Caffeine slows down water molecules
As coffee drinkers know, caffeine (C8H10N4O2) is strikingly effective at stimulating the central nervous system. Huib Bakker from the AMOLF research institute in Amsterdam and his colleagues wondered about the effect of this potent molecule on the most common molecule in the body: water. They found that both caffeine and taurine (C2H7NO3S), a common additive in energy drinks, actually slow the jostling of surrounding water molecules.
The researchers created saturated aqueous solutions of caffeine and taurine at room temperature and analyzed the water molecules’ mobility via two complementary spectroscopy techniques. First, the team applied an oscillating electric field and compared the rotation speeds of water molecules near and far from the dissolved molecules. Then the researchers used polarization-resolved IR pump-probe spectroscopy to quantify the reorientation dynamics. A resonant 100 fs pump pulse excited OH vibrations of water molecules that were oriented parallel to the polarization of the pulse. With a second laser pulse at the same frequency, the researchers determined the amount of slowed-down water molecules near caffeine and taurine by comparing the amount of excited OH groups both parallel and perpendicular to the polarization of the pump pulse.
Bakker and colleagues found that the presence of caffeine slowed the rotation of neighboring water molecules by a factor of five compared with molecules located farther away. One caffeine molecule slows roughly 10 surrounding water molecules, whereas one molecule of taurine slows four. The effect is negligible for human health—the caffeine in a cup of coffee slows about one in a million water molecules in the body, the researchers estimate.
The slowing likely takes place because water molecules encage hydrophobic parts of the caffeine and taurine molecules, thus forming local immobile clathrate-like structures. Consequently, the slowing effect on water is also expected to occur for other dissolved molecules, such as proteins and fatty acids, that contain hydrophobic groups. Understanding the effects of modestly sized molecules such as caffeine and taurine on the molecular dynamics of water is relevant for studying the properties of emulsions, hydrogels, and similar substances. The mobility of water also affects the rate at which other molecules can move and thereby the rate at which they can self-organize and react. (W. J. Smit et al., J. Phys. Commun. 3, 025010, 2019 .)
