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Structured Water Layers Adjacent to Biological Membranes

Michael J. Higgins ^*, Martin Polcik ^*, Takeshi Fukuma ^*, John E. Sader , Yoshikazu Nakayama  and Suzanne P. Jarvis ^*

^* Centre for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin, Dublin, Ireland; Department of Mathematics and Statistics, University of Melbourne, Victoria, Australia; and Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuen-Cho Sakai, Osaka, Japan

Correspondence: Address reprint requests to Michael J. Higgins, E-mail: michael.higgins{at}tcd.ie.

Water amid the restricted space of crowded biological macromolecules and at membrane interfaces is essential for cell function, though the structure and function of this "biological water" itself remains poorly defined. The force required to remove strongly bound water is referred to as the hydration force and due to its widespread importance, it has been studied in numerous systems. Here, by using a highly sensitive dynamic atomic force microscope technique in conjunction with a carbon nanotube probe, we reveal a hydration force with an oscillatory profile that reflects the removal of up to five structured water layers from between the probe and biological membrane surface. Further, we find that the hydration force can be modified by changing the membrane fluidity. For 1,2-dipalmitoyl-sn-glycero-3-phosphocholine gel (L_ß) phase bilayers, each oscillation in the force profile indicates the force required to displace a single layer of water molecules from between the probe and bilayer. In contrast, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine fluid (L) phase bilayers at 60°C and 1,2-dioleoyl-sn-glycero-3-phosphocholine fluid (L) phase bilayers at 24°C seriously disrupt the molecular ordering of the water and result predominantly in a monotonic force profile.

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