Regensburg 2007 – scientific programme

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DS: Fachverband Dünne Schichten

DS 19: Symposium: In situ Optics II

DS 19.3: Invited Talk

Wednesday, March 28, 2007, 18:15–18:45, H34

Water at model membranes: structure, dynamics and biomolecular sensing — •Mischa Bonn — FOM-Institute for Atomic and Molecular Physics, Kruislaan 407; NL- 1098 SJ, Amsterdam

Lipids form the basic building blocks of cell membranes. Thanks to their bipolar nature (lipids consist of a polar head group and a long apolar tail), they possess the ability to self-organize and thus form the boundary of living cells. The interaction of lipids with water drives the self-assembly process and water is therefore an essential ingredient of a biological membrane. It has been a challenge to elucidate the role of water in biomolecular processes, including those occurring at the membrane surface.

Vibrational spectroscopies have been shown to be very useful for the study of water, as the O - H stretch vibration of water is a very sensitive reporter of the local environment of the water molecule. The non-linear vibrational spectroscopic technique of sum frequency generation (SFG) further allows us to distinguish bulk water near the membrane from water physically bound to the membrane. This unique surface specificity enables the detailed study of membrane-bound water. Moreover, we can investigate the femtosecond dynamics of interfacial water molecules, as we use femtosecond laser pulses in our experiments.

We find that, although the SFG spectra of interfacial water at the membrane-water interface very closely resembles that of interfacial water at the air-water and air-quartz interfaces, the vibrational dynamics are markedly different. Whereas for both the air-water and air-quartz interfaces, the surface water molecules exchange energy very rapidly with the bulk, this is not the case for membrane-bound water. This is the first direct experimental evidence that membrane-bound water is an inherent part of the membrane: water at the membrane interface does not just terminate the bulk.

We further show that the preferential adsorption of DNA to a lipid monolayer on water, results in dramatic changes in the interfacial water structure. In this way, the water molecules, which are interrogated with SFG, act as highly sensitive reporters for the presence of DNA, constituting a novel, non-invasive method for label-free detection of DNA with very high (picomolar) sensitivity. The approach we present is very simple and it can easily be extended to be specific towards unique DNA sequences.