Thiolates bonding

Covalent attachment of enzymes to surfaces is often intuitively perceived as being more reliable than direct adsorption, but multisite physical interactions can in fact yield a comparably strong and stable union, as demonstrated by several biological examples. The biotin/streptavidin interaction requires a force of about 0.3 nN to be severed [Lee et al., 2007], and protein/protein interactions typically require 0.1 nN to break, but values over 1 nN have also been reported [Weisel et al., 2003]. These forces are comparable to those required to mpture weaker chemical bonds such as the gold-thiolate bond (1 nN for an alkanethiol, and even only 0.3 nN for a 1,3-aUcanedithiol [Langry et al., 2005]) and the poly(His)-Ni(NTA) bond (0.24 nN, [Levy and Maaloum, 2005]). 

Langry KC, Ratio TV, Rudd RE, McElfresh MW. 2005. The AFM measured force required to rupture the dithiolate linkage of thioctic acid to gold is less than the rupture force of a simple gold-alkyl thiolate bond. Langmuir 21 12064-12067. 

Stokbro K, Taylor J, Brandbyge M, Mozos JL, Ordejon P (2003) Theoretical study of the nonlinear conductance of di-thiol benzene coupled to Au(l-l-l) surfaces via thiol and thiolate bonds. Comput Mater Sci 27(1/2) 151-160... 

According to this model the iron-hydride bond is cleaved upon illumination and the hydride, together with the proton, leaves the complex. This would lead to re-establishment of the nickel thiolate bond which was weakened in the former state, explaining the changes in the EPR spectrum observed after illumination (Pig. 7.16-III). After a flip of the electronic z-axis the selenium could in this state interact with the unpaired electron in an orbital with d -y2 character. To get EPR-active, CO-treated... 

The structure of pseudoazurin from A. faecalis strain S-6 was determined in two laboratories (in part, because it crystallizes so readily excellent crystals are formed in a matter of hours ) (Petratos et al., 1987, 1988a Adman et al, 1989). The crystals, space group P6j, have the interesting property that they are intensely blue when viewed along the sixfold axis, but are nearly colorless when viewed normal to this axis. This is assumed to be due to the fact that the plane of the Cu-Sy-C/3 atoms is perpendicular to the sixfold axis, consistent with the fact that the copper— thiolate bond is responsible for the blue color of the protein. 

Haber Weiss reaction has to be considered. In addition, a shoulder appears at 161.3 eV attributable to the Cu(I)-thiolate bonding. This shoulder is not seen in the oxidised protein, although it might be hidden under the relative broad ascending right side. The similar intensity of either sulphur 2p band at 166.3 and 163.1 should be noticed. The 163.1 eV band of the apoprotein rises sharply suggesting the complete dissappearance of any 161.3 eV contribution. The magnitude of the 167 eV band is still only half to that of the reduced holoprotein. 

Figure 7.3 Gold-thiolate bonds encourage the adsorption of the bisthiol 5 on the surface of gold in the form of a monolayer (a). The subsequent and spontaneous formation of disulfide linkages promotes the deposition of additional molecules of 5 to produce multilayers (b).

Two important reactions of arene oxides in animal tissue are (1) detoxification and (2) formation of conjugates of arene oxides with purine pyrimidine bases of DNA. For both of these reactions to take place, the arene oxide should have a certain intrinsic stability to survive an aromatization reaction. Reaction with the thiolate bond of glutathione is responsible for detoxification, whereas the extent of involvement of arene oxides in the nucleophilic reactions with nonpolarized nitrogen bases of DNA is directly related to their carcinogenic activity. 

Class I and/or II MTs have been described in all animals examined. Mammalian MTs have been some of the most extensively studied of the 61 or 62 amino acids, 20 are cysteine residues. Metal ions are bound to the MT exclusively through thiolate bonds involving all 20 cysteines (see Hamer, 1986). They associate with a wide range of metals in vitro, 18 different metals in the case of rat liver MT (Nielson etal., 1985). Divalent and trivalent metals exhibit saturation binding at 7 mole equivalents forming M7-MT, whereas copper (Cu(I)) and silver (Ag(I)) bind as monovalent ions forming M12-MT. The structure of these molecules is such that two metal-binding domains are formed an a-cluster from the carboxy-terminal portion of the protein, contains 11 cysteines which bind either 4 divalent or 6 monovalent ions the (3-cluster, the amino-terminal... 

The main difficulty in synthesizing an accurate model for type I copper comes from the instability of the copper(II)-thiolate bond. This bond easily undergoes homolysis, causing reduction of the copper(II) ion with formation of disulfide as follows ... 

XH- and 195Pt-NMR investigations have shown that therapeutic nucleophilic agents for cisplatin, such as Na(ddtc) (sodium diethyldithiocarba-mate) and thiourea, can help to remove Pt from certain proteins [35]. The mechanism may be based on the relatively easy reversal of Pt binding to methionine side chains. In contrast, nephrotoxicity, thought to be caused by formation of Pt-cysteine adducts (Pt11 thiolate bonds), cannot be reversed by Na(ddtc) and thiouera. 

In general, iron snlfur clnsters are coordinated to native proteins via cysteine thiolate bonding, often in a Cys-X-Y-Cys motif (X, Y = nonsnlfhr containing amino acids). In clnsters with more than one iron atom, /u.2- or /rs-snlfur linkages connect the iron atoms within the clnster. At the active site of proteins, iron-snlfur clusters perform a variety of functions, including electron transport and enzymatic activity. 

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