Activation of acyl phosphate monoesters by lanthanide ions: Enhanced reactivity of benzoyl methyl phosphate

Article abstract of DOI:10.1021/ja016600v

Acyl phosphate monoesters are intermediates in many biochemical acylation reactions, such as those involving aminoacyl adenylates. Benzoyl methyl phosphate, a typical acyl phosphate monoester, is slowly hydrolyzed in neutral solutions but reacts rapidly with amines. Since biochemical processes of acyl phosphate monoesters involve accelerated reactions with oxygen-centered nucleophiles, we sought catalysts for hydrolysis and methanolysis of benzoyl methyl phosphate to mimic the biochemical outcome. Lanthanide ions are particularly effective catalysts, accelerating reactions much more than comparable levels of magnesium ion. Detailed kinetic analysis of the hydrolysis reactions reveals formation of a 1:1 complex, followed by rapid reaction with a nucleophile. The hydroxide-dependent hydrolysis rate in the europium complex is about 10⁵ times that of free substrate with hydroxide. A mechanism that accounts for the data and observed behavior involves bidentate coordination of the metal ion by the acyl phosphate through phosphate and carbonyl oxygens, lowering the energy of the tetrahedral addition intermediate and the associated transition states. The dependence of the metal ion catalyzed process on the concentration of hydroxide ion is consistent with coordinated hydroxide acting as a nucleophile. The reaction of benzoyl methyl phosphate with methanol to form methyl benzoate and methyl phosphate is 30 000 times more rapid in the presence of 0.0001 M lanthanum triflate (in the absence of the metal ion kobs = 2.1 × 10^-7 s^-1, at 25°C). Thus, the combination of acyl phosphate esters and lanthanide salts appears to be a promising method for biomimetic acylation of hydroxyl groups.

Full text of DOI:10.1021/ja016600v

Published on Web 03/06/2002
Activation of Acyl Phosphate Monoesters by Lanthanide
Ions: Enhanced Reactivity of Benzoyl Methyl Phosphate
Ronald Kluger* and Lisa L. Cameron
Contribution from the DaVenport Chemistry Research Building, Department of Chemistry,
UniVersity of Toronto, Toronto, Ontario, Canada M5S 3H6
Received July 12, 2001
Abstract: Acyl phosphate monoesters are intermediates in many biochemical acylation reactions, such as
those involving aminoacyl adenylates. Benzoyl methyl phosphate, a typical acyl phosphate monoester, is
slowly hydrolyzed in neutral solutions but reacts rapidly with amines. Since biochemical processes of acyl
phosphate monoesters involve accelerated reactions with oxygen-centered nucleophiles, we sought catalysts
for hydrolysis and methanolysis of benzoyl methyl phosphate to mimic the biochemical outcome. Lanthanide
ions are particularly effective catalysts, accelerating reactions much more than comparable levels of
magnesium ion. Detailed kinetic analysis of the hydrolysis reactions reveals formation of a 1:1 complex,
followed by rapid reaction with a nucleophile. The hydroxide-dependent hydrolysis rate in the europium
complex is about 105 times that of free substrate with hydroxide. A mechanism that accounts for the data
and observed behavior involves bidentate coordination of the metal ion by the acyl phosphate through
phosphate and carbonyl oxygens, lowering the energy of the tetrahedral addition intermediate and the
associated transition states. The dependence of the metal ion catalyzed process on the concentration of
hydroxide ion is consistent with coordinated hydroxide acting as a nucleophile. The reaction of benzoyl
methyl phosphate with methanol to form methyl benzoate and methyl phosphate is 30 000 times more
rapid in the presence of 0.0001 M lanthanum triflate (in the absence of the metal ion kobs ) 2.1 × 10-7 s-1
,
at 25 °C). Thus, the combination of acyl phosphate esters and lanthanide salts appears to be a promising
method for biomimetic acylation of hydroxyl groups.
