| Abstract
| - Interactions between hydrophobic chains of lipid monolayers and interactions between hydrophilic headgroups oflipid bilayers (with or without a molecular recognition step) are now well documented, especially for commonly usedlipids. Here, we report force measurements between a new class of fluorinated lipid layers whose headgroups (syntheticligands of retinoid receptors) display a very unusual polar/apolar character and can interact via a combination ofhydrophobic forces and hydrogen bonds. Although these two interactions produce adhesion and are therefore not easilydistinguishable, we show that it is possible to extract both contributions unambiguously. Experiments are performedboth in pure water, where the adhesion is a combination of hydrophobic forces and hydrogen bonds, and in Tris buffer,where the hydrophobic effect is the dominant short-range attractive force. The contribution of hydrophobic forcesscaled down to molecular interactions is deduced from force versus distance profiles, and the same value is foundindependently in pure water and Tris buffer, about 1 kBT. We also show that retinoid lipid layers attract each otherthrough a very long-range (100 nm) exponential force, which is insensitive to the pH and the salinity. The origin ofthis long-range attraction is discussed on the basis of previously proposed mechanisms.
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