Heavy atoms ligands

M(N03)4] anions containing the unusual unidentate niirato ion are also known. Fluoro complexes are even less prevalent, the preference of these cations being for the other halides, cyanide, N- and heavy atom-donor ligands. 

Scheme 8 gives species with extended 109 or starbust -type structures, which are strongly luminescent even with the large ligand L = PCy3.85 The diphenylfluorene derivative shows a remarkable heavy atom effect on the intersystem crossing rate.78... 

In the absence of capping ligands, polymeric alkynylmercury derivatives can be readily prepared. Polymer 57 is formed by reaction of HgC with 9,9-bis(4-ethynylphenyl)-fluorene in a basic methanolic solution. This polymer, whose average molecular weight (/I7W) is only 8,900, shows a strong triplet emission of the organic chromophore enhanced by the presence of the mercury heavy atom.69... 

Fig. 19.7 Three possible orientations of the ligand HUB093 within the SI pocket of the enzyme peptide deformylase. Heavy atoms of the protein are coloured according to secondary chemical shift effects caused by the ring current...

Spin orbit coupling is most effectively increased by using higher Z metals. Heavy atom substituents on the organic ligand have proved of minimal use and may actually make the ligands more photochemically unstable. 

The method chosen to achieve this stabilization is to have the olefinic group form part of a molecule containing a class b donor atom, usually P or As but occasionally 0 or S. It is found, as expected, that the strongest chelate effect occurs when the chelate ring has 5.5 or 6.5 members, that, is, when there are two or three carbon atoms between heavy atom and vinyl group. Typical ligands, with the abbreviations used here, are shown in Tables 1 and 2. This review excludes compounds such as the one in Fig. 1, where no true chelation, considered as the... 

Heavy-atom derivation of an object as large as a ribosomal particle requires the use of extremely dense and ultraheavy compounds. Examples of such compounds are a) tetrakis(acetoxy-mercuri)methane (TAMM) which was the key heavy atom derivative in the structure determination of nucleosomes and the membrane reaction center and b) an undecagold cluster in which the gold core has a diameter of 8.2 A (Fig. 14 and in and ). Several variations of this cluster, modified with different ligands, have been prepared The cluster compounds, in which all the moieties R (Fig. 14) are amine or alcohol, are soluble in the crystallization solution of SOS subunits from H. marismortui. Thus, they could be used for soaking. Crystallographic data (to 18 A resolution) show isomorphous unit cell constants with observable differences in the intensity (Fig. 15). 

Three types of interaction between ligand and protein heavy atoms arising from different atom types. Primary and secondary amines are defined to be donors while oxygen and nitrogen atoms with no bound hydrogens are defined to be acceptors. Crystallographic water molecules and hydroxyl groups are defined to be both donor and acceptor. Carbon and sulfur atoms are defined to be nonpolar... 

Since the discovery in 1964 that the antibiotic valinomydn exhibited alkali cation specificity in rat liver mitochondria, a new area of research has developed, based not only on biological systems but also on model systems such as crown ethers.484 The ability of neutral compounds to form lipid-soluble alkali and alkaline earth complexes was observed in 1951. The structure of the corresponding ligand, the anion of the antibiotic nigericin (78), was characterized as its silver salt in 1968.488 486 Silver was used as a heavy atom crystaUographically, since the Ag+ cation had a radius between that of Na+ and K+, which were the two alkali cations with which nigericin was most active. 

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