Metabolic acyl transfer reactions require initial enzymatic
catalysis to convert carboxylic acids to activated acylating
agents. An important example is ribosomal protein synthesis,
in which amino acids are converted to anhydrides with AMP
from ATP prior to transferring the aminoacyl moiety to a
terminal hydroxyl of tRNA specified by its anticodon. It
occurred to us that acyl phosphate monoesters might also be
biomimetic acylating agents. They hydrolyze very slowly in
neutral solutions but react rapidly with amines to form amides.1,2
It would be especially useful also to have these compounds
acylate hydroxyl groups, emulating the reaction that leads to
the formation of aminoacyl tRNAs. However, since the com-
pounds are inherently unreactive toward hydroxyl groups, we
needed to find catalysts for such a process. Since hydrolysis
involves the attack of an oxygen-centered nucleophile, we
initially examined pathways of ionic catalysis.
carbon, a larger catalyst should connect these sites more easily.
Recently, Neverov and Brown reported that lanthanide ions
accelerate reactions of other activated acyl derivatives with
methanol.4-6 Lanthanide ions have also been reported by
Komiyama and co-workers to promote the hydrolysis of
peptides,7,8 alkyl esters, and amides.9 These are promising
indications for the potential use of lanthanides in enhancing the
reactivity of acyl compounds in general toward oxygen-centered
nucleophiles. Therefore, we examined the effects of lanthanide
ions upon the hydrolysis of the acyl phosphate monoester
benzoyl methyl phosphate (BMP) and the reaction of BMP with
methanol. Our results indicate that lanthanide salts dramatically
increase the reactivity of acyl phosphates toward oxygen-
centered nucleophiles.
Experimental Section
Materials and Methods. Commercial reagents were used as
received. NMR spectra were recorded at 300 MHz for 1H, 75 MHz for
13C, and 121.5 MHz for 31P. Benzoyl methyl phosphate (BMP) was
We have previously reported that some divalent cations
increase the rate of hydrolysis of acyl phosphate monoesters
but the acceleration is not large.3 We reasoned that a more
powerful, water-stable Lewis acid could be a better catalyst. In
particular, since the primary coordination site of metal ions is
the phosphate oxygen atoms and the reaction site is the carbonyl
(4) Neverov, A. A.; Brown, R. S. Can. J. Chem. 2000, 78, 1247-1250.
(5) Neverov, A. A.; Montoya-Pelaez, P. J.; Brown, R. S. J. Am. Chem. Soc.
2001, 123, 210-217.
(6) Neverov, A. A.; McDonald, T.; Gibson, G.; Brown, R. S. Can. J. Chem.
2001, 79, 1704-1710.
(7) Yashiro, M.; Takarada, T.; Miyama, S.; Komiyama, M. J. Chem. Soc.,
Chem. Commun. 1994, 1757-1758.
* Corresponding author: e-mail rkluger@chem.utoronto.ca.
(1) Di Sabato, G.; Jencks, W. P. J. Am. Chem. Soc. 1961, 83, 4393-4400.
(2) Di Sabato, G.; Jencks, W. P. J. Am. Chem. Soc. 1961, 83, 4400-4405.
(3) Kluger, R.; Loo, R. W.; Mazza, V. J. Am. Chem. Soc. 1997, 119, 12089-
12094.
(8) Takarada, T.; Yashiro, M.; Komiyama, M. Chem. Eur. J. 2000, 6, 3906-
3913.
(9) Takarada, T.; Takahashi, R.; Yashiro, M.; Komiyama, M. J. Phys. Org.
Chem. 1998, 11, 41-46.
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J. AM. CHEM. SOC. 2002, 124, 3303-3308
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Kluger and Cameron
prepared by the general procedure reported for acyl phosphate esters.10
Sodium dimethyl phosphate was prepared by the reaction of sodium
iodide in acetone with trimethyl phosphate.10,11 Benzoyl chloride (14.0
g, 0.1 mol) and sodium dimethyl phosphate (14.8 g, 0.1 mol) were
suspended in dry tetrahydrofuran (60 mL, under nitrogen) and refluxed
for 10 h. The solution was cooled and filtered to remove sodium
chloride. Removal of the solvent left benzoyl dimethyl phosphate (1)
as a colorless oil (19 g, 96%). H NMR (CDCl3) δ 3.89 (6H, d, JP-H
) 11.7 Hz), 7.40 (2H, tt, J1 ) 7.24 Hz, J2 ) 1.3 Hz), 7.57 (1H, tt, J1
) 7.4 Hz, J2 ) 1.3 Hz), 7.93-8.0 (m).
range 0.1-0.8 M HCl. The rate coefficients for BMP hydrolysis at
high acid concentrations are affected by the ionic strength of the
solution, so the ionic strength was maintained to the equivalent of 1 M
by addition of potassium chloride.
