After excitation, pyrene emits fluorescence from the excited
1
0
monomer and from the excimer (excited-state dimer). In
pyrene-containing lipids, the intensity of the excimer peak
has been extensively used to study lipid redistribution in
liposomes.8 When the lipids have metal-chelating head-
groups, addition of metal ions to the resultant liposomes
causes the lipids to disperse, leading to a decrease in the
excimer emission intensity. The liposomes can be used as
sensors for transition metal ions.11
b,9
The synthesis of various metal-chelating lipids are reported
12
in the literature; however, reports for the synthesis of
13
pyrene-containing, metal-chelating lipids are relatively few.
For the reported lipids, the metal-chelating headgroups are
usually iminodiacetate (IDA) or nitrilotriacetate (NTA).
Herein, we describe the synthesis of several metal-chelating
lipids with a variety of metal chelating headgroups and
pyrene as the fluorophore. The EDTA and DTPA headgroups
were selected for their ability to complex transition and
lanthanide ions with high affinity.14 The lipids were incor-
porated into liposomes, and the fluorescence property of the
pyrene moiety (excimer-to-monomer ratio) was followed in
Figure 1. Structures of the lipids synthesized.
2
+
2+
the presence of various transition metal ions (Cu , Ni ,
2+
2+
Zn , and Hg ). The results indicated that the liposomes
can be used to sense cupric ions.
affinity (K > 1015 M ) for lanthanide ions. DPTA has
-1
17
The structures of pyrene-containing, metal-chelating lipids
are shown in Figure 1. These lipids are based on racemic
been incorporated into lipid 4 as the metal-chelating head-
group. The ligand DTPA complexes lanthanide ions with
2
,3-diaminopropanoic acid. Lipid 1 has IDA as the metal-
20
-1 18
high affinity (K > 10 M ).
1
2
chelating headgroup. IDA has a strong affinity (K > 10
M ) for various transition metal ions (e.g., Cu , Ni , Co ,
etc.). Lipid 2 has two metal-chelating headgroups (IDA).
The synthesis of the lipids is depicted in Scheme 1.
Commercially available diacid (1,10-decanedicarboxylic acid,
-1
2+
2+
2+
1
5
9
) was selectively converted into monoester 10. Slow
The two IDA groups of this lipid can position two transition
addition of a solution of ethanol (over a 10 h period, 5 mL/
h, using a syringe pump) to a solution of acid chloride from
16
metal ions ∼8 Å apart. Lipid 3 contains the widely used
EDTA as the metal-chelating headgroup. EDTA has a strong
9
in the presence of pyridine afforded 10 in 49% yield after
chromatographic purification. Acid-ester 10 was then coupled
with 1-pyrenemethylamine using BOP reagent in excellent
yield. Selective saponification with LiOH was used to
generate acid 11. Selective amidification of the primary
amine over the secondary amine group in 12 (racemic) was
performed with palmitoyl chloride (1 equiv) in the presence
of triethylamine to give monoamide 13 in 43% yield after
purification. Sequential amidification of monoamide 13 with
11 was carried out by BOP reagent. The ester groups were
hydrolyzed by LiOH‚H O to provide compound 14. Com-
2
pound 14 is the common synthon for the syntheses of lipids
1-4 (Scheme 1).
For the synthesis of IDA-based lipid 1, acid 14 was
combined with reported IDA-amine ester 1516 in the
(
9) (a) Somerharju, P. Chem. Phys. Lipids 2002, 116, 57-74. (b) Song,
X.; Swanson, B. I. Anal. Chem. 1999, 71, 2097-2107. (c) Yamanaka, S.
A.; Charych, D. H.; Loy, D. A.; Sasaki, Y. D. Langmuir 1997, 13, 5049-
5
053. (d) Malony, K. M.; Shnek, D. R.; Sasaki, Y. D.; Arnold, F. H. Chem.
Biol. 1996, 3, 185-192.
(
10) (a) Lehrer, S. S. Methods Enzymol. 1997, 278, 286-295. (b)
Dietrich, C.; Boscheinen, O.; Scharf, K. D.; Schmitt, L.; Tampe, R.
