Hydration number luminescence

Kimura, T. Choppin, G. R. 1994. Luminescence study on determination of the hydration number of Cm(III). Journal of Alloys and Compounds, 213/214,313-317. 

Ln3+ induced water 170 shifts of [Ln(DOTA)] solutions show that the hydration number of the complexes is one across the lanthanide series [59]. The substantial pseudocontact contribution to its LIS indicated that this water ligand has a preferred location in the complex. Two sets of peaks have been observed in H and 13C NMR spectra of [Ln(DOTA)] complexes at room temperature showing the presence of two slowly interconverting structural isomers [60-63]. In the spectra of the paramagnetic complexes, one isomer has larger LIS values than the other. These structural features have been confirmed by luminescence studies [51, 64]. The temperature dependence of the H and 13C NMR spectral features of both the dia- and paramagnetic Ln3+ complexes indicates that the... 

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]. 

Following this study, Korovin and coworkers tested cryptands 23b-f which are used in time-resolved luminescent immunoassays (Mathis, 1993) for the sensitization of Ybm luminescence (Korovin et al., 2002b). From the lifetimes determined in both water and deuterated water, one calculates that the hydration number varies from 2 (23b), to 1.5 (23a, 23e), and finally to 1 (23c, 23d, 23f). Quantum yields were not determined, but luminescence intensities relative to the cryptate with 23a (in water, at room temperature) point to cryptands 23c and 23d being the best sensitizers of the Yb111 luminescence with a seven-fold enhancement, while cryptates with 23e and 23b are only 1.5- to 1.8-times more luminescent. 

The calix[4]azacrowns 41 and 42 (fig. 38), capped with aminopolyamide bridges bind Lnm ions and form both 1 1 and 1 2 (Ln L) complexes with stability constants in the ranges log/3i v 5-6 and log (h 10-11 in acetonitrile for 42, while the stability of the complexes with 41 is about two orders of magnitude smaller. Hydration numbers around 1 were found for the Eum and Tbm complexes, but the ability of the ligands to sensitize Lnm luminescence is veiy weak, except in the Tbm complex with 42. No Er111 luminescence could be evidenced, but some Ndm emission was recorded, which is 12 times larger with ligand 42 than with receptor 41 (Oueslati et al., 2006). 

The three Ndm, Er111, and Ybm chelates display sizeable metal-centered NIR luminescence in HBS-buffered (pH 7.4) aqueous solutions. Their photophysical characteristics are summarized in table 18. The hydration numbers calculated from eqs. (10a) and (9a) are very small, 0.31 and 0.16 for Ndm and Ybm, respectively, and compare well with the results obtained for the tetrapodal ligand H890a. The overall luminescence quantum yields in aqueous solution are comparable to those obtained for H890a, but smaller than those determined for chelates withH890b (compare tables 17 and 18). Upon deuteration of the solvent, from 3- to 10-fold increases are observed in the luminescence quantum yields. Moreover cytotoxicity studies on several cell lines have shown the Ybm chelate to be non-toxic, opening the way for applications in cell imaging (Comby et al., 2007). 

We define the hydration number as the average number of water molecules in the first sphere about the metal ion. The residence time of these molecules is determined generally by the nature of the bonding to the metal ion. For the f-element cations, ion-dipole interactions result in fast exchange between the hydration layer and the bulk solvent. The techniques for studying the nature (number and/or structure) of the hydration shell can be classified as either direct or indirect methods. The direct methods include X-ray and neutron diffraction, luminescence and NMR (nuclear magnetic resonance) relaxation measurements. The indirect methods involve compressibility, NMR exchange and absorption spectroscopy measurements. 

Cryptands have been somewhat deceptive for both coordination chemistry (Sastri et al., 2003) and photophysical properties of the resulting lanthanide complexes despite some commercial uses (Mathis, 1998), in particular of Lehn s Eu cryptate with cryptand 23a (fig. 28). The latter has been tested for the sensitization of the NIR luminescence of Nd and Yb. Characteristic emission from these two ions is seen upon excitation of the bipyridyl chro-mophores at 355 nm. Emission from Yb is reported to be much more intense than the one from Nd and the authors propose that the excitation mechanism depicted in fig. 9 is operative in this case since no transient absorption corresponding to the formation of the triplet state could be detected (Faulkner et al., 2001). Analysis of lifetime measurements in both water (r( F5/2) = 0.52 ps) and deuterated water (5.21 ps) gives a hydration number q = 1.5. Since fitting the luminescence decays to a double exponential function did not improve noticeably the resulting fit, the authors concluded that the non-integer value does not reflect an equilibrium between two different hydration states but, rather, that the distance of close approach of two water molecules is longer note that comparable experiments on Eu and Tb ... 

Kimura T, Kato Y (1998) Luminescence study on determination of the inner-sphere hydration number of Am(III) and Nd(ni). J Alloys Compd 271 867-871... 

The X-ray techniques have limited applications in actinide systems because the high concentration required for the measurements (ca. 2-3 M) does not allow study of elements for which only isotopes of high specific radioactivity are available. Beitz (1991) calculated the hydration number of some curium complexes using luminescence lifetime measurements and assuming a hydration number of nine of the free ion. Electrophoretic measurements using tracer concentrations (Lundqvist 1981, Lundqvist et al. 1981) have provided estimates of the hydrated radii (r ) for Eu, Am, Cm, Es, Fm and Md from Stokes law. The hydrated radii were used, in turn, to calculate hydration numbers, h, by dividing the volume of the hydrated sphere by the volume of a water molecule (si 30 A ). Since the volume occupied by the bare metal ion is small [2-5 A for Ln(III) and An(III)], it was neglected in comparison with the volume of the water molecules. The results are listed in table 3. 

Lanthanide binding to proteins and other biomolecules can make very important contributions to structural and functional analyses. Techniques such as luminescence and NMR spectroscopy can help to provide detailed structural information and/or permit the identification of amino acids in the vicinity of binding sites, the estimation of hydration numbers of sequestered Ln ions and, where more than one metal-binding site exists, interionic distances. At the functional level Ln binding can also provide valuable information, e.g., the fact that Ln ions are better cofactors than Ca for the activation of trypsinogen to trypsin points to a charge-masking role for metal ions in this reaction (Evans 1990). 

As remarked above, the luminescence of Eu " ions is most appropriate as structural probe for the determination of the number of metal ion sites in a compound, their symmetry, their hydration numbers, and their coordination sphere. Especially, the ratio of the intensities of the Dq Fi and Dq... 

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