Guanidinium ions metal complexes

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

Guanidine forms salts with such relatively weak acids as nitromethane, phthalimide, phenol and carbonic acid [20], Interactions between carboxylate anions of proteins and added guanidinium ion are thought [19, 56] to be weaker than the interactions with ammonium ions the role of guanidinium-carboxylate interactions in stabilizing natural protein conformations has been discussed [36c]. A few reports of metal complex formation by guanidines [57-60], and aminoguanidines [61] have appeared. 

Complexing bifluorophores are of limited use for fie detection of metal cations for which many other methods arc available, but they are of potential use for organic cations like guanidinium ion, as shown in scheme 3. 

Compared with the Mn -enzyme binary complex, the ternary complex (en-zyme-Mn -pdTp) has one more water molecule in the second coordination sphere of the metal ion (20). The additional water molecule apparently replaces the Glu-43 carboxylate which is coordinated with the metal ion in the binary complex. This and other evidence suggests that, in the ternary complex, the side chain carboxylate of Glu-43 acts as a general base potentiating the attack of the water molecule on the phosphodiester bond. The guanidinium ion of Arg-87 is also appropriately positioned to both bind and catalytically activate the 5 -phosphate group of the substrate. Since all five Arg residues in the protein have pK values greater than 11.6, Arg-87 may be a candidate for the acidic catalyst that protonates the 5 -ribose alkoxide prior to product release (17). 

Monomer 16 was transformed in a reversible manner from monomer A (the protonated chloride salt 16 HC1) to the 2 1 metal complex B or to the ion-paired dimer C depending on the presence of Fe(II) ions or the pH value (Scheme 6.4). In the presence of both chemical stimuli, larger aggregates D were formed. As the zwitterion is only present in a narrow pH range of 5-7 (pKg of the carboxylic acid and the guanidinium cation are 4—5 and 6-7, respectively), the dimer formation can be controlled in a reversible fashion by protonation/deprotonation. The metal ion can be removed in the presence of a competing ligand such as HEEDTA. 

Positively charged or neutral electron-deficient groups may serve as interaction sites for anion binding. Ammonium and guanidinium units, which form +N-H" X bonds, have mainly been used, but neutral polar hydrogen bonds (e.g., with -NHCO- or -COOH functions), electron-deficient centers (boron, tin, mercury, [3.6, 3.7] as well as perfluoro crown ethers and cryptands [3.8], etc.), or metal-ion centres in complexes also interact with anions. 

In recent years the effects of crown ethers on enzyme reactions in organic solvents have been investigated. Depending on their ring size and structure, crown ethers can form complexes with metal ions, ammonium groups, guanidinium groups, and water, species that are all common in enzymatic reactions. 

BAP has 10 His/monomer, 3 of which are involved in Zn " " coordination (two in the A site and one in the B site). His-372 contributes to catalysis via H-bonding interaction with the Zn-binding Asp-327 (77). The Pj of the E-P complex is anchored partly by the coordination with the A-site metal ion (76), and partly by the guanidinium group of Arg-166 which in turn interacts with the carboxyl group of Asp-101. 

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