Bound lanthanide-induced shifts

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

In subsequent work it was found that the carboxylic acid form of 25 and 27 could be solubilized in chloroform-rf by the addition of triethylamine. Addition of Eu(fod)3 caused the reverse behavior from what had been observed with the ester forms of the CSA, as the enantiomer that associated more favorably with the CSA has the larger lanthanide-induced shifts. This observation was explained by assuming that the carboxylate form of the CSA is bound to the lanthanide ion to create the species [Ln(fod)3CSA] . The substrate then associated with the CSA in its lanthanide-bound form ° . ... 

Lanthanide Shift Reagents.—New theoretical treatments of the origin of the paramagnetic shifts induced in the n.m.r. spectra of substrates bound to... 

Of particular interest in the study of effects of metal ion binding on the conformation of a-lactalbumin are the contributions of Berliner and co-workers. Murakami etal. (1982), in a study of bovine, caprine, human, and guinea pig a-lactalbumin, observed metal ion-induced conformational change resulting in a unique hypsochromic shift and quenching of tryptophan fluorescence. They found that Ca(II) and the lanthanides Tb(III), Eu(III), Gd(III), Yb(III), Pr(III), and Dy(III) could be bound extremely strongly to a specific site. They also found that Mn(II), Ca(II), and Mg(II) could be weakly bound to the same site. 

Ferrocene cryptands have also been studied as cation receptors. Hall ef al. have investigated the coordination of alkaline earth and lanthanide metal cations by cryptand 16 using cyclic voltammetry in CH3GN solution.The cation-induced anodic shift of the Fc/Fc redox couple was found to broadly correlate with the charge density of the cationic guest. Further electrochemical analysis revealed evidence that upon oxidation the 16 Be and 16 Dy complexes readily eject the bound cation from the cryptand cavity, presumably due to electrostatic repulsion. 

The interaction between lanthanide shift reagents containing p-diketones and several methyl 2,3-anhydro-4,6-0-benzylidene-D-hexopyranosides has shown that the carbohydrate may be bound to the reagents in either monodentate or chelated manner. The separation of xylitol and L-arabitol on cation exchange resin in the La form is mentioned in (Chapter 23. The n.m.r. shifts induced by complexation between per-O-methylated aldopyranoses and metal cations (mostly Eu ) have been studied and an order of stability of the complexes has been deduced. Treatment of (dppelPtClj with 2 equiv. of KOSiMej and a slight excess of l,2 5,6-di-0-isopropylidenemannitol has afforded the (dppe) Pt (glycolate) complex. ... 

---