Metals motif

This metallic motif is linked to multiple free bpe molecules incorporated in the structures via O-H- N interactions. These interactions generate 2D H-bonded networks, where either coordinated bpe or H-bonded free bpe molecules are stacked in a parallel fashion. A noteworthy structural feature of such organisations is the presence of... 

The loop region between the two a helices binds the calcium atom. Carboxyl side chains from Asp and Glu, main-chain C =0 and H2O form the ligands to the metal atom (see Figure 2.13b). Thus both the specific main-chain conformation of the loop and specific side chains are required to provide the function of this motif. The helix-loop-helix motif provides a scaffold that holds the calcium ligands in the proper position to bind and release calcium. 

A completely different structural motif has very recently been found in the red-brown phosphide CasPg, formed by direct fusion of Ca metal and red P in the correct atom ratio in a corundum crucible at 1000 C. The structure comprises Ca + cations and Pg anions, the latter adopting a staggered ethane conformation. (Note that P+ is isolobal with C and P with H so that C2H6 = [(P+)2(P"")6] = Ps - -) The internal P-P distance is 230.1pm and the terminal P-P distances 214.9-216.9 pm, while the internal PPP angles are 104.2-106.4° and the outer angles are 103.4-103.7°. 

P4) is closely similar with P-P distances of 216 pm (smaller than for P4 itself, 221pm). Indeed, a whole series of complexes has now been established with the same structure-motif and differing only in the number of valency electrons in the cluster some of these are summarized in Table 13.11. The number of valence electrons in all these complexes falls in the range 30-34 as predicted by R. Hoffmann and his colleagues.Many other cluster types incorporating differing numbers of Group 15 and transition metal atoms are now known and have been fully reviewed. ... 

HIV integrase consists of three distinct domains. The N-terminal domain contains a HHCC motif that coordinates a zinc atom that is required for viral cDNA integration. Three highly conserved amino acids (D,D-35-E) are embedded in the core domain, which form the acidic catalytic triad coordinating one or possibly two divalent metals (Mn + or Mg +). The C-terminal domain (residues 213-288) is responsible for unspecific DNA binding and adopts an overall SH3 fold (Chiu and Davies 2004). The enzyme functions as a multimer and to this end all three domains can form homodimers. 

Fig. 7. Comparison of the cluster binding fold of the water-soluble Rieske fragment from bovine heart 6ci complex (ISF, left PDB file IRIE) with the structure of ru-bredoxin (middle PDB file 7RXN) and with the zinc-ribbon motif (right PDB file ITFl). The metal binding loops are shown as ball-and-stick models of the backbone atoms.

Fig. 6. A schematic view of the [3Fe-4S] Emd [4Fe-4S] cores, as versatile structures. The absence of one site leads to the formation of a [3Fe-4S] core. The cubane structure can incorporate different metals (in proteins, M = Fe, Co, Zn, Cd, Ni, Tl, Cs), and S, N, O may be coordinating atoms from hgands (Li). The versatihty csm be extended to higher coordination number at the iron site and a water molecule can even be a ligand, exchangeable with substrate (as in the case of aconitase (,87)). The most characteristic binding motifs are schematically indicated, for different situations proteins accommodating [3Fe-4S], [4Fe-4S], [3Fe-4S] + [4Fe-4S], and [4Fe-4S] -I- [4Fe-4S] clusters. A disulfide bridge may replace a cluster site (see text).

As we have already seen zinc-finger peptides are well-studied polypeptide motifs that have found many applications in synthetic systems, mostly because of their abihty to bind metal ions and interact with oligonucleotides. In this context the report by lima and Crooke [44] of the hydrolytic cleavage by a zinc-finger peptide devoid of any metal ion is a surprising. The system they studied, a 30-amino acid sequence, is based on a catalytic mechanism very similar to that discussed above... 

Very few reports concerning transformations of ligands with other donor atoms exist (Table V). P—H activation at secondary phosphines is the most common motif, with the metal-metal bonds at the heterometallic faces stabilizing the resulting fragments in each case (Figs. 31. 32, 33).In the formation of both... 

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