J. Am. Chem. Soc. 1996, 118, 12495-12498
12495
Solvent-Accelerated Decarboxylation of
N-Carboxy-2-imidazolidinone. Implications for Stability of
Intermediates in Biotin-Dependent Carboxylations
Jubrail Rahil, Shaochun You, and Ronald Kluger*
Contribution from The Lash Miller Laboratories, Department of Chemistry,
UniVersity of Toronto, Toronto, Canada M5S 3H6
ReceiVed July 17, 1996X
Abstract: The decarboxylation of N-carboxy-2-imidazolidinone has previously been established as a model for the
transfer of carbon dioxide from N(1′)-carboxybiotin. The present paper reports the pH-dependence of the reaction
as well as the acceleration of the reaction in methanol and in acetonitrile. These results suggest that enzymic reactions
of N(1′)-carboxybiotin in a hydrophobic active site with decreased hydrogen bonding can be rapid if the energy of
desolvation is compensated by the energy made available by association of the substrate and protein. In addition,
a report on the decarboxylation of N-carboxy-2-imidazolidinone in organic solvents containing macrocycles (Kluger,
R.; Tsao, B. J. Am. Chem. Soc. 1993, 115, 2089-90) must be reinterpreted on the basis of the inherent instability
of the substrate under the reaction conditions.
The transfer of carbon dioxide from the enzymic intermediate
N(1′)-carboxybiotin to a carbanionic acceptor is a key step in
biosynthetic carboxylation pathways.1 Knowledge of the re-
activity patterns of N(1′)-carboxybiotin can reveal the potential
catalytic functions in an enzymic pathway associated with this
intermediate.2 Caplow and Yager used N-carboxy-2-imidazo-
lidinone (1) to model the reactivity expected for N(1′)-
carboxybiotin.3
to a reactant through desolvation.7,8 The energy for this
desolvation can be derived from coupling of favorable binding
interactions between the substrate and the enzyme. Could such
a mechanism contribute to increased reactivity of N(1′)-
carboxybiotin upon the binding of substrate? In order to
consider this possibility, we have extended the studies of Caplow
and Yager3 to find the effects of the reaction medium on the
stability of N-carboxy-2-imidazolidinone (1).
Experimental Section
General. Chemicals and reagents obtained from commercial
suppliers were of the highest available grade. Liquids were distilled.
UV spectra were recorded with a double-beam spectrophotometer
employing a double-pass monochromator to minimize noise at high
absorbance. 1H NMR spectra were recorded at 200 MHz while 13C
NMR spectra were recorded at 100 MHz (chemical shifts for carbon
refer to dioxane at 67.4 ppm). pH measurements were done with a
meter equipped with a combination electrode. IR spectra were
performed on a FT instrument. Kinetic data were fit to integrated rate
expressions by nonlinear regression on a computer.
They observed that the species is relatively unreactive. If
the reactivity of N(1′)-carboxybiotin is similar, an enzyme would
have to utilize catalytic functions or alter the structure of the
cofactor to make it more reactive. Since the formation of N(1′)-
carboxybiotin on an enzyme may occur in the absence of an
acceptor substrate, this stability may be necessary to prevent
abortive reactions in which carbon dioxide is transferred to
solvent water rather than to a metabolic acceptor. Wallace,
Keech, and co-workers showed that in pyruvate carboxylase, a
typical biotin-dependent enzyme, the reactivity of N(1′)-
carboxybiotin is altered once the acceptor substrate (or its
analogue) is bound.4 The mechanism for such activation is
unknown although several schemes have been suggested involv-
ing structural changes in the protein and cofactor.2,5,6 An
alternative utilization of a structural change in a protein is one
that leads to specific stabilization of a transition state relative
Synthesis. N-carbomethoxycarbonyl-2-imidazolidinone was pre-
pared according to the published procedure.9,10
crystallized from water: mp 173-175 °C; H NMR (CDCl3) δ 3.51
The product was
1
and 3.93 (m, 4H, -CH2CH2-), 3.86 (s, 3H, -OCH3), 6.45 (broad,
1H, NH); 13C NMR (CDCL3) δ 36.84 and 43.31 (N-CH2CH2-N),
53.46 (-OCH3), 152.50 (N-CO-O), 156.31 (N-CO-N); IR (KBr)
1669, 1761 cm-1
.
N-Carboxy-2-imidazolidinone (1) was obtained by hydrolysis of
N-methoxycarbonyl-2-imidazolidinone.3 The ester (0.13 g, 0.9 mmol)
was suspended in 10 mL of water. Potassium hydroxide solution (1.8
mL, 1 M) was added with stirring. After 15 min, the solution was
freeze-dried. NMR analysis revealed that the residue is an 80:20
mixture of N-carboxy-2-imidazolidinone and 2-imidazolidinone. Since
decarboxylation of N-carboxy-2-imidazolidinone is slow under these
conditions, the mixture results from the carbamate ester undergoing
both C-N and C-O cleavage. Other reaction conditions gave lower
proportions of the desired product. All conditions attempted for
separation led to decomposition of the desired material. For 1: 1H
NMR (D2O) δ 3.38 and 3.73 (two multiplets, 4H); 13C NMR (D2O)
X Abstract published in AdVance ACS Abstracts, December 1, 1996.
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S0002-7863(96)02464-X CCC: $12.00 © 1996 American Chemical Society