Chemistry Reference
In-Depth Information
(Su et al. 2005). Mannosyl lipid containing liposomes have also been used for this
purpose (Zhang et al. 2005). The glycolipids are presumed to interact with lectins
present on the surface of the bacterial cells.
12.2.2. Other Ligands
PDA liposomes have been functionalizedwith avarietyof ligands besides carbohydrates.
Liposomes containing a mixture of biotin-terminated diyne lipids and carboxy-
terminated diyne lipids were prepared (Jung et al. 2006). The irradiation time required
for cross-linking decreased as the mole fraction of the biotinylated lipid was increased.
Addition of streptavidin to these liposomes resulted in a chromatic transition from blue
to red. Because of the high concentrations of streptavidin used in this experiment, the
resulting red liposomes precipitated as a result of liposome cross-linking induced by
streptavidin, which contains four binding sites for biotin. A 24-mer target DNA
sequence was detected colorimetrically using two liposomes, each functionalized with
a 12-mer probe sequence complementary to the 5
0
or 3
0
end of the target sequence
(Wang and Ma 2005; Wang et al. 2006). The probe sequences were anchored to the
liposome using a cholesterol conjugate of the oligonucleotide. A diyne lipid terminated
in a hexapeptide sequence containing the Arg-Gly-Asp tripeptide sequence was used
to detect a soluble form of the a
5
b
1
integrin receptor (Biesalski et al. 2005).
Several approaches for using PDA liposomes to detect antibody-antigen inter-
actions were reported. One approach fused peptide epitopes to membrane spanning
a-helical peptides as a method for incorporating antigens into mixed diyne/phospho-
lipid liposomes (Kolusheva et al. 2001). A synthetic peptide anchor as well as one
derived from a phage coat protein were successfully used as membrane anchors. A
variety of peptide epitopes were detected, and the specificities obtained from the col-
orimetric assay compared favorably to the results from a more conventional ELISA
assay. A second strategy conjugated the antibodies directly to PDA liposomes
using succinimide ester bioconjugation chemistry (Su et al. 2004). Anti-human
immunoglobulin (Ig) antibody from goat was conjugated to PDA liposomes that
were subsequently photopolymerized. Addition of human Ig to a solution of these
liposomes induced a chromatic transition in a concentration-dependent fashion.
Mixed PDA/phospholipid vesicles, most frequently prepared with a 3:2 ratio of
tricosadiynoic acid (5) and dimyristoyl phosphatidylcholine (6, DMPC), have been
used extensively for sensing applications. These vesicles have been characterized
in detail using a variety of techniques, including small-angle X-ray scattering
(SAXS), differential scanning calorimetry (DSC), and electron spin resonance
(Kolusheva et al. 2003). These studies suggest that the phospholipids and diacety-
lenes assemble into distinct domains. Formation of ternary liposomes using choles-
terol as the third component reduces the thermochromic response of the vesicles,
consistent with decreased phospholipid fluidity upon cholesterol addition.
12.2.3. Small Molecules
PDA vesicles have been used to screen small molecules that are known to interact
with membranes. Screening of almost 40 compounds against PDA/DMPC liposomes
