Harnessing thorium(IV) as a catalyst: RNA and phosphate diester cleavage by a thorium(IV) macrocyclic complex

Article abstract of DOI:10.1039/b005866l

A robust macrocyclic tetraamide complex of Th(IV) binds phosphate diesters and promotes cleavage of RNA and phosphate diesters at 37 °C in the pH range 5.00-7.90.

Full text of DOI:10.1039/b005866l

Harnessing thorium(IV) as a catalyst: RNA and phosphate diester cleavage by
a thorium(IV) macrocyclic complex†
Congfang Wang, Seema Choudhary, Claire B. Vink, Elizabeth A. Secord and Janet R. Morrow*
Chemistry Department, State University of New York at Buffalo, Amherst, NY 14260-3000, USA
Received (in Cambridge, UK) 20th July 2000, Accepted 8th November 2000
First published as an Advance Article on the web
A robust macrocyclic tetraamide complex of Th(IV) binds
phosphate diesters and promotes cleavage of RNA and
phosphate diesters at 37 °C in the pH range 5.00–7.90.
involves ring inversion and cooperative pendent group rotation
which interconverts the two carbons of the macrocyclic ring, the
two sets of lower ring protons, the two sets of upper ring protons
and the methylene protons in analogous lanthanide(III) com-
plexes.13,14 Consistent with such a fluxional process occurring,
the 13C NMR spectrum of Th(1)4+ in D2O at 22 °C has only
three resonances‡ suggesting that, on the 13C NMR timescale,
the ethylene carbons are in fast exchange as has been observed
for La(1)3+.13
Tetravalent cations such as Ce(IV), Th(IV) and Zr(IV) are among
the most active catalysts for phosphate diester cleavage.1–7
There are several reports that these metal ions promote the
hydrolysis of phosphate diesters with poor leaving groups
including dimethylphosphate and DNA.1–5 Yet, the basis for
this exceptional reactivity is not well understood. Studies
reported to date on Ce(IV) or Th(IV) employ free metal ion or
weak metal ion complexes and most studies are carried out at
acidic pH to prevent precipitation of metal hydroxide com-
plexes.1–7 The complicated solution speciation of these systems
makes mechanistic analysis difficult. Few attempts have been
made to prepare well defined, catalytically active complexes of
these metal ions.4,6,7 Here, we report an unusual example of a
Th(IV) complex of a neutral macrocycle which is highly
resistant to dissociation in aqueous solutions. The complex
efficiently promotes RNA cleavage and transesterification of a
phosphate diester at neutral pH. Rate constants for RNA
cleavage compare favorably to other metal ion macrocyclic
complex catalysts including those containing lanthanide
ions.8–13
The Th(1)4+ complex is resistant to dissociation in aqueous
solution. Dissociation of Th(1)4+ was studied in the presence of
excess Cu(II) by monitoring formation of the Cu(1)2+ complex
by UV–VIS spectroscopy.13 No dissociation of the complex
(1.00 mM) was observed at pH 6.00 in a buffered solution in the
presence of a 10-fold excess of Cu(II) over a period of one week
at 37 °C. Remarkably, incubation of the Th(1)4+ complex in the
presence of an equivalent of EDTA at 37 °C and pH 6.00 did not
induce dissociation of the complex over a period of a week. This
inertness to metal ion release is reminiscent of the Eu(1)3+
complex which did not release Eu3+ over a period of 6 weeks
under similar conditions.13 That this resistance to dissociation is
