Spectroscopic Study of the Interaction of Nd3+ with Amino Acids: Phenomenological 4f-4f Intensity Parameters

Journal of the Brazilian Chemical Society

Malta, Oscar¹  Massabni, Ana M.G.¹  Stucchi, Elizabeth B.¹  Leite, Sergio R. de A.¹  Universidade Federal de Pernambuco, Recife, Brazil¹  Jericó, Soraya¹  Carubelli, Célia R.¹  Universidade Estadual Paulista, Araraquara, Brazil¹ 

关键词: neodymium;    amino acids;    transition intensity parameters;   

DOI  :  10.1590/S0103-50531998000500014

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The study of the chemical bonding between trivalent lanthanide ions (Ln3+) and amino acids or peptides has its origin in the interest in using these ions as structural probes in biological systems, particularly in those systems which contain Ca2+ in their structure1,2. The Ca2+ ion is optically inactive and is, therefore, not suitable for providing information, through optical spectroscopic measurements, about the chemical environment in which it is embedded. On the other hand, almost all Ln3+ ions are know to exhibit rich optical spectra, either in absorption or emission, and it occurs that, due to the similarity between ionic radii, they may substitute Ca2+ ions in the chemical structures. There is strong evidence that the bonding between Ln3+ ions and amino acids is made with the oxygens of the carboxylate group, and that the bonding via the nitrogen of the amino group is unlikely to occur at least in a range of pH values up to 5.63,4. In this paper we examine the relation between the basicity of the carboxylate groups of glycine (Gly), L-aspartic acid (Asp), L-glutamic acid (Glu), L-histidine (His), DL-malic acid (Mal) and AspartameTM (APM) (Table 1), and the intensity parameters of 4f-4f transitions in their compounds with Nd3+ ion. The intensity parameters are qualitatively interpreted in terms of the forced electric dipole and dynamic coupling mechanisms of 4f-4f intensities5-10. Both mechanisms are dependent on the chemical environment around the Ln3+ ion. The basicity of the carboxylate groups is considered according to pK values and partial atomic charges on the oxygens which were calculated from molecular mechanics and the semi-empirical PM3 quantum chemical method.   Experimental The samples were prepared from aqueous solutions of Nd(ClO4)3 and the amino acids glycine, L-aspartic acid, L-glutamic acid and L-histidine, the dipeptide AspartameTM or the DL-malic acid. The absorption spectra were measured in a Carl Zeiss M-40 UV-visible spectrophotometer between 11000 cm-1 and 30000 cm-1. Solutions of Nd(ClO4)3 with pH 5.0-5.5 were prepared from Nd2O3 (Aldrich, 99.99%) and standardized by EDTA / xylenol orange titration. Solutions of the ligands were standardized by titration with NaOH / phenolphtalein or by potentiometric titration. The concentration of all solutions were around 5.00 x 10-2 mol L-1. We have firstly examined the behavior of the 4I9/2 Â® 4G5/2, 2G7/2 hypersensitive transitions of Nd3+, between 16600 cm-1 and 18200 cm-1, as a function of the molar ratio Nd3+: Ligand and the pH of the solution, which was varied from 1 to 5.5 for each ratio. The solutions of the Nd(ClO4)3 and the ligand were mixed in a quartz cell of 1.00 cm optical pathway which was coupled to a quartz bulb with 20 mL capacity. Volumes were measured with calibrated pipetes. The molar ratio Nd3+ / Ligand was varied from 1:1 to 1:10. The pH was adjusted by addition of acid or base and measured directly in the cell with a combined glass microelectrode. Concentrations were corrected for the volume. Wavenumber scan was made with variable slits and constant energy beam at the photodetector and the absorbance values were read to the fourth decimal place in a digital display. All the measurements were made at 25 ± 1 °C. Doubly destilated water was used as reference. The samples with the ratio Nd3+ / Ligand and pH that gave the highest absorption in the region of hypersensitivity were used to obtain the entire spectra from which the spectroscopic parameters were obtained. The best ratio Nd3+ / Ligand is 1/4 except for the Nd3+ / Aspartame, in which

【 授权许可】

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