Biomedical Engineering Reference
In-Depth Information
angle ψ, the resulting SHG polarization anisotropy (SPA) data can be fitted with Equation 5.12 in the
following form:
I
SHG
(
ψ
)
∝
sin
2
2 +
ψ
(
sin
2
ψ γ
+
cos
2
ψ
)
2
(5.14)
with
χ
χ
( )
2
N
β
cos
3
ϑ
2
yyy
yxx
γ
≡
=
=
(5.15)
( )
2
(
N
/
2
)
β
cos
ϑ
sin
2
ϑ
tan
2
ϑ
As an example, Figure 5.3b shows three different
I
SHG
profiles for samples characterized by different
values of the angle ϑ. Therefore, SPA data can be used to access information on the structural distribu-
tion of HRS emitters in the sample.
5.4 Membrane SHG imaging
For imaging applications, it is therefore highly desirable that the radiating molecules be organized
rather than unorganized because much more signal is generated for the same number of molecules.
As indicated above, the plasma membrane provides a plane for the ordered distribution of those HRSs
which possess the physical property of embedding into the membrane bilayer itself. A molecular design
strategy for obtaining this condition is shown in Figure 5.4: hydrocarbon (hydrophobic) side chains are
grafted at one end of the dye and a polar head group at the other. The HRS capability, on the other hand,
is conferred by a push-pull design with donor and acceptor moieties spanned by a uni-axial charge
transfer path.
When perfused onto a lipid bilayer, the dye molecules insert themselves into the membrane with the
same orientation: the hydrophobic side chains as far as possible from the aqueous surroundings, that
is, directed into the membrane interior; and the polar head group pointing outward, into the aqueous
surroundings (see Figure 5.5).
(a)
O
O
-
S
O
N
+
N
(b)
O
O
-
S
O
N
N
+
e
-
FIgurE 5.4
Chemical structure of the voltage-sensitive dye, di-4-ANEPPS. The dye is shown in two configura-
tions (panels a and b) differing for charge distribution which is transferred intra-molecularly by a global shift of all
the π-electron bonds from the donor to the acceptor ends of the molecule. The structure also shows the polar head
and hydrophobic tail conferring to the molecule its ability to partition inside the membrane.
