Table 2 Kinetics of the uranyl ion catalysed hydrolysisa
317 nm (appearance of p-nitrophenol). Pseudo-first-order rate constants
(measured over 20 h) are reported as means of 2 or 3 runs (r > 0.997), with
reproducibility within ±10%. The pH, adjusted with 0.1 mol dm23 HCl, was
buffered by the metal cations and varied no more than 0.2 pH units during
the course of reaction. The percentage cleavage was determined from the
concentration of liberated p-nitrophenoxide ion, measured at pH 12 (400
nm), after 20 h of reaction.
∑ Substrate 5 (mp 165 °C, decomp.) was prepared by phosphorylation of
hexadecanol with 4-nitrophenyl phosphorodichloridate (CH2Cl2, Et3N,
0–25 °C, 3.5 h), followed by methanolysis to hexadecyl methyl p-nitro-
phenyl phosphate. The methyl group was removed by reaction with LiBr in
refluxing acetone (48 h), affording white crystalline 5 (Li salt), which was
characterized by NMR and elemental analysis.
** HEPES buffer (2 mmol dm23) is present in these runs because it is
necessary in the preparation of liposomal 3 (ref. 12). The buffer alone does
not solubilize UO22+ in the presence of substrates 1, 3 or 5, nor does it alter
the reactivity of the cation toward 2 or 4. Control experiments also show that
HTMED alone does not induce cleavage of the substrates.
% Cleaved
at 20 h
Substrate
k
obs/s21
krel
1
2
3
4
5
9.5 3 1026
1.5 3 1025
1.1 3 1024
1.7 3 1026
1.1 3 1024
5.6
8.8
65
1.0
65
84
95
95
72
92
a
Conditions: [substrate] = 1 3 1024 mol dm23, [HTMED] = 1 3 1024
mol dm23, [UO22+] = 1 3 1023 mol dm23, 2 3 1023 mol dm23 HEPES
buffer, 0.01 mol dm23 KCl, pH 4.9 ± 0.1, 37 °C. Kinetic data were obtained
at 317 nm.
2+
coaggregation. We are thus able to measure the UO2
hydrolytic rate constants collected in Table 2.**
Note first that the UO22+ catalysed cleavages of 2 and 4 are
slower by factors of ca. 15 and 8, respectively, relative to
reactions in the absence of HTMED (cf. Table 1 and above),
presumably because the electrophilic character of the uranyl
cation is attenuated by chelation with HTMED. Most im-
portantly, however, Table 2 reveals the additional reactivity
inherent in the liposomal 3 and micellar 5 phosphodiester
substrates, both of which are hydrolysed 65 times more rapidly
than 4 in the presence of UO22+. This aggregate catalysis is
undoubtedly due to the binding of the metal cations to the
anionic aggregates, assisted by the HTMED which probably
forms part of a coaggregate. The cleavage of liposomal 3 by the
lanthanide, Eu3+, is similarly enhanced by a factor of 56,
relative to 4.12
References
1 (a) R. Breslow and D.-L. Huang, Proc. Natl. Acad. Sci. USA, 1991, 88,
4080; (b) J. R. Morrow, L. A. Buttrey and K. A. Berback, Inorg. Chem.,
1992, 31, 16 and references cited therein.
2 H.-J. Schneider, J. Rammo and R. Hettich, Angew. Chem., Int. Ed.
Engl., 1993, 32, 1716; K. G. Ragunathan and H.-J. Schneider, Angew.
Chem., Int. Ed. Engl., 1996, 35, 1219; J. R. Morrow, K. Aures and
D. Epstein, J. Chem. Soc., Chem. Commun., 1995, 2431.
3 B. K. Takasaki and J. Chin, J. Am. Chem. Soc., 1993, 115, 9337;
R. Breslow and B. Zhang, J. Am. Chem. Soc., 1994, 116, 7893;
B. K. Takasaki and J. Chin, J. Am. Chem. Soc., 1995, 117, 8582.
4 N. Takeda, M. Irisawa and M. Komiyama, J. Chem. Soc., Chem.
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5 S. Hashimoto and Y. Nakamura, J. Chem. Soc., Chem. Commun., 1995,
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M. Komiyama, N. Takeda, Y. Takahashi, H. Uchida, T. Shiiba,
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The UO22+-mediated hydrolyses of 3 and 5 at pH 5 (Table 2)
occur at similar rates to the Eu3+ reaction with 3 (kobs = 2 3
1024 s21, pH 5.6, 25 °C) at similar reactant concentrations.12
2+
Relative to substrate 2 in the absence of UO2 (Table 1), the
actinide plus aggregate catalysis affords a kinetic advantage of
> 3300 in the hydrolysis of substrates 3 and 5.