The pH dependence of the lanthanide ion-catalyzed hydrolysis of
BMP was determined by extrapolation of buffer plots to zero
concentration for four pH values. MES buffer was used at pH 5.5, 6,
and 6.7, and EPPS buffer was used at pH 7. At higher pH values, water-
insoluble lanthanide hydroxide species form, interfering with kinetic
determinations.4,12-17 The dependence of rate on hydroxide of the
lanthanide free reaction was determined at concentrations of added
hydroxide ion ranging from 0.002 to 0.008 M in the absence of buffer.
Ionic strength was maintained at 1 M (KCl) for these studies.
1
31P NMR (CDCl3) δ -4.50. ESI MS calculated 215.1, found (m/z)
215.
Mass Spectral Analysis. BMP is a mixed anhydride and the same
products result from addition of water to either the phosphoryl or
carbonyl group. Acyl phosphate monoesters in general are much more
reactive at their carbonyl group.1,18 However, since the lanthanide ions
in principle could accelerate reactions at either site, we conducted a
lanthanide-catalyzed hydrolysis of BMP using 18O-enriched water. A
stock solution of 0.02 M EPPS and 0.01 M EuCl3 was prepared with
doubly distilled, deionized water. One milliliter of this solution was
mixed with 1 mL of 10 atom % 18O-enriched water in which 25 mg of
BMP had been dissolved, to give a final solution that is 0.01 M in
EPPS buffer, 0.005 M in EuCl3, and about 0.05 M in BMP, with the
water containing 5 atom % 18O. The reaction was stirred at room
temperature for 24 h, acidified, and then extracted three times with
ether. The solution was dried over magnesium sulfate, filtered, and
concentrated to give a white powder. The same procedure was repeated
with doubly distilled deionized water in place of the 10 atom % 18O
water to provide a standard. The masses and relative abundance of the
resulting benzoic acid products were analyzed by mass spectrometric
measurements of the intensities of parent ions. We note that the
exchange of water into benzoic acid is an extremely slow process, even
in strongly acidic solutions at high temperatures,19,20 so that the isotopic
oxygen incorporation we observe in benzoic acid does not arise from
exchange into this product.
Benzoyl dimethyl phosphate was dissolved in acetone and an acetone
solution of an equimolar amount of sodium iodide was added. The
solution stood overnight, leading to crystallization of the product, the
sodium salt of benzoyl methyl phosphate (BMP) in 71% yield, mp >
1
260 °C. H NMR (D2O) δ 3.55 (3H, d, JP-H ) 11.6 Hz), 7.27 (1H, t,
J1 ) 7.9 Hz), 7.45 (1H, br d, J ) 8.8 Hz), 7.74-7.82 (2H, m).
31P NMR (D2O) δ -4.47. MS (negative FAB) calculated 229.1,
found (m/z) 229.
Kinetic Studies. The hydrolysis of benzoyl methyl phosphate (BMP)
to give methyl phosphate and benzoic acid parallels the change in
absorbance at 240 nm due to the difference in absorbance of BMP and
benzoate. All rates were determined at 25 °C, pH 7 (0.01 M EPPS
buffer) unless noted otherwise. Fresh solutions of buffer and inorganic
materials were prepared and the pH was adjusted at 25 °C. Before each
kinetic run, the buffer and salt solutions were transferred into
spectrometer cells and brought to a volume of 2.85 mL by addition of
water. The cells were then kept in a water-jacketed compartment at 25
°C for a minimum of 30 min. A stock solution (0.001 M) of BMP was
prepared immediately before each kinetic run and 0.15 mL was
transferred to give a final BMP concentration of 5 × 10-5 M and a
final reaction volume of 3 mL. At least three kinetic runs were
performed for each set of experimental conditions. The data were fit
to the equation for a single-exponential decay to determine the observed
first-order rate coefficients.
