APPLIED PHYSICS LETTERSVOLUME 74, NUMBER 318 JANUARY 1999

Extended x-ray absorption fine structure study on the cerium„IV…-induced
DNA hydrolysis: Implication to the roles of 4 f orbitals in the catalysis
Hidemi Shigekawa,a) Masahiko Ishida, and Koji Miyake
Institute of Materials Science, and Center for TARA, CREST, Japan Science and Technology Corporation (JST),
University of Tsukuba, Tsukuba 305-8573, JapanRyu Shioda
Electro Technical Laboratory, Tsukuba 305-8568, Japan

Yoshitoki Iijima
Center for TARA, University of Tsukuba, Tsukuba 305-8573, Application & Research Center, JEOL LTD,Akishima, Tokyo 196-8558, Japan

Takamitsu Imai, Hideyuki Takahashi, Jun Sumaoka, and Makoto Komiyamab)
Department of Chemistry and Biotechnology, Graduate School of Engineering, University of Tokyo,Tokyo 113-8656, Japan~Received 18 August 1998; accepted for publication 10 November 1998!
At various Ce~IV!/DPP ~DPP: diphenyl phosphate! molar ratios, the structure of Ce clusters inaqueous solutions was studied by extended x-ray absorption fine structure measurements. Although
the Ce–Ce bond was strong in the absence of DPP, the corresponding signal was significantly
reduced when the Ce~IV!/DPP molar ratio was 1. The result indicates the existence of a strong
interaction between Ce~IV! ions and DPP, and suggests the formation of a Ce–DPP structure insolution. With an increase in the molar ratio, the signal of Ce–Ce bonds increased again, andexceeded that observed in the absence of DPP, suggesting the formation of a Ce–Ce–DPP structure.
Ce~IV! ions in the complexes were found to have ;0.67 4f electrons. On the other hand, nomeasurable charge transfer was observed in the case of Ce~III! ions. These results demonstrate therole of the 4 f orbital of Ce~IV! ions for its enormous activity for DNA hydrolysis. © 1999
American Institute of Physics. @S0003-6951~99!01603-4#

Interest in the nonenzymatic hydrolysis of DNA has
been growing rapidly, mainly because it could provide anovel tool for future biotechnology and molecular biology.1
However, DNA is strongly resistant to hydrolysis ~the intrinsic half life of its phosphodiester linkage at pH 7 and 25 °C
is estimated to be 200 million years!. It was only a few years
ago that the hydrolytic activity of the Ce~IV! ion for DNA
was demonstrated, when DNA hydrolysis was performed under physiological conditions.2 The activity of Ce~IV! is significantly greater than those of other lanthanide~III! and nonlanthanide ions. However, the following points have remain
to be clarified: ~1! Why is the Ce~IV! ion far superior to othermetal ions? ~2! What is the structure of the complex formed
between the Ce~IV! ion and DNA? ~3! What kind of chargetransfer interaction takes place in the complex? and ~4! Why
does Ce~IV! have to exist in excess compared to the phosphodiester linkage for efficient catalysis? Information related
to these points is crucial to the understanding of the catalytic
mechanism and to design catalysts with improved activity.
In this study, the structure of Ce~IV! ions in complexes
with diphenyl phosphate ~DPP! is analyzed by extended
x-ray absorption fine structure ~EXAFS! measurements. The
results of the electronic structure obtained from the study of
x-ray absorption near edge structure ~XANES! and x-ray

photoemission spectroscopy ~XPS! will be discussed together.
DPP was used as a model of DNA. Figure 1~a! shows
the chemical structure of DNA and that of DPP. The twomolecules have similar phosphodiester linkages. Therefore, asimilar complex is expected to be formed by both compounds with Ce~IV! ions @Fig. 1~b!#.3 EXAFS and XANES
experiments were performed using beam line 12C of thePhoton Factory in Tsukuba.4 The fluorescence yield from Ce
atoms was measured using a 19-element solid-state detector.
The samples were prepared by mixing DPP withCe~NH4!2~NO3!6 in water at the required molar ratios, andsubjected to analysis after being frozen in liquid nitrogen.
First, we discuss the results of EXAFS measurements.
Figure 2 shows a series of Fourier-transformed Cea!

FIG. 1. ~a! Chemical structures of the phosphodiester linkage in DNA andDPP. ~b! Schematic model for the Ce/DPP complex.

Electronic mail: hidemi@ims.tsukuba.ac.jp; http://dora.ims.tsukuba.ac.jp
Electronic mail: mkomi@chembio.t.u-tokyo.ac.jpb!

0003-6951/99/74(3)/460/3/$15.00
460© 1999 American Institute of Physics
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Appl. Phys. Lett., Vol. 74, No. 3, 18 January 1999

FIG. 2. Series of Fourier transformed Ce L 3 -EXAFS spectra at different
molar ratios @~a!–~e!#, and ~f! comparison of the spectra between ~b! and ~e!.

