows that the problem is more related to protection from water and dimer formation than to strain. 

The hexathioether ligand [18]aneSe, the S-analog of 18-crown-6 ([18]aneOe), can encapsulate ions to form highly stable complexes [M([18]aneSe)] + (M = Fe Co,Ni,Cu (61) Pd (62) Pt). The complexes of these ligands with d metal ions such as Pd PP and Au tend to show long-range M- -S interactions in the range 2.8-3.1 A. This is reminiscent of the coordination at Cu sites in blue copper proteins such... 

In this section we look at ways in which Nature carries out redox chemistry with reference to blue copper proteins, iron-sulfur proteins and cytochromes. The redox steps in Photosystem II were outlined in the discussion accompanying equation 22.54. We have already discussed two topics of prime importance to electron transfer in Nature. The first is the way in which the reduction potential of a metal redox couple such as Fe +/Fe + can be tuned by altering the ligands coordinated to the metal centre. Look back at the values of for Fe +/Fe + redox couples listed in Table 8.1. The second is the discussion of Marcus-Hush theory in Section 26.5 this theory applies to electron transfer in bioinorganic systems where communication between redox active metal centres may be over relatively long distances as we shall illustrate in the following examples. 

Another source of interest came from biochemistry. Research on the blue copper proteins revealed unusual electronic properties (redox potential and kinetics, EPR and optical behavior) that were suspected of arising from interaction of the copper ion with a thioether group from methionine [7]. While crystallographic studies established a weak interaction (Cu -- - S 2.9 A) [8,9,10], its influence on the electronic properties of the Cu site is now considered questionable. Nevertheless, the controversy regarding the blue eopper proteins, like the analogy to phosphines, served to focus attention on the broad issue of how thioether coordination affects the electronic structure of transition metal ions. Homoleptic thioether complexes provide the best way of assessing these consequences, since no other groups obscure the effect of thioether coordination. 

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