The dependence of rate on lanthanide concentration was determined
over a range 0.001-0.008 M for neodymium and europium triflates,
0.001-0.009 M for europium chloride, and 0.001-0.010 M for
ytterbium triflate. (The concentration was limited by the solubility of
the individual lanthanides in water at pH 7.) A solution 0.001 M in
lanthanide salt was used for studying rate variation due to changes in
buffer, salts, and acidity. The effects of magnesium ion concentration
on the rate of BMP hydrolysis were determined by the method of initial
rates due to the very slow hydrolysis. The second-order rate constant
(kMg) was determined from the linear dependence of the observed first-
order rate coefficients on magnesium ion concentrations between 0.05
and 0.4 M. The magnesium ion dependence was studied under
conditions where the ionic strength was allowed to vary with the
concentration of magnesium and also with ionic strength maintained
at I ) 1 with potassium chloride. The second-order rate constant for
specific acid catalysis was also determined by initial rates over the
Methanolysis. The rates of methanolysis of BMP were determined
by monitoring the decrease in UV absorbance at 240 nm, 25 °C. For
the methanol reactions, all stock solutions were prepared in 99.8%
methanol. Apparent first-order rate coefficients were determined with
the assumptions of initial rates and pseudo-first-order conditions.
Lanthanide salts were added to the methanolic solutions and the rates
were observed as in the reactions in water. Lanthanum triflate stock
solution (0.001 M) was used. The solution was prepared simply by
dissolving the lanthanum salt in methanol. The formation of methyl
benzoate was confirmed by HPLC analysis of the reaction mixtures
with a co-injection of authentic methyl benzoate. UV scans of the
product mixtures show a shift of λmax to 227 nm, which is the λmax of
methyl benzoate in methanol.
Methanolysis was followed by high-performance liquid chromatog-
raphy (HPLC) analysis, detected at 230 nm on a C18 reverse-phase
column, eluting with a 90:10 mixture (v/v) of water and acetonitrile.
Methanolic solutions of 0.01 M La(OTf)3 and 0.1 M triethylamine were
(12) Suzuki, Y.; Nagayama, T.; Sekine, M.; Mizuno, A.; Yamaguchi, K. J. Less-
Common Met. 1986, 126, 351-356.
(13) Butcher, W. W.; Westheimer, F. H. J. Am. Chem. Soc. 1954, 76, 2420-
2424.
(14) Gomez-Tagle, P.; Yatsimirsky, A. K. J. Chem. Soc., Dalton Trans. 1998,
2957-2960.
(15) Jurek, P. E.; Jurek, A. M.; Martell, A. E. Inorg. Chem. 2000, 39, 1016-
1020.
(16) Morrow, J. R.; Buttrey, L. A.; Berback, K. A. Inorg. Chem. 1992, 31, 16-
20.
(17) Briggs, P. J.; Satchell, D. P. N.; White, G. F. J. Chem. Soc. B 1970, 1008-
1012.
(18) Bentley, R. J. Am. Chem. Soc. 1949, 71, 2765-2767.
(19) Bender, M. L. J. Am. Chem. Soc. 1951, 73, 1626.
(20) Bunton, C. A.; James, D. H.; Senior, J. B. J. Chem. Soc. 1960, 3364-
3367.
(10) Zervas, L.; Dilaris, I. J. Am. Chem. Soc. 1955, 77, 5354-5357.
(11) Kluger, R.; Grant, A. S.; Bearne, S. L.; Trachsel, M. R. J. Org. Chem.
1990, 55, 2864-2868.
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Acyl Phosphate Monoester Activation by Lanthanide Ions
A R T I C L E S
Figure 1. Saturating dependence of the observed first-order rate coefficient
for hydrolysis of BMP on the concentration of neodymium ion, 25 °C, pH
7, 0.01 M EPPS.