Finally, we note that both the exo- and endo-liposomal p-
nitrophenylphosphate functional groups of 3 are quantitatively
cleaved by UO22+-HTMED in a uniphasic kinetic process at
both 25 and 37 °C (Tc of 3 is 42 °C12); there is no evidence of
the exo-liposomal-specific cleavage observed with ‘naked’
Eu3+ at 25 °C.12 Presumably, the difference originates in the
obligatory presence of HTMED, which can chelate UO22+ and
rapidly ( > khydrol) transport it across the liposomal bilayer to
mediate endo-liposomal cleavage. Alternatively, the HTMED
molecules could disrupt the integrity of the liposomal mem-
brane, permitting uranyl cation permeation. Eu3+ cleavage of
liposomal 3 also becomes complete and uniphasic in the
presence of HTMED for related reasons.12
7 A. Tsubouchi and T. C. Bruice, J. Am. Chem. Soc., 1994, 116, 11 614;
J. Am. Chem. Soc., 1995, 117, 7399.
8 S. J. Oh, K. H. Song and J. W. Park, J. Chem. Soc., Chem. Commun.,
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9 M. Irisawa, N. Takeda, and M. Komiyama, J. Chem. Soc., Chem.
Commun., 1995, 1221.
10 N. H. Williams and J. Chin, Chem. Commun., 1996, 131.
11 B. Linkletter and J. Chin, Angew. Chem., Int. Ed. Engl., 1995, 34, 472;
J. Chin, Acc. Chem. Res., 1991, 24, 145.
12 R. A. Moss, B. D. Park, P. Scrimin and G. Ghirlanda, J. Chem. Soc.,
Chem. Commun., 1995, 1627.
13 R. C. Courtney, R. L. Gustafson, S. J. Westerback, H. Hyytiainen,
S. C. Cheberek, Jr. and A. E. Martell, J. Am. Chem. Soc., 1957, 79,
3030.
14 M. Kainosho and M. Takahashi, Nucleic Acid Res. Symp. Ser., 1983, 12,
181; K. E. Rich, R. T. Agarwal and I. Feldman, J. Am. Chem. Soc., 1970,
92, 6818; I. Feldman and K. E. Rich, J. Am. Chem. Soc., 1970, 92,
4559.
15 P. E. Nielsen, C. Hiort, S. H. Sonnichsen, O. Buchardt, O. Dahl and
B. Norden, J. Am. Chem. Soc., 1992, 114, 4967; N. E. Mollegaard,
A. I. H. Murchie, D. M. J. Lilley and P. E. Nielsen, EMBO J., 1994, 13,
1508
16 M. Shimazu, K. Shinozuka and H. Sawai, Angew. Chem., Int. Ed. Engl.,
1993, 32, 870; H. Sawai, K. Higa and K. Kuroda, J. Chem. Soc., Perkin
Trans. 1, 1992, 505.
17 T. Ihara, H. Shimura, K. Ohmori, H. Tsuji, J. Takeuchi and M. Takagi,
Chem. Lett., 1996, 687.
18 D. M. Brown and D. A. Usher, J. Chem. Soc., 1965, 6558.
19 J. Burgess, Metal Ions in Solution, Halsted Press, New York, 1978,
pp. 267–270.
20 P. Scrimin, P. Tecilla and U. Tonellato, Tetrahedron, 1995, 51, 217;
Y. Y. Lim, E. H. L. Tan and L. H. Gan, J. Coll. Interface Sci., 1993, 157,
442; F. M. Menger, L. H. Gan, E. Johnson and H. D. Durst, J. Am. Chem.
Soc., 1987, 109, 2800.
We are grateful to Professor Paolo Scrimin (University of
Padua) for a gift of HTMED, and to both Professors Scrimin and
John Brennan (Rutgers University) for helpful discussions. We
thank the U.S. Army Research Office for financial support.
Footnotes
† Martell (ref. 13) reported the rapid cleavage of the fluorophosphonate
Sarin by the 1,8-dihydroxynaphthalene-3,6-disodium sulfonate (DNS)
complex of UO22+. Phosphodiester substrates, however, are much less
reactive than fluorophosphonates. Indeed, we find substrates 2 and 3 to be
quite unreactive to the UO22+–DNS complex at either pH 5 or 7
(khydrol << 1x1026 s21), with hydrolysis < 50% complete after 5 days at
37 °C.
‡ After our current work had been completed, it was reported that the
actinide Th4+ accelerates the hydrolyses of various nucleotide phospho-
monoester and -diester bonds in acidic aqueous solutions (ref. 17).
§ UO22+ precipitates as polynuclear metal hydroxide gels at pH ! 5.3 (ref.
19) restricting us to pH ca. 5.0. Below pH 4.0, no hydrolysis of the
phosphodiester substrates was observed over 24 h.
¶ Hydrolyses were followed spectrophotometrically between 200–600 nm;
and kinetics were measured at both 290 nm (disappearance of substrate) and
Received, 10th January 1997; Com. 7/00260B
564
Chem. Commun., 1997