a kinetic effect and not due to an exceptionally high formation
constant for Th(1)4+ relative to Cu(1)2+ is suggested by
competition experiments containing free 1, Th(NO3)4 and
Cu(NO3)2. When 1 (1.00 mM) was incubated simultaneously
with Th(NO3)4 (1.00 mM) and Cu(NO3)2 (10.0 mM) at pH 6.00,
37 °C, all of 1 was complexed as Cu(1)2+.
The ability to bind phosphate diesters is an integral part of
catalysis; lanthanide(III) macrocyclic complexes that do not
bind to phosphate diesters are not catalysts for cleavage.10,13
Binding was studied in solutions containing 5.00 mM diethyl-
phosphate and up to 40 mM Th(1)4+ at pH 6.90, 21 °C with 0.10
M tetramethylammonium chloride, 5.0 mM triethylphosphate
as a reference and 0.100 M Hepes buffer. Two 31P NMR
resonances were observed for free (0.81 ppm) and bound
phosphate diester (24.61 ppm), consistent with slow exchange
kinetics on the NMR timescale. Fitting of the binding isotherm
obtained by plotting the fraction of bound phosphate diester vs.
concentration of free Th(1)4+ gave a binding constant of 17
M21. Th(1)4+ also bound the RNA analog 2 under similar
conditions, but only a few percent was bound at pH 6.90 in the
presence of 15 mM Th(1)4+. In contrast, under similar
conditions, diethylphosphate binding to Eu(1)3+ and La(1)3+ is
too weak to determine binding constants and dissociation of
diethylphosphate from La(1)3+ is rapid on the 31P NMR
timescale.13 The stronger binding and slower exchange rates of
Th(1)4+ are consistent with the higher charge of this actinide
complex. These results are consistent with previous studies
showing that ligand exchange rates for Th(IV) are slower than
those of trivalent cations.15
The tetraamide macrocycle 1 binds to 12 members of the
lanthanide(III) series with Sm(III) and Eu(III) being the most
strongly bound.12 Given that the ionic radius and coordination
geometry of Th(IV) complexes are similar to those of the middle
lanthanide(III) ions, 1 was studied as a ligand for Th(IV).
Th(1)(NO3)4 was prepared in dry methanol from the free base
form of 1 and Th(NO3)4.‡ NMR studies were carried out to
characterize Th(1)4+ in solution. The six broad 1H NMR
resonances observed at room temperature in D2O sharpened but
1
remained slightly broadened at 4 °C (ESI†).§ The H NMR
spectrum is analogous to that observed for La(1)3+ which has 1
coordinated as an octadentate ligand with donor groups
coordinated in a square antiprismatic arrangement.¶ The two
sets of amide pendent group methylene protons appear as two
doublets, the two sets of equatorial ethylene protons give rise to
two doublets and the two sets of axial ethylene protons give rise
to two triplets. Unfortunately, it was not possible to study the
fluxional process further for Th(1)4+ due to ligand decomposi-
tion at high temperatures and the insolubility of the complex in
solvents other than water. However, this fluxional process likely
1
† Electronic supplementary information (ESI) available: H NMR spectra
for [Th(1)](NO3)4 in D2O at 21 and 4 °C. See http://www.rsc.org/suppdata/
cc/b0/b005866l
Although exchange of bound and free phosphate diester is
slow on the NMR timescale, ligand exchange is not likely the
DOI: 10.1039/b005866l
Chem. Commun., 2000, 2509–2510
This journal is © The Royal Society of Chemistry 2000
2509