Figure 3. Dependence of the observed first-order rate coefficient for
hydrolysis of BMP on the concentration of ytterbium ion, 25 °C, pH 7,
0.01 M EPPS.
Scheme 1
Table 1. Rate and Equilibrium Constants Associated with the
Lanthanide-Catalyzed Hydrolysis of BMPa
-1
catalyst
k2, s-1
K , M
k2K , M-1 s-1
1
1
Nd(OTf)3
Eu(OTf)3
EuCl3
Yb(OTf)3
MgCl2
(4.0 ( 0.1) × 10-3
(4.3 ( 0.4) × 10-3
(6.1 ( 0.2) × 10-3
(2.9 ( 0.1) × 10-3
(2.2 ( 0.2) × 102
(3.7 ( 1) × 102
(2.2 ( 0.2) × 102
(8.0 ( 0.6) × 102
0.88
1.6
1.3
2.3
8.1 × 10-6 b
a Based on a Michaelis-Menten mechanism, 25 °C, pH 7, 0.01 M EPPS
buffer. b Second-order rate constant determined from the linear correlation
of the observed first-order rate coefficient for BMP hydrolysis and
magnesium concentration.
Figure 2. Effects of europium chloride (b) and europium triflate (O)
concentration on the observed first-order rate coefficient for hydrolysis of
BMP, 25 °C, pH 7, 10 mM EPPS buffer.
shows no sign of curvature at 0.4 M metal. This shows that the
interaction of magnesium ions with BMP is very weak compared
to the interaction with lanthanide ions, which saturates at 0.01
M metal. Table 1 contains a summary of the derived rate
constants under conditions of lanthanide ion catalysis, as well
as the measured second-order rate constant for magnesium
catalysis:
prepared. Increasing amounts of these solutions were added to methanol
to give a final volume of 0.5 mL. Then 0.5 mL of 0.001 M BMP in
methanol was added to the reaction mixture and immediately subjected
to HPLC analysis.
Results
Metal Ion Catalysis. The addition of lanthanide salts
significantly accelerates the hydrolysis of BMP (which is
pseudo-first-order in BMP). The magnitude of the observed first-
order rate coefficient is dependent on the lanthanide ion at low
concentrations but the dependence decreases at higher metal
ion concentrations (Figures 1-3). The data fit the general
Michaelis-Menten catalytic scheme (Scheme 1), where lan-
thanide ion is a catalyst, permitting separation of the association
(K1) and catalysis (k2) parameters. The reaction conducted in
18O-enriched water resulted in incorporation of the of 18O into
the resulting benzoic acid as evidenced by enhancement of the
parent ion + 2 peak in the mass spectrum of benzoic acid (m/z
124) to the extent of isotopic enrichment of the water. Therefore,
the lanthanide ions accelerate C-O cleavage, indicating that
these ions promote reaction at the carboxyl group of BMP.
The derived values for the catalytic rate constants, k2, and
association constants for the substrate lanthanide complexes,
K1, were obtained by a computer-generated least-squares fit of
kobs to eq 1. The second-order rate constant for magnesium-
promoted BMP hydrolysis is 8.1 × 10-6 M-1 s-1. The
magnesium dependence is independent of ionic strength and
kobs ) K1k2[Ln]/(1 + K1[Ln])
(1)
Salt Effects. Increasing concentrations of the EPPS buffer
do not promote the rate of hydrolysis of benzoyl methyl
phosphate in the presence of metal ions. In fact, we observe
that increased EPPS slows the rate of the reaction with inhibition
saturating at approximately 0.5 M EPPS. Sensitivity of metal-
catalyzed reactions to buffer concentration has been observed
for other reactions involving coordination to phosphorus oxy-
gens.16,21
The rate of the lanthanide-promoted hydrolysis of BMP is
subject to a nonspecific depressing salt effect. The effect of
mono- and divalent cations is a linear function of Debye-
Hu¨ckel ionic strength parameters from a plot of log kobs values
vs the Debye-Hu¨ckel term, xI/(1 + xI) at constant lanthanide
concentration (see Figure 4 and eq 2).22 This is consistent with
the expectation that the catalytically active species (lanthanide
(21) Gellman, S. H.; Petter, R.; Breslow, R. J. Am. Chem. Soc. 1986, 108, 2388-
2394.
(22) Roigk, A.; Hettich, R.; Schneider, H.-J. Inorg. Chem. 1998, 37, 751-756.
