Liposomes for DNA Nanotechnology: Preparation, Properties, and Applications - DNA Nanotechnology: From Structure to Function - page 66

of papers have been published to link DNA-functionalized liposomes to planar
supported bilayers [ 37 , 41 ]. There are also reports on assembling liposomes using
other types of interactions in the absence of DNA [ 42 - 46 ]. Several examples on
DNA-directed assembly of liposomes are given below.
Beales and Vanderlick assembled liposomes using cholesteryl-TEG DNA [ 26 ].
Approximately 155 DNAs were on each liposome. Resultant aggregates appeared
as white fluffy flocculates, which changed into a clear solution upon heating, and
cooling showed the same white fluffy flocculates reappearing. By varying the
amount of cholesteryl-TEG DNA added after liposome preparation, they showed
that only 39 DNAs per liposome were required for aggregation. Hadron and
Hatz used fluorophore-labeled membranes to control the formation of DNA-based
liposome assemblies using the streptavidin-biotin system. They incorporated a
phospholipid with a PEG spacer and biotin head group during liposome formation
to ensure sufficient distance between liposome surface and the active biotin group.
Reacting this with streptavidin allows a biotin-modified ssDNA to attach to the
liposome surface. The advantage of this method is the high detachment resistance
of the DNA-lipid complex, and therefore, there was no intermembrane transfer.
Aggregation was only achieved in the presence of both streptavidin and cDNA.
Using this controlled method, they assembled three liposomes with different
fluorophore-labeled streptavidins to control the assembly architecture [ 47 ].
In majority of the above-mentioned examples, the DNA sequences were designed
to be directly complementary so that aggregation takes place even without linker
DNA. We reason that using a linker DNA as shown in Fig. 4.5 d can provide
better control of the resulting aggregates. In Fig. 4.5 f, a cryo-TEM micrograph is
presented, where LUVs are packed together. To assist visualization of the aggre-
gates, 1% rhodamine-labeled lipid was used (Fig. 4.5 e). Using such a construct,
it is possible to compare DNA-linked AuNPs and liposomes side by side. Such a
comparison is interesting since the DNA ligands are fixed on AuNPs but mobile
on liposomes. Melting studies have been performed on DOPC, DPPC, and AuNPs
linkedbythesameDNA[ 36 ]. Essentially the same DNA melting transitions were
observed, suggesting that the mobility of the DNA ligands is not important in
determining the melting property of such nanostructures. In addition, the same T m
was observed with either neutral DOPC or negatively charged DOPG, which was
attributed to the 10-nm separation provided by DNA and the high-salt conditions
that screen the electrostatic repulsion between liposomes. In the case of changing
particle size, AuNPs showed higher T m with larger size, but the liposome T m was not
affected. In addition, introducing nucleotide spacers significantly increased AuNPs
T m but had no effect on liposome T m . These differences suggest the soft nature of
liposome being better at accommodating the crowded environment.

4.6.2 Assembly of Liposomes and AuNPs

It is also convenient to form aggregates containing both AuNPs and liposome
(Fig. 4.5 i), where the AuNPs are more separated than in pure AuNP aggregates