L 3 -EXAFS spectra of the k 3 2 x function at different molar
ratios; ~a! Ce 10 mM; ~b! Ce 100 mM; ~c! Ce 10 mM, DPP
10 mM; ~d! Ce 10 mM, DPP 5 mM; and ~e! Ce 10 mM, DPP
2 mM. Two large peaks at about 0.18 and 0.36 nm exist inthe spectra. By comparison with the crystal structure of CeO2~also shown in Fig. 2!, these peaks are attributed to Ce–Oand Ce–Ce bonds, respectively. CeO2 has a structure similar
to CaF2, and Ce–O and Ce–Ce bond length in the nearest
neighbors obtained by the x-ray diffraction method ~our EXAFS measurement! are 0.234 ~0.235! and 0.383 ~0.384! nm,respectively. The bond length obtained from the curve fittingof EXAFS x function with the theoretical phase shifts andbackscattering amplitude calculated with FEFF.5
As shown in Figs. 2~a! and 2~b!, Ce–Ce bonds exist inthe Ce 10 mM and Ce 100 mM samples without DPP. Therefore, Ce ions themselves form some cluster structure in solution. However, as is clearly shown in Fig. 2~c!, the signalof Ce–Ce bonds is markedly reduced to almost zero, when
the Ce/DPP ratio is 1:1 ~Ce 10 mM, DPP 10 mM!. Theobserved results indicate the existence of a strong interaction
between Ce~IV! ions and DPP molecules in solution, andsuggest the possibility of the formation of a Ce–DPP complex with a molar ratio of 1:1.
When the molar ratio of Ce~IV! ions to DPP was increased, the Ce–Ce bond appeared again and increased asshown in Fig. 2~d! ~Ce 10 mM, DPP 5 mM! and ~e! ~Ce 10mM, DPP 2 mM!. This result indicates that this Ce–Ce bondis caused by the additional Ce ions. The observed Ce–Cebonds may be ascribed to the formation of a Ce–Ce–DPP
structure in solution, as shown in Fig. 1~b!. However, sinceShigekawa et al.

461

FIG. 3. Ce L 3 -XANES spectra obtained for ~a! Ce/DPP ~Ce 10 mM, DPP 5mM!, ~b! Ce~III!/DPP ~Ce 10 mM, DPP 5 mM!, and ~c! Ce~IV!1Ce~IV!
samples.

Ce–Ce bonds exist in Ce solution without DPP from thebeginning as shown in Figs. 2~a! and 2~b!, we have to clarify
whether the observed Ce–Ce bonds that appear in Figs. 2~d!
and 2~e! are related to the structure including DPP.
A comparison between Figs. 2~b! ~Ce 100 mM! and 2~e!
~Ce 10 mM, DPP 2 mM! is shown in Fig. 2~f!. The Ce–Cebond exists in both cases; however, the signal increases almost twofold, when DPP molecules exist in solution. Fromthe results obtained, as shown in Fig. 2~f! and the finding thatthe Ce–Ce bond almost disappears when the molar ratio ofCe/DPP is 1, we concluded that the Ce–Ce bonding in Figs.
2~d! and 2~e! is related to the formation of a structure including DPP molecules. It remains to be clarified whether theeffective Ce–Ce bond in the complex corresponds to theformation of the (Ce)n-DPP structure or the simple Ce–Ce–DPP structure. Further experiments are necessary to clarify
this point. Nevertheless, these results are quite consistent
with the structure in the model shown in Fig. 1~b!. The
Ce–Ce bond length obtained for the Ce/DPP complexes
shown in Figs. 2~d! and 2~e! is 0.35260.005 nm.
In order to examine the electronic structure, we analyzed
the valence of the Ce ions in the complex form by measuring
the Ce L 3 -XANES spectra. Double white lines on the spectral edges, which are due to the electronic structures related
to 4 f 1 ~13! and 4 f 0 ~14!, are clearly observed in the spectra. Figure 3~a! shows the spectrum obtained for a Ce/DPP
sample ~Ce 10 mM, DPP 5 mM!. From the analysis of theratio between these two peaks, the number of 4 f electrons on

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462

Shigekawa et al.

Appl. Phys. Lett., Vol. 74, No. 3, 18 January 1999

FIG. 4. XPS spectra for DPP, Ce~IV! without DPP and Ce/DPP complex
samples.

