A R T I C L E S
6-
tions of infectious prions in blood and cerebrospinal fluid.14
Selective interaction of POMs with basic fibroblast growth
factor, a globular single-chain heparin-binding polypeptide
synthesized by different cell types, has also been demonstrated.15
Scheme 1. Chemical Structures of HPNP and [Mo7O24
]
The broad spectrum of biological activity combined with
generally low cytotoxicity makes POMs attractive as potential
therapeutic agents. However, considering the large number of
biologically active POMs, there is very limited information on
their mode of interaction on the molecular basis. It has been
generally accepted that noncovalent binding influenced by the
size, shape, and charge density of polyoxometalates is the main
factor governing their biological activity, but on the other hand,
very little is known about the reactivity of POMs toward
biological molecules and their building blocks.
In our quest to gain more insight into the origins of the biological
activity of this important class of compounds, we have focused
our attention on the polyoxomolybdate [Mo7O24]6-, also known
as heptamolybdate. The well-established solution chemistry of
polyoxomolybdates forms a good basis for studying interactions
with biological model systems on a molecular level. Moreover,
the first studies on the antitumor activity of POMs in general,
revealed that the in ViVo antitumoral activity of the polyoxomo-
lybdate [Mo7O24]6-, on several types of tumor cells is comparable
to that of some commercial drugs.16 The most recent studies have
shown that [Mo7O24]6- exhibits potent antitumor activity against
pancreatic cancer cells which are extremely resistant toward most
of the known therapeutic agents.18-20
ters.29,30 The discovery of the phosphodiesterase activity of
[Mo7O24]6- represents the first example of a phosphodiester bond
cleavage promoted by a highly negatively charged polyoxo-
metalate cluster. This encouraged us to further explore the
reactivity of [Mo7O24]6- toward 2-hydroxypropyl-4-nitrophenyl
phosphate (HPNP), which is commonly used as an RNA model
system by means of 1H, 31P, and 95Mo NMR, UV-vis, Raman,
and Mo K-edge extended X-ray absorption fine structure
(EXAFS) spectroscopy. EXAFS has been proven to be a very
valuable technique in studying structures of polyoxometalates,
since it can provide information about the coordination sphere
of metal ions both in the solid state and in solution. A recent
paper by Balula et al. gives a comprehensive overview of a
number of studies which have been carried out in the past 10-15
years.31
In this study we demonstrate that contrary to the hydrolysis
of bis(p-nitrophenyl)phosphate (BNPP), the transesterification
of HPNP can be achieved under catalytic conditions, and we
give a full account on the mechanism of this novel reaction
(Scheme 1).
Several studies have revealed that molybdate binds to
nucleotides exclusively via the phosphate group, leading to the
formation of a pentamolybdodiphosphate type of structure.21-25
At high temperatures the slight hydrolysis of mononucleotides
in the presence of molybdate has been observed.26 Polyoxo-
molybdates have also been shown to catalyze the hydrolysis of
the labile “high energy” phosphoanhydride bonds in ATP.27,28
Various studies implicate the interaction of molybdate with
Experimental Section
Materials. Disodium p-nitrophenyl phosphate and 1,2-epoxypro-
pane were obtained from Aldrich Ltd. Aqueous ammonia and
barium hydroxide (Acros) were used to adjust pH in the synthesis
of HPNP. The pH of the solutions for the NMR studies was adjusted
with D2SO4 and NaOD, both from Acros. D2O with 0.05 wt %
3-(trimethylsilyl) propionic acid (Acros) as an internal standard was
used as a solvent. HPNP was prepared from p-nitrophenyl phosphate
and 1,2-epoxypropane according to a published procedure.32
NMR Spectroscopy. 1H, 31P, and 13C NMR spectra were
recorded on a Bruker Avance 300 spectrometer and on a Bruker
Avance 400 spectrometer. Trimethyl phosphate was used as a 0
ppm 31P reference. 95Mo NMR spectra were recorded on a Bruker
Avance 600 (39 MHz) spectrometer.