9
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Figure 4. Observed first-order rate coefficients as a function of ionic
strength: 1 × 10-3 M EuCl3 (0, Li+; 9, Mg2+; b, Ca2+; 25 °C, pH 7,
0.010 M EPPS).
Figure 5. Dependence on pH (indicating measured value or 14 + log
[hydroxide]) of the observed first-order rate coefficients for the hydrolysis
of BMP, 25 °C. (9) 1 × 10-3 M europium, (b) saturating europium, (0)
no europium, µ ) 1 (KCl).30
chelate, see Discussion) dissociates in the presence of other ions,
as they provide electrostatic stabilization of the unassociated
species.
x
I
log k ) log k0 + m
(2)
(
)
x
1 +
I
Figure 6. HPL chromatograms for the reaction of BMP in methanol. The
peak for BMP is indicated by i, and the methanolysis product, methyl
benzoate, is indicated by ii. Conditions: 0.0005 M BMP in methanol with
0.005 M La(OTf)3, 25 °C.
Acid and Base Catalysis. The hydrolysis of BMP is subject
to specific hydronium ion catalysis with a second-order rate
constant of 3.1 × 10-6 M-1 s-1 (obtained from the linear
dependence of the observed first-order rate coefficients on the
concentration of added acid in solutions of 0.1 M HCl and
higher, at constant ionic strength, I ) 1). At these high acidities,
lanthanide ion does not promote the hydrolysis of BMP,
indicating that the proton is a superior catalyst under these
conditions. The hydrolysis in the absence of metal ions is
promoted by hydroxide, kOH ) 3.4 × 10-1 M-1 s-1. In the case
of the europium ion-promoted reaction, the hydroxide-dependent
reaction is much faster (kOH ) 1.9 × 104 M-1 s-1), with a
second-order rate constant that is 5 orders of magnitude greater
than that for the dissociated species (Figure 5).
Methanolysis. Since we undertook these studies with the
objective of using acyl phosphate esters for biomimetic acylation
of hydroxyl groups, we examined the effects of lanthanum
triflate in methanol on the formation of methyl benzoate from
BMP. The initial first-order rate coefficient for uncatalyzed
methanolysis of BMP at 25 °C is 2.1 (( 0.7) × 10-7 s-1. This
slow rate of methanolysis of BMP is dramatically accelerated
by lanthanide ions. We find that low concentrations of La(III)
have a very large catalytic effect. Addition of 0.0001 M
lanthanum triflate gives an observed initial first-order rate of
formation of methyl benzoate that is accelerated 30 000-fold
(kobs ) 7.7 (( 0.3) × 10-3 s-1). We observe greater accelera-
tions at higher lanthanide concentrations and in more basic
(23) Schneider, H. J.; Theis, I. J. Org. Chem. 1992, 57, 3066-3070.
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Acyl Phosphate Monoester Activation by Lanthanide Ions
A R T I C L E S
Scheme 2
Scheme 3
solutions, but the reactions are kinetically complex, (presumably
due to changes in speciation at different lanthanide concentra-
tions and sspH).4-6,24
The acyl phosphate dianions form bidendate complexes via the
carbonyl group and phosphate, producing six-membered chelate
rings whose formation is promoted by low solvent polarity. This
is consistent with our observation that increasing ionic strength
slows the reaction involving lanthanide ion. Since BMP is
monoanionic, the carbonyl will compete more effectively with
the phosphate as a coordinating site than in unesterified acyl
phosphate dianion derivatives. Furthermore, lanthanide ions are
superior to smaller divalent metal ions for this mode of
coordination, on the basis of size, charge, and oxophilicity. The
importance of the high charge-to-size ratio of the metal ions in
the catalysis of acyl phosphate monoester hydrolysis is illustrated
by the trend in association constants within the lanthanide series
(formation of the catalytically active complex increases with
the charge density of the lanthanides).
HPLC analysis of reaction solutions of BMP in methanol
permits direct observation of the formation of methyl benzoate,
clearly showing the large catalytic effect of lanthanide ions. In
the absence of lanthanide, the system does not detect a
significant amount of methyl ester present after 22 h. However,
in the presence of 5.0 × 10-3 M La(OTf)3 (Figure 6), after 15
min in methanol, more than half of the BMP is converted to
the methyl ester. When we increase the basicity of the solution
by the addition of triethylamine (0.001 M), the reaction is faster,
with 95% conversion of BMP to methyl benzoate at the initial
injection.
