Liposomes for DNA Nanotechnology: Preparation, Properties, and Applications - DNA Nanotechnology: From Structure to Function - page 67

(Fig. 4.5 c). The color change is also from red to purple upon assembly (Fig. 4.5 h)
[ 48 ]. In this work, calcein was loaded into DPPC liposomes at a self-quenching
concentration, so that liposome leakage was monitored by calcein fluorescence
increase. Since the highest DPPC leakage is around its T c of 41 ı C, it is a common
practice to use associated AuNPs to harvest radiation energy to heat up DPPC
and induce its leakage [ 49 - 52 ]. UV irradiation resulted in a slow calcein release
from free DPPC liposome [ 48 ]. In the case where AuNPs were assembled close to
the liposome surface, reduced calcein leakage was observed. This was attributed
to AuNPs absorbing UV radiation and turning it into heat. The heat was not
effectively transferred to the liposome since they were separated by 8nmdefined
by the linker DNA. Most other systems containing liposome/AuNPs showed that
AuNPs promoted radiation-induced liposome leakage. In those systems, the AuNPs
were either adsorbed right onto the liposome surface via electrostatic attraction or
embedded into the bilayer if the AuNP surface was hydrophobic [ 49 - 52 ].

4.7 DNA-Directed Liposome Fusion

Membrane fusion is a fundamental process in biology, influencing a diverse range of
biological processes such as neurotransmission, endocytosis, and infection by virus.
Vesicle fusion is believed to occur via three steps [ 32 ]. First docking allows the
membranes to come close to one another. Second, the outer leaflet merges resulting
in stalk formation and a hemi-fused state. Third, content mixing is achieved via
transient pore formation or the merging of the two membranes. Although the
mechanism is not completely understood, the SNARE family protein (soluble N -
ethylmaleimide-sensitive factor attachment protein receptors) is believed to play a
major role for membrane fusion [ 53 ]. Given the complexity of the protein system,
H o ok et al. aimed to produce a simplified system using DNA hybridization to induce
liposome fusion [ 54 , 55 ].AsshowninFig. 4.6 , two types of liposomes were mixed,
each was functionalized with two cholesterol-modified DNAs forming a duplex
with an overhang. The DNA was designed such that hybridization was achieved
in a zipper-like fashion. Fusion was evaluated by monitoring the lipid mixing of
both the inner and outer leaflet of the bilayer. The fusion rate was dependent on the
number of DNA strands inserted into the membrane and in the presence of DOPE
and cholesterol in the liposome composition.
Boxer et al. studied this problem using a slightly different approach, where each
liposome was functionalized with just one type of ssDNA [ 32 , 33 ]. The DNAs
on two types of liposomes are complementary to each other and thus can bring
the liposomes very close to each other. They found much greater lipid mixing
than content mixing, suggesting that the latter is the rate-limiting step for such
DNA-directed fusion process. Although no zipping mechanism was engineering,
fusion was also observed. Fusion increased as the number of DNA per vesicle was
increased, and fusion was low when noncomplementary DNA conjugates were used
instead.