Reverse chelate effect

X. Systems Containing More Than One Metal Ion — The Reverse Chelate Effect References... 

The chelation effect also brings about a stabilization of the — 1 state of the peptide model complexes as indicated by the thermal stability and redox behavior. Only [Fe(Z-cys-Pro-Leu-cys-OMe)2] exhibits a relatively reversible redox couple in the cyclic voltammogram measurement, but the others do not (20). The bulkiness of side chains of the X and Y residues in Cys-X-Y-Cys probably restricts the adoption of the inherent by preferable conformation (ift = 0°), resulting in a more restricted orientation of Fe-S-C. In fact, the X-ray analysis of native rubredoxin shows that two of the Fe-S torsion angles are restricted and the other two are normal, i.e., conformationally more stable. 

An intriguing question is whether the use of bidentate Lewis acids may be advantageous for the selective recognition of anions. Early studies by Shriver, Katz, Koester, and Wrackmeyer show that a bidentate organoboron Lewis acid may coordinate a nucleophile in a bridging fashion as illustrated for Katz s hydride-sponge in Scheme 24. The relatively short distance of ca. 3 A between the boron centers of 1,8-diborylated naphthalenes allows for effective coordination of nucleophiles in a reverse chelate fashion. Many other... 

The Cram chelation model (sec. 4.7.B) is an example where the chelation effects of the heteroatom influence the rotamer population and, thereby, the selectivity of the reduction. Zinc borohydride [Zn(BH4)2], effectively chelates the carbonyl oxygen and alcohol oxygen atoms in the reduction of 42 and leads to intermediate 43. Transfer of hydride to the carbonyl gave primarily the anti diastereomer, 45 (4 96, 44/45). When the chelating hydroxyl group was blocked as a tert-butyldiphenylsilyl ether (in 46 - sec. 7.3.A.i), reduction with Red-Al (sec. 4.3) led to a reversal in selectivity (96 4, 47/48).The ability to chelate a heteroatom varies with the reagent used. Lithium aluminum hydride shows less selectivity, due in part to poorer coordination with the heteroatom and reduction of 42 gave a 27 73 mixture of 44 and 45,... 

The first dissociation (1) is expected to be slower than a similar dissociation of ammonia, because the ethylenediamine ligand must bend and rotate to move the free amine away from the metal. The reverse reaction associated with the first dissocation is fast. Indeed, the uncoordinated nitrogen is held near the metal by the rest of the ligand, making reattachment more likely. This kinetic chelate effect dramatically reduces aquation reaction rates. 

Other studies examined whether declines in blood lead concentration from the peak at around 2 years of age are associated with improvements in cognition later in life (Rogan et al., 1991 Ruff et al., 1993, 1996 Tong et al., 1998 Liu et al., 2002 Dietrich et al., 2004 Chen et al., 2005). Overall, these studies support the view that children are vulnerable to lead exposures occurring anytime from early childhood through school age. Further, chelation therapy does not appear to reverse the effects of lead in children with blood lead concentrations below about 45 pg dl (Rogan et al., 2001 CDC, 2002 Liu et al., 2002 Dietrich et al., 2004 AAP, 2005). 

The effect of Pb on AQP4 appears to depend on the activity of Ca -calmoduUn-dependent protein kinase II (CaMK-II), but not PKC (Gunnarson et al., 2005). In addition, treatment with a chelator, but not simple Pb washout, reversed the effect, suggesting that Pb enters and acts inside the astrocytes. AQP4 was not upregu-lated or redistributed in the brains of the lead-treated rats. 

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