Water lends protons a helping hand

Protons in water are extremely mobile. AMOLF researchers have shown that a surprisingly large number of twenty water molecules is involved in proton transport, and that there are two types of water molecules that assist the proton in this process.

Until now it was not known how many water molecules facilitate the transport and which role the different water molecules fulfill. Proton transport is essential for energy management in (living) cells. The researchers published their results in the May 15th edition of the prestigious journal Physical Review Letters.

Proton transfer in aqueous systems is an omnipresent and important process, which forms the basis for the generation and storage of energy in living organisms and lies at the heart of the operation of fuel cells. Protons in water are extremely mobile – much more so than common ions. This is the result of the fact that proton transport in water does not take place through normal diffusion, but through a process where hydrogen bonds between water molecules are converted into covalent bonds, and vice versa (see figure 1 and animation 1). Thus, it is not the proton itself that is transported; only the charge of the proton is transported; and not its mass. The charge is basically passed on from water molecule to water molecule.

Proton transport is hence based on a subtle interplay between the proton and the surrounding water molecules. An important question is how many water molecules are exactly involved in the transfer of the proton. In theoretical studies, it was found that not only the direct neighbors of the proton play a role in proton transport, but also more remote water molecules. How many water molecules are involved and what their role exactly is, was however not known.

Researcher Klaas-Jan Tielrooij and colleagues in AMOLF’s Ultrafast Spectroscopy research group have clarified this by using a very short (about one picosecond) electrical pulse (Terahertz pulse) and investigating how the presence of protons in water affects the reorganization of water molecules. These experiments make use of the fact that in the water molecule, the oxygen atom has a small negative charge and the hydrogen atoms a small positive charge: water has a large dipole moment. Due to this, water molecules are sensitive to an externally applied electric field. The water molecules that interact with the proton are less capable of following the externally applied field (the Terahertz pulse), because the proton strongly affects the (re-)orientation of the water.

Indeed this was the case: the measurements show that each proton cause about twenty water molecules to no longer follow the external field. It turns out that of these twenty water molecules, four are strongly bound to the proton: the proton is present as a hydronium ion (a water molecule with an additional proton, H3O+), which is surrounded by three strongly bound water molecules. In the proton transfer one of these three water molecules takes up the charge of the proton. This process requires the reorientation and reorganization of the surrounding water molecules, since the newly formed hydronium ion should also be surrounded by three water molecules. These measurements show that about fifteen water molecules need to rearrange to facilitate proton transfer (see figure 2 and animation 2). Hence, the water molecules in the environment of the proton take on two different roles.

A follow-up question immediately presents itself: what happens with the rate and the mechanism of proton transfer if there are not twenty water molecules available in the environment? This is the case, for instance, in the pore of a membrane or if the neighboring water molecules are hindered in their movement by the presence of other molecules or ions. Based on the present results it can be expected that for these systems proton transfer will either be much slower or will take place through a different mechanism. Tielrooij and colleagues will be studying this in the near future.

Reference:
Structure dynamics of the proton in liquid water probed with Terahertz time-domain spectroscopy, Klaas-Jan Tielrooij, Rutger Timmer, Huib Bakker en Mischa Bonn, Phys. Rev. Lett. 102, 198303 (2009)

More information:
Klaas-Jan Tielrooij, Tel. 020 – 6081225 or prof .dr. Mischa Bonn, Tel. 020 – 608 1234

Figure 1 The mechanism through which protons move through water. Hydrogen bonds (dashed line) are converted into covalent bonds (solid lines). White is a hydrogen atom and red an oxygen atom.

Figure 2 Proton transport, where the hydronium ion (a water molecule with an additional proton, H3O+) is surrounded by three strongly bound water molecules. Reorganization of many surrounding water molecules facilitates the transport.