rate determining step in the cleavage studies described below.
Binding of Th(1)4+ (15 mM) to diethyl phosphate (10 mM), pH
6.90, 0.10 M Hepes buffer at 22 °C, is complete within the 90
s it takes to record the 31P NMR spectrum. In comparison,
cleavage of RNA analog 3 by 15 mM Th(1)4+ in 0.10 M Hepes
at 22 °C has a half-life of 4.5 min.
phosphate diesters. In conclusion, we show here that it is
feasible to prepare a Th(IV) complex of a neutral macrocycle
which is highly resistant to dissociation in neutral aqueous
solutions and is active in the cleavage of phosphate diesters and
RNA. We demonstrate that the catalytic power of a tetravalent
cation can be harnessed in a macrocyclic complex.
We are grateful to the American Chemical Society Petroleum
Research Fund (34358-AC) and the National Science Founda-
tion (CHE-9986332) for support of this work.
Th(1)4+ promotes transesterification of the phosphate diester
3 to form the cyclic phosphate diester and 4-nitrophenylate as
determined by use of 31P NMR and UV–VIS spectroscopy.∑ At
pH 7.30, 37 °C with 1.00 mM complex and 0.100 mM 3, the
reaction exhibited good first-order kinetics in 3 for greater than
four half-lives. Transesterification of 3 is first-order in Th(1)4+
complex in the concentration range 0.60–2.00 mM with a
second-order rate constant of 0.65 M21 s21. Addition of 10%
EDTA (based on complex concentration) to reaction solutions
did not reduce the pseudo-first-order rate constant, suggesting
that the reaction is not catalyzed by a small amount of free
Th(IV) ion. Transesterification of 3 by Th(1)4+ is essentially
independent of pH in the pH range 5.0–7.9. Phosphate diester
cleavage by metal ion complexes is typically pH dependent
owing to the formation of metal hydroxide complexes.6,8–11
That cleavage of 3 by Th(1)3+ is not pH dependent suggests that
the speciation of the Th(IV) complex does not change in this pH
range. Pseudo-first-order rate constants for Th(1)4+ and analo-
gous Ln(III) complexes are listed in Table 1. Th(1)4+ is 40-times
more active than La(1)3+, the most active lanthanide complex in
the series. In addition, Th(1)4+ promotes cleavage of adenylic
acid oligomers more rapidly than does La(1)3+ under similar
conditions.
Notes and references
‡ Equimolar amounts of TCMC and Th(NO3)4 were heated to reflux in dry
methanol for 1.5 h. The methanol was removed and the complex was
isolated as a white solid in 43% yield following precipitation twice from a
methanol–diethyl ether mixture. Anal. Calc. for C20H42N12O17Th [Th(1)-
(Et2O)](NO3)4: C, 25.16; H, 4.43; N, 17.60. Found: C, 25.02; H, 4.38; N,
17.27. FAB MS: m/z 818 [Th(1)(NO3)3+]. The 13C NMR spectrum of
Th(1)4+ in D2O showed resonances at 52.5, 57.3, and 177.3 ppm assigned
to carbons of the ethylene moiety, methylene and carbonyl carbons,
respectively.
§ Only one major diastereomeric form of the complex is observed by 1H or
13C NMR spectroscopy. In contrast, Eu(1)3+ has two diastereomers present
in solution and solid state (see refs. 13 and 14).
¶ In the solid state, [La(1)(CF3SO3)(EtOH)](CF3SO3)2 is a ten-coordinate
complex with an unusual 1,5,4 geometry while [Eu(1)(H2O)](CF3SO3)3 is
a nine-coordinate complex with a monocapped distorted square antiprism
geometry.13
∑ The kinetics of transesterification of 3 were monitored by use of UV–VIS
spectroscopy. At pH values > 6.0, the production of 4-nitrophenylate was
monitored at 412 nm. At pH values < 6.0, the decrease in the absorbance of
3 at 300 nm was monitored.
How does Th(1)4+ promote RNA cleavage and why is the
complex more efficient than analogous Ln(1)3+ complexes?
Given that the ionic radii of Th(IV) and Eu(III) are nearly
identical, it is likely that Th(1)4+ is nine-coordinate with a single
site for the binding substrate, analogous to Eu(1)3+.13 Thus,
Th(1)4+ activates the phosphate diester to cleavage most
probably through interaction at a single coordination site. The
greater reactivity of the Th(IV) complex compared to its
lanthanide(III) analogs is attributed to the greater Lewis acidity
of the Th(IV) center as suggested by its strong interaction with
1 J. Sumaoka, Y. Azuma and M. Komiyama, Chem. Eur. J., 1998, 4,
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2 R. A. Moss and K. G. Ragunathan, Chem. Commun., 1998, 1871.
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119, 9323.
Table 1 Pseudo-first-order rate constants for cleavage of RNA and
phosphate diester 3 by thorium(IV) and lanthanide(III) complexes of 1 at
37 °C
8 B. N. Trawick, A. T. Daniher and J. K. Bashkin, Chem. Rev., 1998, 98,
939.
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2000.
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Chem., Int. Ed., 1994, 33, 773; (b) S. Amin, D. A. Voss, Jr., W. DeW.
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Complex
Substratea,b,c
kobs/1024 s21
Th(1)4+
Th(1)4+
La(1)3+
La(1)3+
Eu(1)3+
Eu(1)3+
3
A10
3
A12–A18
3
A12–A18
7.5a
9.2
0.16
1.6
NRd
NRd
a For substrate 3 conditions were pH 7.3, 1.00 mM complex, 0.100 mM 3,
10 mM Hepes buffer, 100 mM NaNO3. b For A10 conditions were pH 7.4,
0.200 mM complex, 5 mM Hepes buffer, 0.013 mM A10 (adenosine
concentration). c For A12–A18 conditions were pH 7.6, 0.200 mM complex,
5 mM Hepes buffer, 0.08 mM A12–A18 (adenosine concentration).13 d No
cleavage observed under the conditions given above.13
15 A. Fratiello, R. E. Lee and R. E. Schuster, Inorg. Chem., 1970, 9,
391.
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Chem. Commun., 2000, 2509–2510
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