A further mode of activation that is likely to be provided by
the lanthanide ion is ionization of coordinated water, producing
a nucleophilic hydroxide species that is positioned within the
chelate for reaction with the carbonyl center (Scheme 3). In
the scheme we show only the ionizing water ligand in the
lanthanide hydration sphere. Although the pKa of water is
depressed by about 7 units upon binding to a lanthanide ion,28
it is still 1-2 units higher than would be necessary to have it
be 50% dissociated under the conditions of our measurements
(pH 7). Hydroxide ion would act as a Brønsted base, accepting
the proton of the lanthanide-bound water. This mechanism
accounts for both the first-order hydroxide dependence and the
acceleration provided by the lanthanide of the base-catalyzed
process.
Discussion
Lanthanide ions are powerful catalysts for the hydrolysis and
methanolysis of BMP in neutral solutions. Our detailed kinetic
studies focus on the hydrolysis reactions. The saturating
dependence of the observed first-order rate coefficients for
hydrolysis upon lanthanide concentration indicates that catalysis
occurs through initial formation of a complex between the ion
and BMP. Since metal ions normally coordinate at phosphate
oxygens, we could expect a similar coordination structure to
form between lanthanides and BMP. However, such coordina-
tion would not have a significant effect on the specific reactivity
of the carbonyl center. On the other hand, bidentate coordination
via the carbonyl and phosphate oxygens could lead to stabilized
tetrahedral intermediates from addition of water or hydroxide
ion to the carbonyl, as well as in the associated transition states
(Scheme 2). Such a pattern is analogous to those Sigel found
for association of acetyl phosphate and divalent metal ions.25-27
(25) Sigel, H.; Da Costa, C. P.; Song, B. S.; Carloni, P.; Gregan, F. J. Am.
Chem. Soc. 1999, 121, 6248-6257.
(26) Sigel, H.; Da Costa, C. P. J. Inorg. Biochem. 2000, 79, 247-251.
(27) Da Costa, C. P.; Song, B.; Gregan, F.; Sigel, H. J. Chem. Soc., Dalton
Trans. 2000, 899-904.
(28) Burgess, J. In Metal Ions In Solution; Ellis Horwood Limited: New York,
1978; pp 259-289.
(24) Neverov, A. A.; Brown, R. S. Inorg. Chem. 2001, 40, 3588-3595.
9
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A R T I C L E S
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The activation toward hydrolysis of the acyl phosphate
monoester that is provided by complexation with the lanthanide
ions is very significant, indicating that this can be a productive
system for utilizing the otherwise stable acyl phosphate mo-
noesters as acylating agents in biomimetic chemistry. Lanthanide
ions have already proven to be catalysts in the alcoholysis of
phosphodiesters,29 esters,6 and amides.4,5 The rates for the
uncatalyzed hydrolysis, with khydr ) 8.6 × 10-7 s-1 (obtained
by extrapolation of the linear magnesium ion dependence) and
methanolysis (kMeOH ) 2.1 × 10-7 s-1) of BMP are similar.
lanthanum triflate is greater than the rate enhancement of
hydrolysis achieved by a 10-fold higher concentration of
lanthanide. This suggests that alcoholysis may be a preferred
process in mixed solvents in the presence of lanthanides. On
the basis of our observation of a facilitated reaction with metal
ions, we have begun a complete study of lanthanide catalysis
in the reactions of alcohols with acyl phosphate monoesters and
with aminoacyl phosphate esters. We are currently developing
systems that target hydroxyl groups in complex biological
molecules.
The rate enhancement of methanolysis achieved by 10-4
M
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for continued support
through an operating grant.
(29) Morrow, J. R.; Aures, K.; Epstein, D. J. Chem. Soc., Chem. Commun. 1995,
2431-2432.
(30) We were unable to determine the pH dependence at higher hydroxide ion
concentration due to the formation of water-insoluble lanthanide hydroxide
gels.
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