Stereochemical label, ligand

The CHEMiCAL ABSTRACTS stereochemical notation system is composed of four parts the site symmetry term, the configuration number, the chirality label, and the ligand stereochemical label. 

Re and Si Labels used in stereochemical descriptions of heterotopic faces. If the CIP priority of the three ligands a, b, and c is assigned as a > b > c, the face that is oriented clockwise toward the viewer is called Re, while the face with a counterclockwise orientation of a — b — c is called Si, as shown in Figure 1-34 ... 

Stereochemical probes of the specificity of substrates, products, and effectors in enzyme-catalyzed reactions, receptor-ligand interactions, nucleic acid-ligand interactions, etc. Most chirality probe studies attempt to address the stereospecificity of the substrates or ligands or even allosteric effectors. However, upon use of specific kinetic probes, isotopic labeling of achiral centers, chronfium-or cobalt-nucleotide complexes, etc., other stereospecific characteristics can be identified, aU of which will assist in the delineation of the kinetic mechanism as well as the active-site topology. A few examples of chirality probes include ... 

Heterofunctionalized phosphines, with different denticities and/or donor sets, were successfully applied, at macroscopic and noncarrier added level, to the stabilization of the [M=0]3+, trans-[M02]+, [M=N]2+ and/ac-[M(CO)3]+ cores. The unique electronic and/or stereochemical properties of the tridentate PNS phosphine allowed the synthesis of 3+1 mixed-ligand complexes with an unprecedented in vivo robustness. Phosphines as bifunctional chelate ligands have been successfully applied for labeling different biologically active substrates, namely peptides and CNS-receptor avid molecules. 

Harris, Robb and co-workers have thoroughly studied the kinetics of ligand substitutions of the aquachlororhodium(III) complexes.1301 1202 1211-1213 The reactions studied are summarized in Scheme 40, and the activation parameters for the labeled reactions are recorded in Table 87. The stereochemical course of these reactions is dominated by the frans-labilizing effect of chloride ligands. For... 

A classic example of a CKlII) photosubstitution is the well-studied complex, Cr(NH3)5Clp , for which NHj photoaquation is the principal result of excitation of ligand-field bands, and Cl aquation represents only about 2% of the overall total . The stereochemical origin of the labilized NHj is trans to the coordinated Cl as shown by isotope-labeling experiments, although the final reaction product has the cis configuration ... 

It is also known from experiments with [y- OJATP and 0-labeled transition-state analogs of metaphosphate monoanion that P is coordinated to Mn(II) in the active complex and that the remaining three ligands to Mn(II) are water molecules (2i). The stereochemical results by this technique are in agreement with those obtained using the epimers of ADPaS and ADP/3S as substrates and various metal ions as activators (27). 

As an example of a molecule with diastereotopic ligands, consider the amino acid L-phenylalanine. The two protons at C-3 are diastereotopic, since substitution of either of them would generate a molecule with two chiral centers. Because the chiral center already present is 5, the two diastereomers would be the 2S,3R and the 25,35 stereoisomers. As in the case of enantiotopic protons, diastereotopic protons are designated pro-R or pro-S. The enzyme phenylalanine ammonia lyase catalyzes the conversion of phenylalanine to trans-cinnamic acid by a process involving anti elimination of the amino group and the 3-pro-S hydrogen. This stereochemical course has been demonstrated using deuterium-labeled L-phenyl-alanine as shown" ... 

Stereoehemical Labelling.—Several years ago, kinetic studies on substitution at [Cr(OH2)5l] + were discussed in terms of a high trans effect of the iodide ligand in this complex. The original kinetic evidence and conclusions were convincing but not absolutely conclusive. Recently kinetic studies in aqueous dimethyl sulphoxide have made ingenious use of the dimethyl sulphoxide as a stereochemical signpost to confirm and amplify the earlier mechanistic hypotheses. ... 

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