Biomedical Engineering Reference
In-Depth Information
Fig. 1.30
Structure of AQP1 together with a scheme of the molecular water pump. (
a
) Ribbon
diagram of the structure of AQP1. Two NPA regions are in green tube form. The ar/R region
is around the Arg residue. The Arg and Asn residues are in licorice form. The Arg residue has
a
e
net charge, and the two Asn residues on NPA motifs can be considered to carry an effective
charge of
C
0.5
e
on each residue (see text for details). The X-ray structure of AQP1 is obtained
from Protein Data Bank (PDB) (reprinted from [
18
]. Copyright 2008 Institute of Physics, UK) (
b
)
snapshot of the system, side view. The
green spheres
are the carbon atoms of the nanotube and
the graphite sheets. The
blue points
are the positive charges. The figure is not drawn to scale that
the sizes of water molecules and charges are enlarged (reprinted from [
43
]. Copyright 2007 Nature
Publishing Group)
C
the nanotube and the external charge, , is defined as the angle between the dipole
orientation of the water molecule and the line connecting the oxygen of the water
molecule and the external charge. It is clear that < > approaches 90
o
when
r
is
large enough, and the departure of < > from 90
o
shows the ordering of the water
orientations. We can even predict the electrostatic interaction energy of the external
charge with a water molecule at
r
, denoted by
E
theory
, from this < > distribution
with the dipole moment for each water molecule (0.489
e
A in the TIP3P water
model). As is shown in the figure (red symbols •-•),
E
theory
agrees well with the
simulation result
E
simulation
(the black symbols
).
With this observation, we can manipulate a drop of water in the nanotube by a
charge outside the nanotube. When there is a biomolecule in the water drop, we
can even manipulate its position. As the charge moves along, the water-peptide
mixture will follow it inside the nanotube. As the example shown in Fig.
1.32
,
we have shown the
x
-coordinate of the COM of the peptide and the peptide-water
mixture as a function of time together with the
x
-coordinate of the external charge.
We can see that the peptide-water mixture shows controllable movement by the
manipulation of external charge [
42
]. As we know, encapsulating the molecules into
-