Raman and UV-Visible Spectroscopy. FT-Raman spectra were
recorded on a Bruker IFS-66 with a FRA106 Raman module (Nd:
YAG laser). Typically 200 scans were taken with a resolution of 4
cm-1. UV-vis absorption spectra have been measured on a Varian
Cary 5000 spectrophotometer.
EXAFS Spectroscopy. EXAFS measurements were performed
in transmission mode using a Si(111) double crystal monochromator
on the Dutch-Belgian Beamline (DUBBLE, BM26A) at the
European Synchrotron Radiation Facility (ESRF, Grenoble, France).
Preliminary measurements were done during project 26 01 743
between March 6th and 10th, 2006. The ESRF storage ring was
then operating under uniform filling mode. Energy resolution dE/E
phosphate groups in ATP as one of the key factors in
25,27,28
understanding the antitumor activity of [Mo7O24]6-
.
In
our initial studies we examined the interactions between
heptamolybdate and commonly used DNA model phosphodies-
(14) Lee, I. S.; Long, J. R.; Prusiner, S. B.; Safar, J. G. J. Am. Chem. Soc.
2005, 127, 13802.
(15) Wu, Q.; Wang, J.; Zhang, L.; Hong, A.; Ren, J. S. Angew. Chem.,
Int. Ed. 2005, 44, 4048.
(16) Fujita, H.; Fujita, T.; Sakurai, T.; Seto, Y. Chemotherapy 1992, 40,
173.
(17) Yamase, T.; Fujita, H.; Fukushima, K. Inorg. Chim. Acta 1988, 151,
15.
(18) Ogata, A.; Mitsui, S.; Yanagie, H.; Kasano, H.; Hisa, T.; Yamase, T.;
Eriguchi, M. Biomed. Pharmacother. 2005, 59, 240.
(19) Yanagie, H.; Ogata, A.; Mitsui, S.; Hisa, T.; Yamase, T.; Eriguchi,
M. Biomed. Pharmacother. 2006, 60, 349.
(20) Ogata, A.; Yanagie, H.; Ishikawa, E.; Morishita, Y,; Mitsui, S.;
Yamashita, A.; Hasumi, K.; Takamoto, S.; Yamase, T.; Eriguchi, M.
Br. J. Cancer, 2008, 98, 399.
(21) Katsoulis, D. E.; Lambriandou, A. N.; Pope, M. T. Inorg. Chim. Acta
1980, 46, L55.
(22) Geraldes, C. F. G. C.; Castro, M. C. C. A. J. Inorg. Biochem. 1986,
28, 319.
(23) Piperaki, P.; Katsaros, N.; Katakis, D. Inorg. Chim. Acta 1982, 67,
37.
(24) Hill, L. M. R.; George, G. N.; Duhm-Klair, A. K.; Young, C. G.
J. Inorg. Biochem. 2002, 88, 274.
(29) Cartuyvels, E.; Absillis, G.; Parac-Vogt, T. N. Chem. Commun. 2008,
85.
(25) Kwak, W.; Pope, M. T.; Scully, T. F. J. Am. Chem. Soc. 1975, 97,
5735.
(30) Van Lokeren, L.; Cartuyvels, E.; Absillis, G.; Willem, R.; Parac-Vogt,
T. N. Chem. Commun. 2008, 2774.
(26) Cartuyvels, E.; Van Hecke, K.; Van Meervelt, L.; Go¨rller-Walrand,
C.; Parac-Vogt, T. N. J. Inorg. Biochem. 2008, 102, 1589.
(27) Weil-Malherbe, H.; Green, R. H. Biochem. J. 1951, 49, 3286.
(28) Ishikawa, E.; Yamase, T. J. Inorg. Biochem. 2006, 100, 344.
(31) Balula, M. S. S.; Santos, I. C. M. S.; Gamelas, J. A. F.; Cavaleiro,
A. M. V.; Binsted, N.; Schlindwein, W. Eur. J. Inorg. Chem. 2007,
1027, and references therein.
(32) Brown, D. M.; Usher, D. A. J. Chem. Soc. 1965, 6558.
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