Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Tu-0940
Anisotropic Hydration of Biological Molecules Visualized by Three-
Dimensional Scanning Force Microscopy
Hitoshi Asakawa 1 , Yasumasa Ueda 1 and Takeshi Fukuma 1,2
1 Frontier Science Organization, Kanazawa University, Japan
2 PRESTO, JST, Japan
Hydration of biological molecules has grave impact on important biological processes
such as membrane fusions and protein activities. However, molecular-scale details of
local hydration structures have remained to be understood due to the lack of a method
able to visualize their three-dimensional (3D) distribution at sub-nanometer resolution. In
order to resolve this issue, we have recently developed a novel technique referred to as
3D scanning force microscopy (3D-SFM). Combined with frequency modulation atomic
force microscopy (FM-AFM) in liquid, the method allows us to record 3D volume data of
frequency shift (Δf ) induced by tip-sample interaction force in less than 1 min. We are
able to slice the obtained 3D-SFM image in any directions to produce 2D cross-sectional
Δf images.
In this study, we have investigated hydration structures formed at the interface between
a model biological membrane and aqueous solution. A dipalmitoylphosphatidylcholine
(DPPC) bilayer was formed on a cleaved mica surface as a model biological membrane in
HEPES buffer solution. The XY cross-sectional Δf image inserted in Fig. 1(a) was
obtained by slicing the 3D-SFM image near the tip position closest to the membrane
surface. The image shows the pseudo-hexagonal packing of DPPC molecules. The
periodic Δf variation corresponding to the DPPC molecule is also found in a crosssectional
image obtained by slicing the 3D-SFM image with a plane perpendicular to the
membrane surface (Fig. 1(b)). The image reveals formation of multiple hydration layers
on the DPPC bilayer. In addition, we found that the first hydration layer shows
anisotropic distribution around the DPPC headgroups elongated in one direction in a 3D
space as indicated by the white arrows. The result demonstrates the capability of 3D-SFM
for visualizing 3D distribution of water molecules interacting with a biological molecule.
a) b)
Figure 1: 3D-SFM images of the interface between a DPPC bilayer and HEPES/NaCl (10/100
mM, pH 7.4) buffer solution. (a) Schematic illustration of a membrane/water interface. Inset: XY
cross-sectional Δf image, (b) Z cross-sectional Δf image.
46