Application of luminescence and absorption spectroscopy and the x-ray method to the study of Ln 3+ ions interaction with aminoacids

Abstract

Lanthanide ions have found increasing use as metal ion probes for spectroscopically inert Ca(II), and substitution of Ln(II) for Ca(II) enables the performance of absorption, luminescence and magnetic resonance studies on systems of biological interest. In peptide and protein complexes, the Ca(II) ion is normally bound by aspartate and glutamate residues so it is crucial to the full understanding of Ln(III) binding by these materials. In our previous papers we have examined the interaction of lanthanide ions with L-asparagine, L-glutamine, L-aspartic and L-glutamic acid in aqueous solutions. We have suggested the appearance of the dimeric forms in the latter case [1–4]. Now we report the results of our spectroscopic studies of the crystals of the Nd, Ho and Eu compounds with glycine and L-glutamic acid in solid state and of the X-ray investigations of the [Nd(gly) 3][ClO 4] 3·5H 2O crystal. Several lanthanide ion complexes with glycine and L-glutamic acid were synthesized and yield in the form of monocrystals. Absorption spectra of the crystals were measured on a Cary 14 spectrophotometer at room temperature and 5 K. Luminescence spectra of Eu(III) and Nd(III) compounds were recorded at the same temperature. E 1 and E 2 selective excitation energies of the Nd(III) ion level revealed that the Nd(III) ion could appear in two different symmetry positions in the complexes with glutamic acid. From the measured absorption spectra the oscillator strengths of f–f transitions were estimated. The relation between f–f transition intensity and position of the crystallographic axis of crystals has been stated. The f–f transitions were analysed on the basis of the Judd theory taking the dependence of intensity on the crystallographic axis position into account. Intensity analysis of f–f transitions revealed quite a considerable difference in the intensity distribution of bands, especially for hypersensitive transitions. Drastic differences in Judd parameters have been found (Table I). t001 The τλ Parameters Values. Compound τ 2 × 10 9 τ 4 × 10 9 τ 6 × 10 9 [Nd(Gly) 3][ClO 4 3·5H 2O a 3.65 ± 0.85 5.48 ± 0.78 11.49 ± 1.10 b 3.25 ± 0.68 5.67 ± 0.63 10.70 ± 0.88 c 1.83 ± 0.93 6.17 ± 0.86 10.40 ± 1.20 a (mean) 2.83 ± 0.76 5.92 ± 0.71 10.80 ± 0.99 [Nd(Glu)][ClO 4] 2·7H 2O 4.72 ± 0.73 5.44 ± 0.68 12.44 ± 0.95 The crystal structure data determined by us for [Nd(Gly) 3][ClO 4] 3·5H 2O compound with space group P1, Z = 4, symmetry for Nd +3; I-1 and C.N. = 9 (Fig. 1) confirm our suggestion that in lanthanide ▪ ion complexes with aminoacids dimeric and polymeric structures may be observed.