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Water structure at hydrophobic surfaces probed with surface sum-frequency generation

I-16

Water structure at hydrophobic surfaces probed with surface sum-frequency generation


Huib J. Bakker1, Simona Strazdaite1, Konrad Meister1

1FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands

We study the structure of water molecules at different oil surfaces (hexane, heptane, and polydimethylsiloxane), and at an anti-freeze protein surface with surface sum-frequency generation (SFG) spectroscopy. In the experiment we combine broadband femtosecond IR pulses (bandwidth of ~200 cm-1) with relatively narrow-band light at 800 nm (bandwidth of 15 cm-1). The IR pulses have a sufficiently large bandwidth to measure the complete SFG spectrum of the OH(OD) stretch vibrations of H2O(D2O). We observe that the SFG intensity is 4-10 times higher at a water-oil interface than at a water/air interface, which indicates that water molecules are more strongly oriented at the interface with oil than at the interface with air.[1] We also observe that the SFG spectrum is red shifted compared to that of the water/air interface, which points at a strengthening of the hydrogen bonds. These observations indicate that the methyl and methylene groups of oil that protrude into the water phase act as a template for the water network to fold around, thus enhancing the water structure and the hydrogen-bond interaction. With increasing temperature the SFG intensity decreases and the SFG spectrum shows a significant blue-shift, indicating that the ordering of the water decreases (see Figure). Structuring of water also forms an essential element of the working mechanism of anti-freeze proteins. We use SFG to study the properties of water at the ice-binding surface of antifreeze protein III at temperatures well above the freezing point. The SFG spectrum shows a relatively narrow peak centered at 3250 cm-1, highly similar to the infrared and Raman spectrum of ice. We thus find evidence for the presence of highly structured ice-like water layers at the surface of the protein at temperatures well above the freezing point [2]. Decreasing the temperature to the biological working temperature of the protein (-2 to 0 °C) increases the amount of ice-like water, while a single point mutation in the ice-binding site is observed to completely disrupt the ice-like character of the water layer, in accordance with the elimination of the antifreeze activity. These observations indicate that not the protein itself, but rather the ordered ice-like protein hydration shell is responsible for the recognition and binding to ice.

References:

[1] S. Strazdaite et al., J. Chem. Phys. 140, 054711 (2014)

[2] K. Meister, S. Strazdaite, A.L. DeVries, S. Lotze, L.L.C. Olijve, I.K. Voets, and

[3] . Bakker, Proc. Nat. Acad. Sci. USA 111, 17732 (2014).