Lanthanide-induced shifts applications

Hofer, O., The Lanthanide Induced Shift Technique Applications in Conformational Analysis, 9, 111. 

The recently introduced lanthanide-induced shift (LIS) technique has also found useful application in configurational studies of sulfur compounds. For example, the absolute configurations of a new type of bridged steroidal sulfoxide 203, which is epimeric at sulfur, were... 

Lanthanide-induced Shift Technique - Applications in Conformational Analysis/Hb/er 9 111... 

Conformational Analysis, Applications of the Lanthanide-induced Shift... 

Lanthanide-induced Shift Technique— Applications in Conformational... 

The first applications of lanthanide induced shifts (LIS) in 13C NMR appeared in 1971. (7-10) Shortly thereafter it was shown (11) that Eu(dpm)3 induced chemical shift changes in cholesterol which extended from the coordination site at C-3 clear across the ring system, the assignments used being those of Roberts group. (2) Although the LIS... 

The angular and distance information provided by the lanthanide induced shift has found widespread application from the determination of solution structures of Ln chelates [18,19] to gaining structural information on proteins, nucleotides and amino acids [19], More recently anion binding to coordinatively unsaturated lanthanide complexes has been effectively signalled as the observed lanthanide induced shift has been directly correlated to the nature of the donor atom in the axial position [8,20,21], It is the polarisability of the axial donor that ranks the second order crystal field coefficient, B02, and hence determines the magnitude of the observed shift. Values of the mean shift of the four most-shifted axial protons of the 12-Nq ring for [Yb.la]3+ are collated in Table 2. 

Lanthanide-induced Shift Technique—Applications in Conformational Analysis... 

Lanthanide shift reagents have been used to study dynamic NMR processes. One of the classic applications involves the study of rotational barriers of various amides " thioamides ", oximes " and amide oximes ". Lanthanide-induced shifts enabled the distinction of cis- and trans- or syn- and awri -isomers, and variable-temperature studies in the presence of the chelate were performed to determine the barrier to rotation. 

The equilibrium constant for the complexation of cholesterol with Eu(fod)3 has been evaluated from measurements on a series of solutions at varying total concentrations but identical molar ratio. The same analysis provides values for the chemical shifts of protons in the uncomplexed steroid, and in the steroid-europium complex the latter value is not accessible by direct measurement, since the complex is always in equilibrium with the free steroid. The mathematical equations presented in this paper should be generally applicable. In another paper the validity of various procedures for interpreting lanthanide-induced shifts is explored comments on cholesterol are included. No one mathematical model at present available is considered to have general validity. 

Alternatively, the sulfites can be oxidized to the sulfates 85 in 45 min at 0°C by employing a 350 ppm RUO4 solution in water, but in only 30% yield <1992NJC107>. Diastereotopically labeled 0-sulfates 84 and 85, obtained by oxidation of diastereoisomeric cyclic sulfites with Ru 04, were shown by application of lanthanide-induced shift reagents on the NMR signals to be formed with retention of configuration at sulfur <1983CC1392>. 

More recently, Abraham and coworkers [18] reexamined the conformational behaviour of thiane-l-oxide (3) by analysis of lanthanide-induced shifts (LIS) in the appropriate NMR spectra. Their results compare reasonably well with those obtained by Barbarella et al. [17], suggesting the applicability of the LIS method to sulfoxide conformational analysis. 

The pinnacle of the application of lanthanide shift reagents seemed to be in their assistance in the elucidation of the structure of molecules in solution. It must be borne in mind, however, that the intrinsic dipolar shift, 4d, due to the central lanthanide ion is the only part of the observed shift useful in this respect and then only in cases of effective axial symmetry is it readily applicable. Therefore great care should be exercised in the use of lanthanide induced shifts to solve structural problems. Generally the following types of problems may successfully be approached. 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

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