Ce in the Ce/DPP complex was calculated to be 0.67
60.02.
For comparison, a Ce L 3 -XANES spectrum for the caseof Ce~III! ions prepared from CeCl3 is shown in Fig. 3~b!.
Only one peak exists and no measurable decrease in the valence, from 13 to 12, is observed in spite of the formationof the complexes. Similar results were obtained for otherlanthanide ~III! ions.
Figure 3~c! shows the Ce L 3 -XANES spectrum obtained
for the Ce~III!1Ce~IV! mixture. Since one Ce~III! peak andtwo peaks originating from Ce~IV! can be identified separately, the possibility that the two peaks in Fig. 3~a! comefrom the contamination of Ce~III! is ruled out. All the Ce~IV!
ions retain their tetravalent states throughout the presentanalysis, as confirmed by potentiometric titration.2 This result strongly suggests the formation of the hybrid orbital by
Ce~IV! ions.
If the observed change in electronic charge on Ce~IV!
ions plays an important role in DNA hydrolysis, the superiority of Ce~IV! over other lanthanide ions may be related toits high potential for withdrawing electrons from its counterpart in chemical reaction. This mechanism may stabilize theCe/DPP structure, which is necessary for DNA hydrolysis.
In order to confirm the valence of Ce~IV! in the Ce/DPPcomplex, the Ce 3d structure was studied by XPS, which isrelated to the charge transfer of 4 f electrons.6,7 Figure 4shows the Ce 3d spectra obtained for a sample of Ce/DPP
52. Three 3d signals exist, indicated by f 0 , f 1 , and f 2 ,which correspond to the three electronic states, namely, with0, 1, and 2 4 f electrons, caused by the hybridization of thefinal states. Each of them has two components due to spinorbital interaction. Since the transition between 4 f 0 and 4 f 1in the final states is negligible, the electronic charge on Ce in
Ce/DPP was calculated from the ratios of f 0 /( f 1 1 f 2 ), to be
0.6960.02. This value is in good agreement with that obtained by XANES analysis.
Another experiment was performed to examine the electronic structure. Figure 5 shows the spectra around the Fermi
level obtained for DPP, the Ce~IV! hydroxide cluster withoutDPP and Ce/DPP complex samples. No density of states exists in the spectra around the Fermi level for the DPP andCe~IV! hydroxide cluster samples. Since there exists a large
O peak around 4 eV, it is difficult to see the valence electronic structure. However, the density of states is clearly observed in the spectrum around the Fermi level for the Ce/
DPP complex. This result suggests the change in theFIG. 5. Ce 3d spectra obtained for a specimen of Ce/DPP52.

electronic structure of Ce~IV! ions through the formation of acomplex with DPP, as consistent with the proposed model.
In the complex formation between Ce~IV! and DPP, hybrid orbitals are efficiently formed between the 4 f orbitals ofCe~IV! and the phosphate residue, and significant chargetransfer may occur through them. From resonant photoemission spectroscopy and XPS studies on some Ce~IV! compounds, a charge of 0.6–1.0 4 f electrons was confirmed toexist on Ce ions.8,9 The large amount of charge transfer is
induced by the formation of the hybrid orbitals caused by thecharacteristics of the spreading 4 f orbitals in Ce. For example, in the case of CeO2, a large amount of charge observed on Ce is caused by partial charge transfer from O 2 p
to 4 f through the formation of a hybrid orbital between
Ce 4 f and O 2 p. Therefore, a similar electronic structure
may be formed in the Ce~IV!/DPP complex. In contrast, lanthanide ~III! ions bind to the phosphodiester linkage of DPP
via electrostatic attraction, rather than the formation of covalent bonds. The 13→12 transition is thermodynamicallyunfavorable.
In conclusion, the formation of a Ce–Ce–DPP structure
was confirmed by EXAFS in a Ce/DPP solution with a molar
ratio higher than 2. The formation of 4 f hybrid orbitals, aswell as the charge transfer through them, is considered to be
the reason for the superiority of the Ce~IV! ion in terms of its
DNA hydrolytic activity compared to other lanthanide ~III!
ions.

A review: M. Komiyama, J. Biol. Chem. 118, 665 ~1995!.
Y. Matsumoto and M. Komiyama, Nucleic Acids, Symp. Ser. 27, 33~1992!; T. Shiiba et al., J. Mol. Catal. 84, L21 ~1993!; M. Komiyamaet al., Chem. Lett., 1025 ~1994!; B. K. Takasaki and J. Chin, J. Am.
Chem. Soc. 116, 1121 ~1994!; M. Komiyama et al., J. Chem. Soc., PerkinTrans. 2 2, 269 ~1995!.
3
H. Shigekawa et al., Appl. Phys. Lett. 68, 1433 ~1996!. It is not yet
clarified if DPP is complexing with both of the two Ce~IV! ions in thebimetallic cluster @as depicted in Fig. 1~b!# or it is bidentately coordinatedto one of them.
4
M. Nomura et al., KEK Report 95-15 ~1996!, KEK Report 98-4 ~1998!, J.
Synchrotron Radiat. 5, 851 ~1998!.
5J. J. Rehr and R. C. Albers, Phys. Rev. B 41, 8139 ~1996!.
6A. Fujimori, Phys. Rev. B 28, 4489 ~1983!.
7See Ref. 3 and Y. Iijima et al., Jpn. J. Appl. Phys., Part 1 32, 5080 ~1993!.
8A. Bianconi et al., Phys. Rev. B , 806 ~1987!.
9Y. Baer and Ch. Zurcher, Phys. Rev. Lett. 39, 956 ~1977!.
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