Biochemistry 1996, 35, 1100-1105. (c) Ng, K.; Pack, D. W.; Sasaki, D.
Y.; Arnold, F. H. Langmuir 1995, 11, 4048-4055.
(11) (a) Sasaki, D. Y.; Waggoner, T. A.; Last, J. A.; Alam, T. M.
Langmuir 2002, 18, 3714-3721. (b) Waggoner, T. A.; Last, J. A.; Kotula,
P. G.; Sasaki, D. Y. J. Am. Chem. Soc. 2001, 123, 496-497. (c) Sasaki, D.
Y.; Padilla, B. E. J. Chem. Soc. Chem. Commun. 1998, 1581-1582. (d)
Yamanaka, S.; Charych, D.; Loy, D. A.; Sasaki, D. Y. Langmuir 1997, 13,
5
049-5053. (e) Sasaki, Y. D.; Shnek, D. R.; Pack, D. W.; Arnold, F. H.
Angew. Chem., Int. Ed. Ingl. 1995, 34, 905-907. (f) Singh, A.; Tsao, L. I.;
Markowitz, M.; Gaber, B. P. Langmuir 1992, 8, 1570-1577. (g) Kunitake,
T.; Ishikawa, Y.; Shimomura, M.; Okawa, H. J. Am. Chem. Soc. 1986,
1
1
08, 327-329. (h) Shimomura, M.; Kunitake, T. J. Am. Chem. Soc. 1982,
04, 1757-1759.
3
presence of BOP/Et N. The hydrolysis of ester group and
subsequent precipitation by lowering the pH to 3.0 provided
(12) (a) Roy, B. C.; Fazal, M. D.; Arruda, A.; Mallik, S.; Campiglia, A.
D. Org. Lett. 2000, 2, 3067-3070. (b) Dorn, I. T.; Neumaier, K. R.; Tampe,
R. J. Am. Chem. Soc. 1998, 120, 2753-2763. (c) Wisner, E. R.; Aho-
Sharon, K. L.; Bennet, M. J.; Penn, S. G.; Lebrilla, C. B.; Lantz, M. H. J.
Med. Chem. 1997, 40, 3992-3996. (d) Storrs, R. W.; Tropper, F. D.; Li,
H. Y.; Song, C. K.; Kuniyoshi, J. K.; Sipkins, D. A.; Li, K. P. C.; Bednarski,
M. D. J. Am. Chem. Soc. 1995, 117, 7301-7306.
the lipid. Lipid 2 was synthesized by coupling of acid 14
1
6
with amine 16 and subsequent ester hydrolysis. Acid 14
1
3
was combined with EDTA-amine 17 in the presence of
BOP reagent and followed by hydrolysis to afford lipid 3 in
(
13) Roy, B. C.; Peterson, R.; Mallik, S.; Campiglia, A. D. J. Org. Chem.
000, 65, 3644-3651.
14) Other examples of nonpolymerizable lanthanide chelating lipids:
2
(
(17) (a) Latva, M.; Kankare, J.; Haapakka, K. J. Coord. Chem. 1996,
38, 85-99. (b) Franklin, S. J.; Raymond, K. J. Inorg. Chem. 1994, 33,
5794-5804.
(18) (a) Chong, H. S.; Garmestani, K.; Bryant, L. H.; Brechbiel, M. W.
J. Org. Chem. 2001, 66, 7745-7750. (b) Wu, S. L.; Horrocks, W. D. Anal.
Chem. 1996, 68, 394-401.
Hovland, R.; Glogard, C.; Aasen, A.; Klavenees, J. J. Chem. Soc., Perkin
Trans. 2 2001, 929-933. Also see ref 11a.
(15) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum
Press: New York, 1975; Vol. 2, pp 67-68.
(16) Roy, B. C.; Mallik, S. Org. Lett. 2001, 3, 1877-1879.
12
Org. Lett., Vol. 5, No. 1, 2003