Photon Factory Activity Report 2014 #32 (2015) B

BL-27B/2014G128

Structural study of selective complexation of Sr(II) by XAFS method
Kazuhiro Akutsu1,*, Hiroki Iwase1, Shiho Yada2, and Tomokazu Yoshimura2
1
Comprehensive Research Organization for Science and Society (CROSS), 162-1 Shirakata, Tokai,Ibaraki, 319-1106, Japan
2
Research Group of Chemistry, Division of Natural Sciences, Nara Women’s University,
Kitauoya-higashimachi, Nara, 630-8506, Japan
1 Introduction
Strontium-90 (90Sr) having long radiological half-life isradioactive in nature and it falls under the category of
most hazardous of radioactive fallout [1]. Investigation of
the effects of 90Sr on environment and human body are
one of the most important issues for the management ofradioactive elements. Fluorescent probes are sufficiently
sensitive to detect a tiny amount of metal ions such as
strontium ion (Sr(II)) because of its exquisite sensitivity
and selectivity. Therefore, many kinds of fluorescentprobes for Sr(II) detection have been developed [2, 3].
Recently, we synthesized a Sr(II) selective fluorescentprobe,
N-(2-hydroxy-3-(1H-benzimidazol-2-yl)-phenylmethyl)-1-aza-18-crown-6-ether (Hbice), and studied its
fluorescence properties and complexation behavior withSr(II). In present study, the local structure of the Sr(II)Hbice complex in DMSO is determined using EXAFS
analyses to reveal the species of Sr(II) complex which
formed in DMSO solution.
2 Experiment
Hbice was synthesized according to a proceduredescribed previously [4]. An aqueous solution of SrCl2(1.0 M), 18-crown-6 ether (0.02 M), and a DMSO
solution of Hbice (0.02 M) were prepared. 0.01 M ofSrCl2, Sr(18-crown-6 ether) complex, and Sr(Hbice)complex solutions were prepared by adding 20 μl of SrCl2solution, 500 μl of ligand solution, and appropriate
amount of water or DMSO in a cell. All of XAFS spectra
were measured in fluorescence mode at the BL-27B
station in the KEK-PF. The Sr K-edge XAFS spectra wererecorded within 15.77 – 17.05 keV. Each sample wasmeasured for 1 h. The EXAFS spectra were analyzed
using the WinXAS 3.1 program [5]. Theoretical phaseshifts and backscattering amplitude functions weregenerated using the FEFF8.00 code. The amplitude of thereduction factor, S02, was fixed at 1.0. All of XAFS
measurements were performed at room temperature.
3 Results and Discussion
We measured the XAFS spectra of SrCl2 and Sr(18crown-6 ether) complex as a standard sample for theXAFS analysis. Fig. 1 shows the k3-weighted Sr K-edge
EXAFS oscillation of Sr(II) complexes in solution, andthe corresponding magnitude of the Fourier-transformed
data. Because O/N and Cl can behave as ligands for Sr(II)in this system, both Sr-O/N and Sr-Cl models were usedto fit the EXAFS data. The Fourier-transformed spectra

exhibited two peaks at 2.0 and 3.0 Å in the data of Sr(18crown-6 ether) and Sr(Hbice) complex. The larger peak at
2.0 Å can be clearly assigned to a Sr-O/N interaction [6].

Fig. 1: (a) k3-weighted Sr K-edge EXAFS oscillation ofSr(II) complexes in solution, and (b) the corresponding
magnitude of the Fourier-transformed data. Solid linesand dashed lines indicate the experimental data andtheoretical fit, respectively. The phase shift in theFourier-transformed data was not corrected.
Table 1: EXAFS structural parameters obtained by curve
fitting for Sr(II) complexes.
SampleInteraction
RCN
σ2
SrCl2
Sr–O
2.56
7.7
0.013
Sr–O
2.61
7.00.011
Sr(18-crown-6 ether)
Sr–C
3.54
9.4
0.003
Sr–O/N
2.54
6.9
0.014
Sr(Hbice)
Sr–C
3.66
9.20.005
R(Å): bond distance; CN: coordination number; σ2(Å2):
Debye-Waller factor squared.

Photon Factory Activity Report 2014 #32 (2015) B

In addition, other peak at 3.0 Å can also be clearly
assigned to a Sr-C interaction [6, 7], which strongly
proposed that carbon atoms of these ligands are in thesecond coordination sphere of Sr(II). Therefore, it can besuggested that both 18-crown-6 ether and Hbice directlyinteract with Sr(II), and Cl is not in the first coordination
sphere of Sr(II) complexes. Table 1 shows the EXAFS
structural parameters obtained from this analysis. The
fitting results showed that the local structure of Sr(Hbice)complex is similar with that of Sr(18-crown-6 ether)complex. This result suggested that the primary
coordination sphere of Sr(II) consists only of O/N atomsof Hbice, that is, the Sr(II):Hbice = 1:1 complex isformed in this system. Since the Hbice molecule ismonovalent anion, [Sr(Hbice)]Cl species would beformed in DMSO solution in this system. To understandSr(II) recognition mechanism in detail, more study is inprogress.
Acknowledgement
We thank Mr. T. Izumi (JAEA) for his support for theXAFS experiments. Sample preparation was conducted at
the User Experiment Preparation Lab III that is provided
by Comprehensive Research Organization for Scienceand Society (CROSS).
References[1] S. Kaur, et al., Org. Biomol. Chem. 12, 8230 (2014).
[2] H.-F. Ji, et al., Org. Biomol. Chem. 4, 770 (2006).
[3] K.-H. Leung, et al., RSC Advances 2, 8273 (2012).
[4] K. Akutsu, et al., Abstract of the 95th CSJ Annual
Meeting, Funabashi, Japan, 4F6-08 (2015).
[5] T. Ressler, J. Synchrotron Radiat. 5, 118 (1998).
[6] M. P. Jensen, J. Am. Chem. Soc., 124, 10664 (2002).
[7] Y. Okamura et al., Anal. Chem., 84, 9332 (2012).
* k_akutsu@cross.or.jp